[{"id":1,"s":"Cell Membrane","t":"Phospholipid Translocators","u":"Floppase","q":"A healthy nerve cell keeps phosphatidyl choline and sphingomyelin in the outer leaflet through one-way traffic by:","o":["Flippase","Floppase","Phospholipase","Racemase","Scramblase"],"a":1,"p":"Floppase moves lipids from inner → outer leaflet (ATP-dependent). 'FLOP out' — floppase flops lipids outward. ATP-dependent, one-way, outer leaflet."},{"id":2,"s":"Cell Membrane","t":"Phospholipid Translocators","u":"Floppase","q":"A healthy cell maintains phosphatidylcholine and sphingomyelin in the outer leaflet for structural asymmetry. Which enzyme is responsible?","o":["Flippase","Floppase","Phospholipase","Racemase","Scramblase"],"a":1,"p":"Floppase (ATP-dependent) moves PC and SM outward. PI in the INNER leaflet is the signal transduction lipid (PI→PIP2→DAG+IP3), not PC."},{"id":3,"s":"Cell Membrane","t":"Phospholipid Translocators","u":"Flippase","q":"An endometrial cell undergoes apoptosis during menstrual shedding. Which enzyme is inactivated by cytosolic calcium to facilitate apoptosis?","o":["Flippase","Floppase","Kinase","Phospholipase","Scramblase"],"a":0,"p":"Flippase is Ca²⁺-INACTIVATED → PS migrates to outer leaflet → apoptosis signal. Compare: Scramblase is Ca²⁺-ACTIVATED."},{"id":4,"s":"Cell Membrane","t":"Phospholipid Translocators","u":"Flippase","q":"A coronavirus-infected cell undergoes apoptosis. Which ATP-dependent enzyme keeping PS and PE in the inner leaflet is inactivated by cytosolic calcium?","o":["Flippase","Floppase","Kinase","Phospholipase","Scramblase"],"a":0,"p":"Flippase (ATP-dependent) keeps PS and PE in the inner leaflet. Ca²⁺ surge inactivates it → lipid asymmetry lost → cell death signal sent."},{"id":5,"s":"Cell Membrane","t":"Phospholipid Translocators","u":"Flippase","q":"A senescent cell undergoing apoptosis has which ATP-dependent enzyme inactivated, leading to loss of PS and PE from the inner leaflet?","o":["Flippase","Floppase","Kinase","Phospholipase","Scramblase"],"a":0,"p":"Flippase inactivation in senescent cells → PS externalisation → macrophage 'eat me' signal → clearance."},{"id":6,"s":"Cell Membrane","t":"Phospholipid Translocators","u":"Flippase","q":"A healthy cell avoids apoptosis via an ATP-dependent enzyme keeping PS and PE in the inner leaflet through one-way traffic. Which enzyme?","o":["Flippase","Floppase","Kinase","Phospholipase","Scramblase"],"a":0,"p":"Active flippase = PS/PE confined to inner leaflet = no 'eat me' signal = cell survival. Loss of flippase activity triggers apoptosis."},{"id":7,"s":"Cell Membrane","t":"Phospholipid Translocators","u":"Scramblase","q":"A senescent cell undergoes apoptosis. Which ATP-independent enzyme flips PS to the outer leaflet via two-way bidirectional traffic?","o":["Flippase","Floppase","Kinase","Phospholipase","Scramblase"],"a":4,"p":"Scramblase: ATP-independent, Ca²⁺-activated, bidirectional. Randomises lipid distribution during apoptosis, externalising PS."},{"id":8,"s":"Cell Membrane","t":"Phospholipid Translocators","u":"Scramblase","q":"A toxin-exposed cell undergoes apoptosis. Which ATP-independent enzyme activated by calcium flips PS to the outer leaflet?","o":["Flippase","Floppase","Kinase","Phospholipase","Scramblase"],"a":4,"p":"Scramblase ACTIVATED by Ca²⁺ (unlike flippase which is INACTIVATED). Mnemonic: Scramblase Scrambles — activated = chaos."},{"id":9,"s":"Cell Membrane","t":"Phospholipid Translocators","u":"Scramblase","q":"Which of the following disrupts plasma membrane asymmetry during apoptosis?","o":["Flippase","Floppase","Kinase","Phospholipase","Scramblase"],"a":4,"p":"Membrane asymmetry MAINTAINED by flippase+floppase, DISRUPTED by scramblase during apoptosis."},{"id":10,"s":"Cell Membrane","t":"Phospholipid Translocators","u":"Scramblase","q":"Which of the following shifts phosphatidylserine to the outer leaflet signalling apoptosis?","o":["Flippase","Floppase","Kinase","Phospholipase","Scramblase"],"a":4,"p":"PS externalisation by scramblase = 'eat me' signal. Detected by Annexin V assay in flow cytometry."},{"id":11,"s":"Cell Membrane","t":"Membrane Properties","u":"Flexibility","q":"Membrane flexibility may be defined by which property of the cell membrane?","o":["Bending","Mosaicism","Resealing","Rigidity","Viscosity"],"a":0,"p":"Three key membrane properties: Bending=flexibility; Resealing=self-healing; 2D fluidity=lateral movement. Mosaic pattern is a structural description not a property."},{"id":12,"s":"Cell Membrane","t":"Membrane Lipids","u":"Lipid Rafts","q":"Lipid rafts are a collection of cholesterol and which other lipid?","o":["Glycoglycerolipid","Glycophosphatidylinositol","Glycosphingolipid","Phosphatidylcholine","Phosphatidylserine"],"a":2,"p":"Lipid rafts = cholesterol + glycosphingolipids. Ordered microdomains less fluid than surrounding membrane. Important in signalling and pathogen entry."},{"id":13,"s":"Cell Membrane","t":"Membrane Lipids","u":"Phosphate Group","q":"In membrane phospholipids, the phosphate group is directly esterified to which of the following?","o":["Diacylglycerol","Fatty acid","Glycerol","Monoacylglycerol","Triacylglycerol"],"a":2,"p":"Phosphate esterified to sn-3 hydroxyl of glycerol backbone forming phosphatidic acid. DAG is what remains AFTER phosphate head is cleaved by phospholipase C."},{"id":14,"s":"Cell Membrane","t":"Membrane Lipids","u":"Hydrocarbon Tails","q":"Which property of hydrocarbon tails of membrane phospholipids makes the membrane more fluid?","o":["Branched chains","Long chains","Saturated chains","Short chains","Trans-unsaturated chains"],"a":3,"p":"Fluidity increases with: short chains, cis-unsaturated bonds, high temperature. Long, saturated, and trans chains pack tightly → rigidity."},{"id":15,"s":"Cell Membrane","t":"Membrane Lipids","u":"Cholesterol","q":"Which part of cholesterol is hydrophilic and aligns with polar phospholipid heads in the cell membrane?","o":["Hydroxyl group","Phenylalanine ring","Side chain","Steroid nucleus","Tryptophan ring"],"a":0,"p":"Cholesterol is amphipathic: -OH = hydrophilic (faces heads); steroid rings + side chain = hydrophobic (in bilayer)."},{"id":16,"s":"Cell Membrane","t":"Membrane Proteins","u":"Glycoproteins","q":"Which of the following proteins are involved in cell-to-cell recognition?","o":["Glycoproteins","Lipoproteins","Metalloproteins","Nucleoproteins","Phosphoproteins"],"a":0,"p":"Glycoproteins form the glycocalyx — responsible for cell-cell recognition, immune responses, ABO blood groups. Oligosaccharide chains always face outward."},{"id":17,"s":"Cell Membrane","t":"Membrane Lipids","u":"Cholesterol","q":"Cholesterol in the phospholipid bilayer is distributed in which of the following manner?","o":["Cytosolic only","Equal in both leaflets","External only","Inner leaflet only","Outer leaflet only"],"a":1,"p":"Cholesterol = EQUAL in both leaflets. Unlike phospholipids which are asymmetrically distributed. Cholesterol is the only major lipid equally distributed."},{"id":18,"s":"Cell Membrane","t":"Membrane Asymmetry","u":"Inner Leaflet","q":"Which lipid is characteristically found in the internal monolayer of the phospholipid bilayer?","o":["Glycolipid","Phosphatidylcholine","Phosphatidylserine","Sphingomyelin","Triacylglycerol"],"a":2,"p":"Inner leaflet: PS, PE, PI. Outer leaflet: PC, SM, glycolipids. PS is 'Private' (inner). In apoptosis PS moves out → Annexin V detects it."},{"id":19,"s":"Cell Membrane","t":"Membrane Asymmetry","u":"Outer Leaflet","q":"Which lipid is found in the external monolayer of the phospholipid bilayer?","o":["Phosphatidylethanolamine","Phosphatidylinositol","Phosphatidylcholine","Phosphatidylserine","Triacylglycerol"],"a":2,"p":"Outer leaflet: PC + SM + glycolipids. Mnemonic: 'PC-SM goes out to face the world'."},{"id":20,"s":"Cell Membrane","t":"Membrane Proteins","u":"Peripheral Proteins","q":"Which proteins can be separated from membranes using simple salt solutions?","o":["GPI-anchored","Integral","Lipid-anchored","Peripheral","Transmembrane"],"a":3,"p":"Peripheral proteins held by ionic/H-bonds → removed by salt or pH change. Integral proteins need detergents. Peripheral proteins include spectrin, ankyrin, actin in RBCs."},{"id":21,"s":"Cell Membrane","t":"Membrane Proteins","u":"Integral Proteins","q":"Which class of membrane proteins forms the pore-forming and regulatory subunits of ion channels spanning the lipid bilayer?","o":["GPI-anchored","Integral","Lipid-anchored","Peripheral","Receptor"],"a":1,"p":"Ion channel subunits are INTEGRAL membrane proteins. Peripheral proteins sit on the surface and cannot form channel subunits."},{"id":22,"s":"Cell Membrane","t":"Membrane Proteins","u":"Peripheral Proteins","q":"Which proteins tether the plasma membrane to cytoskeletal fibrils?","o":["GPI-anchored","Integral","Lipid-anchored","Peripheral","Transmembrane"],"a":3,"p":"Spectrin and ankyrin (peripheral proteins) anchor RBC membrane to actin cytoskeleton. Defects → hereditary spherocytosis."},{"id":23,"s":"Cell Membrane","t":"Membrane Proteins","u":"Peripheral Proteins","q":"Which proteins take part in vesicle trafficking and membrane fusion?","o":["GPI-anchored","Integral","Lipid-anchored","Peripheral","Transmembrane"],"a":3,"p":"Peripheral proteins (SNAREs, clathrin) orchestrate vesicle budding and fusion — key in endocytosis, exocytosis, and Golgi trafficking."},{"id":24,"s":"Cell Membrane","t":"Membrane Proteins","u":"GPI-Anchored","q":"Which class of membrane proteins preferentially partitions into lipid rafts via a glycolipid anchor on the outer leaflet?","o":["GPI-anchored","Integral","Lipid-anchored cytosolic","Peripheral","Transmembrane"],"a":0,"p":"GPI-anchored proteins (CD55, CD59, 5'-nucleotidase, folate receptor) partition into lipid rafts. Caveolin stabilises caveolae but is an INTEGRAL hairpin protein — not GPI-anchored."},{"id":25,"s":"Cell Membrane","t":"Membrane Asymmetry","u":"Apoptosis Signal","q":"Migration of which lipid to the external monolayer signals apoptosis?","o":["Cholesterol","Phosphatidylcholine","Phosphatidylinositol","Phosphatidylserine","Sphingomyelin"],"a":3,"p":"PS externalisation = universal 'eat me' signal. Macrophages recognise it via TIM-4 receptors. Detected by Annexin V assay."},{"id":26,"s":"Cell Membrane","t":"Membrane Proteins","u":"Glycoproteins","q":"Oligosaccharide chains of glycoproteins in the cell membrane are oriented in which direction?","o":["Both sides","Inwards","Laterally","Outwards","Randomly"],"a":3,"p":"Oligosaccharide chains always face outward forming the glycocalyx. Orientation determined during synthesis in the ER lumen which becomes extracellular."},{"id":27,"s":"Cell Membrane","t":"Membrane Lipids","u":"Blood Groups","q":"Which of the following contributes to blood group type on RBC surfaces?","o":["Glycerolipids","Glycolipids","Phospholipids","Sphingolipids","Triacylglycerols"],"a":1,"p":"ABO blood groups determined by glycolipid sugar sequences on RBCs. A and B antigens differ only in terminal sugar (GalNAc vs Gal)."},{"id":28,"s":"Cell Membrane","t":"Membrane Proteins","u":"Integral Proteins","q":"Active transport of ions across the cell membrane is primarily carried out by which class of proteins?","o":["GPI-anchored","Integral","Lipid-anchored","Peripheral","Receptor"],"a":1,"p":"Ion pumps (Na⁺/K⁺-ATPase, Ca²⁺-ATPase) and ion channels are integral transmembrane proteins spanning the bilayer."},{"id":29,"s":"Cell Membrane","t":"Membrane Lipids","u":"Cholesterol","q":"In a membrane with unsaturated hydrocarbon tails, the presence of cholesterol makes the membrane more:","o":["Fluid","Permeable","Rigid","Soluble","Viscous"],"a":2,"p":"Cholesterol buffers fluidity: RIGIDIFIES unsaturated (fluid) membranes; FLUIDISES saturated (rigid) membranes. Net effect = optimal stability across temperature range."},{"id":30,"s":"Cell Membrane","t":"Membrane Transport","u":"Osmosis","q":"Which of the following can cross the cell membrane through osmosis?","o":["Amino acids","Glucose","Nucleotides","Proteins","Water"],"a":4,"p":"Water crosses by osmosis through aquaporins or directly through the bilayer. Glucose, amino acids, nucleotides need specific transport proteins."},{"id":31,"s":"Cell Membrane","t":"Membrane Properties","u":"Fluidity","q":"Membrane fluidity may be increased if which of the following is increased?","o":["Long chain fatty acids","Saturated fatty acids","Trans fatty acids","Unsaturated fatty acids","Cholesterol content"],"a":3,"p":"Unsaturated (cis) fatty acids have kinks → prevent tight packing → increased fluidity. Trans fatty acids behave like saturated → rigidity."},{"id":32,"s":"Cell Membrane","t":"Membrane Properties","u":"Movement","q":"Which of the following best describes the predominant rapid movement of phospholipids within the cell membrane?","o":["Flip-flop","Lateral","Rotational","Transverse","Vertical"],"a":1,"p":"Phospholipids move LATERALLY within their own leaflet very rapidly. Flip-flop between leaflets is extremely slow without translocases."},{"id":33,"s":"Cell Membrane","t":"Membrane Transport","u":"Marker Enzymes","q":"Which of the following is a marker enzyme for the plasma membrane?","o":["Cyt C oxidase","Glutamate dehydrogenase","Na⁺/K⁺ ATPase","Succinate dehydrogenase","Transaminase"],"a":2,"p":"Plasma membrane markers: Na⁺/K⁺ ATPase and 5'-nucleotidase. Mitochondria inner: Cyt C oxidase, Succinate DH. Matrix: Glutamate DH."},{"id":34,"s":"Cell Membrane","t":"Membrane Transport","u":"Marker Enzymes","q":"Which of the following is a marker enzyme for the plasma membrane?","o":["5'-Nucleotidase","Cyt C oxidase","Glutamate dehydrogenase","Isocitrate dehydrogenase","Succinate dehydrogenase"],"a":0,"p":"5'-Nucleotidase is a GPI-anchored ectoenzyme on the outer leaflet — a key plasma membrane marker alongside Na⁺/K⁺ ATPase."},{"id":35,"s":"Cell Membrane","t":"Membrane Transport","u":"GLUT Transporters","q":"Intracellular glucose is 2 mM, extracellular is 5 mM. Which transporter facilitates glucose entry into the cell?","o":["GLUT1","GLUT4","GLUT5","Na⁺/K⁺ ATPase","SGLT1"],"a":0,"p":"GLUT1: ubiquitous, constitutive, always on surface, low Km. GLUT4: insulin-dependent. GLUT5: fructose only. SGLT1: sodium-coupled active transport."},{"id":36,"s":"Cell Membrane","t":"Membrane Transport","u":"GLUT Transporters","q":"An adipocyte carries out insulin-dependent glucose transport. Insulin's role is to promote:","o":["Antiport","Cotransport","Exocytosis","Symport","Translocation"],"a":4,"p":"Insulin → PI3K/Akt → GLUT4 vesicles fuse with membrane → GLUT4 translocates to cell surface → ↑ glucose uptake. Impaired in Type 2 DM."},{"id":37,"s":"Cell Membrane","t":"Membrane Transport","u":"GLUT Transporters","q":"A 56-year-old man with type 2 diabetes has fasting glucose 150 mg/dL. Glucose transport is impaired in which cell type?","o":["Brain cells","Eye cells","Intestinal cells","Liver cells","Skeletal muscle cells"],"a":4,"p":"GLUT4 (insulin-dependent) in skeletal muscle, adipose, cardiac muscle. In T2DM, GLUT4 translocation fails. Brain uses GLUT1/3 — insulin-independent."},{"id":38,"s":"Cell Membrane","t":"Membrane Transport","u":"SGLT","q":"The energy to power glucose transport against its concentration gradient into intestinal cells is derived from:","o":["ATP hydrolysis","GTP hydrolysis","Na⁺ gradient","Osmotic pressure","Proton gradient"],"a":2,"p":"SGLT = secondary active transport. Na⁺ gradient (from Na⁺/K⁺ ATPase) drives glucose uptake. Na⁺ and glucose move in same direction = symport."},{"id":39,"s":"Cell Membrane","t":"Membrane Transport","u":"SGLT","q":"Inhibiting SGLT2 in kidneys of type 2 diabetes patients would increase which of the following?","o":["Glucosuria","Glycogenesis","Glycolysis","Gluconeogenesis","Glycogenolysis"],"a":0,"p":"SGLT2 inhibitors (gliflozins) block renal glucose reabsorption → glucosuria → ↓ blood glucose. Cardioprotective and nephroprotective."},{"id":40,"s":"Cell Membrane","t":"Membrane Transport","u":"Diffusion","q":"Lung alveoli increase surface area for gaseous exchange. Gases cross the alveolar membrane by:","o":["Active transport","Cotransport","Facilitated diffusion","Osmosis","Simple diffusion"],"a":4,"p":"O₂ and CO₂ are nonpolar gases → simple diffusion. Fick's law: Rate ∝ surface area × concentration gradient / thickness."},{"id":41,"s":"Cell Membrane","t":"Membrane Properties","u":"Interactions","q":"The entire structure of the fluid mosaic model cell membrane is held together by:","o":["Covalent bonds","H-bonds","Hydrophobic interactions","Ionic bonds","Peptide bonds"],"a":2,"p":"Hydrophobic effect drives bilayer self-assembly. Fatty acid tails cluster away from water; polar heads face water. Thermodynamically driven — no covalent bonds hold the bilayer."},{"id":42,"s":"Cell Membrane","t":"Membrane Lipids","u":"Cholesterol","q":"Which of the following is distributed equally in both monolayers of the cell membrane?","o":["Cholesterol","Glycolipid","Phosphatidylcholine","Phosphatidylserine","Sphingomyelin"],"a":0,"p":"Cholesterol = equal in both leaflets. Glycolipids = outer only. PS = inner only. PC = predominantly outer."},{"id":43,"s":"Cell Membrane","t":"Membrane Lipids","u":"Cholesterol","q":"Which part of cholesterol in the membrane is oriented towards the phosphate heads of phospholipids?","o":["Hydroxyl group","Isooctyl side chain","Ring A","Ring C","Steroid nucleus"],"a":0,"p":"Cholesterol's -OH hydrogen-bonds with phospholipid head groups at the membrane-water interface. Steroid rings + side chain are in the hydrophobic core."},{"id":44,"s":"Cell Membrane","t":"Membrane Proteins","u":"Peripheral Proteins","q":"Which of the following is a peripheral membrane protein found on the cytosolic face of RBCs?","o":["Band 3","GPCR","GPI","Ion channel","Spectrin"],"a":4,"p":"Spectrin is a peripheral protein on the cytosolic face of RBCs, linking to actin and ankyrin to maintain biconcave shape. Defects → hereditary spherocytosis."},{"id":45,"s":"Cell Membrane","t":"Membrane Proteins","u":"Peripheral Proteins","q":"A 27-year-old woman with mild anaemia, jaundice, splenomegaly, and spherocytes is diagnosed with hereditary spherocytosis. Her RBC membrane protein defect is in which class?","o":["Channel proteins","GPI-anchored","Integral","Peripheral","Transmembrane"],"a":3,"p":"Hereditary spherocytosis = defect in spectrin, ankyrin, or band 3 (peripheral/anchoring proteins). Loss of membrane-cytoskeleton attachment → spherocytes → haemolysis."},{"id":46,"s":"Cell Membrane","t":"Membrane Proteins","u":"Peripheral Proteins","q":"Which of the following peripheral membrane proteins links spectrin to band 3 in red blood cells?","o":["Actin","Ankyrin","Caveolin","Fibronectin","Glycophorin"],"a":1,"p":"Ankyrin links spectrin to Band 3 (integral protein). Ankyrin mutations are the most common cause of hereditary spherocytosis (~50% of cases)."},{"id":47,"s":"Cell Membrane","t":"Membrane Proteins","u":"Peripheral Proteins","q":"Which of the following is a peripheral membrane protein forming junctional complexes in the RBC cytoskeletal network?","o":["Actin","Band 3","Caveolin","Fibronectin","Glycophorin"],"a":0,"p":"Short actin protofilaments act as junctional complexes crosslinking spectrin tetramers. Actin is a peripheral protein on the cytosolic face."},{"id":48,"s":"Cell Membrane","t":"Membrane Proteins","u":"Peripheral Proteins","q":"Which proteins can be dislodged from membranes by conditions that disrupt ionic and H-bond interactions?","o":["GPI-anchored","Integral","Lipid-anchored","Peripheral","Transmembrane"],"a":3,"p":"Peripheral proteins bind via ionic/H-bonds → removed by high salt, high pH, or EDTA. Integral proteins require detergents (SDS, Triton X)."},{"id":49,"s":"Cell Membrane","t":"Membrane Proteins","u":"GPI-Anchored","q":"Which lipid anchor is covalently linked to proteins on the extracellular face of the plasma membrane?","o":["Farnesyl","Geranylgeranyl","GPI","Myristoyl","Palmitoyl"],"a":2,"p":"GPI (glycosylphosphatidylinositol) anchors linked to C-terminus of outer leaflet proteins — CD55, CD59, 5'-nucleotidase, folate receptor. PNH = PIGA mutation → defective GPI synthesis."},{"id":50,"s":"Cell Membrane","t":"Membrane Proteins","u":"Lipid-Anchored","q":"Proteins prenylated on the cytosolic side (e.g. Ras, Rho GTPases) are anchored via thioether linkage to which of the following?","o":["Farnesyl","GPI","Myristoyl","Palmitoyl","Phosphatidylserine"],"a":0,"p":"Prenylation = THIOETHER linkage between isoprenoid (farnesyl 15C or geranylgeranyl 20C) and cysteine at CAAX motif. Examples: Ras, Rho, Rac. Ras mutations = most common oncogenic mutations in human cancer."},{"id":51,"s":"Cell Membrane","t":"Membrane Transport","u":"Aquaporins","q":"A patient develops excessive urination. Investigation shows kidney collecting duct cells cannot reabsorb water. Which channel protein is deficient?","o":["Aquaporin","Connexin","Porin","Prestin","Voltage-gated channel"],"a":0,"p":"Aquaporins are water-specific integral membrane channels. Without aquaporins → water stays in tubule → dilute urine → polyuria (Diabetes Insipidus). AQP2 is the regulated collecting duct channel."},{"id":52,"s":"Cell Membrane","t":"Membrane Transport","u":"SGLT","q":"A medical student is studying intestinal glucose absorption. Glucose is absorbed from the gut lumen into intestinal cells against its concentration gradient. Which energy source directly powers this transport?","o":["ATP","Electrochemical gradient","GTP","Na⁺ gradient","Proton gradient"],"a":3,"p":"SGLT1 uses Na⁺ gradient as direct energy source — secondary active transport. Na⁺ flows in down its gradient dragging glucose (symport). Na⁺ gradient maintained by Na⁺/K⁺ ATPase using ATP."},{"id":53,"s":"Cell Membrane","t":"Membrane Transport","u":"SGLT","q":"A 45-year-old man with type 2 diabetes is prescribed a drug blocking glucose reabsorption in the kidney proximal tubule. Which transporter does this drug inhibit?","o":["GLUT1","GLUT2","GLUT4","SGLT1","SGLT2"],"a":4,"p":"SGLT2 responsible for ~90% of renal glucose reabsorption. SGLT2 inhibitors (gliflozins — empagliflozin, dapagliflozin) block this → glucosuria → ↓ blood glucose. Also cardioprotective and nephroprotective."},{"id":54,"s":"Cell Membrane","t":"Active Transport — Pump Classes","u":"ABC Class","q":"A 6-year-old boy with recurrent chest infections and greasy stools has sweat chloride 90 mEq/L. The defective CFTR protein belongs to which transporter family?","o":["ABC transporters","Aquaporins","GLUT family","P-class pumps","V-class pumps"],"a":0,"p":"CFTR is an ABC transporter — ATP-Binding Cassette family. Functions as cAMP-regulated Cl⁻ channel. ΔF508 mutation → misfolds in ER → proteasomal degradation → absent from membrane."},{"id":55,"s":"Cell Membrane","t":"Active Transport — Pump Classes","u":"P Class","q":"A researcher blocks a membrane pump in cardiac muscle cells causing intracellular Na⁺ to rise then Ca²⁺ to rise. Which pump class does this pump belong to?","o":["ABC class","F class","Ion channel","P class","V class"],"a":3,"p":"Na⁺/K⁺ ATPase is a P-class pump (Phosphorylated intermediate). Inhibition → ↑ intracellular Na⁺ → ↓ Na⁺/Ca²⁺ exchanger → ↑ Ca²⁺. Mechanism of digoxin and ouabain."},{"id":56,"s":"Cell Membrane","t":"Active Transport — Pump Classes","u":"V Class","q":"A student notes lysosomes maintain an internal pH of approximately 4.5. Which pump class acidifies the lysosomal lumen?","o":["ABC class","F class","Ion channel","P class","V class"],"a":4,"p":"V-class pumps (Vacuolar H⁺ ATPase) pump H⁺ INTO lysosomes → acidification → pH ~4.5 → activates acid hydrolases. Also found in osteoclasts for bone resorption."},{"id":57,"s":"Cell Membrane","t":"Active Transport — Pump Classes","u":"F Class","q":"During oxidative phosphorylation, protons accumulate in the mitochondrial intermembrane space. Which pump class harnesses this gradient to synthesise ATP?","o":["ABC class","F class","Ion channel","P class","V class"],"a":1,"p":"F-class pumps (F₀F₁ ATP synthase): H⁺ flows FROM intermembrane space INTO matrix through F₀ → rotation of F₁ → ATP synthesis. ETC Complexes I, III, IV pump H⁺ INTO intermembrane space. F-class uses that gradient to make ATP."},{"id":58,"s":"Cell Membrane","t":"Active Transport — Pump Classes","u":"ABC Class","q":"Cancer cells overexpress a membrane transporter that pumps chemotherapy drugs out of the cell causing treatment failure. Which transporter family is responsible?","o":["ABC transporters","Aquaporins","GLUT family","P-class pumps","SGLT family"],"a":0,"p":"MDR1 (P-glycoprotein/ABCB1) is an ABC transporter → pumps chemotherapy drugs out → multidrug resistance. Expressed normally in gut, liver, blood-brain barrier."},{"id":59,"s":"Cell Membrane","t":"Membrane Transport","u":"Symport","q":"A physiologist notes that Na⁺ and glucose move in the same direction across the intestinal epithelium. This type of transport is classified as:","o":["Antiport","Facilitated diffusion","Primary transport","Simple diffusion","Symport"],"a":4,"p":"Symport = two molecules in SAME direction. SGLT1: Na⁺ and glucose both move into cell. Antiport = opposite directions: Na⁺/Ca²⁺, Na⁺/H⁺. Uniport = single molecule one direction (GLUT transporters)."},{"id":60,"s":"Cell Membrane","t":"Membrane Transport","u":"Diffusion","q":"A student notes that oxygen crosses the alveolar membrane without using any protein. Which type of transport does this represent?","o":["Active transport","Facilitated diffusion","Osmosis","Primary transport","Simple diffusion"],"a":4,"p":"O₂ and CO₂ are small nonpolar gases → dissolve directly in the phospholipid bilayer → simple diffusion. Driven purely by concentration gradient. No protein, no energy required."},{"id":61,"s":"Cell Membrane","t":"Membrane Asymmetry","u":"Inner Leaflet","q":"A cell biologist examines a red blood cell and notes different phospholipids on inner and outer leaflets. Which phospholipid is characteristically found on the inner leaflet?","o":["Glycolipid","Phosphatidylcholine","Phosphatidylserine","Sphingomyelin","Triacylglycerol"],"a":2,"p":"Inner leaflet: PS, PE, PI. Outer leaflet: PC, SM, glycolipids. PS is 'Private' — confined to inner leaflet by flippase in healthy cells. In apoptosis PS moves out → Annexin V detects it."},{"id":62,"s":"Cell Membrane","t":"Phospholipid Translocators","u":"Scramblase","q":"In apoptosis calcium is released into the cytosol. This activates an enzyme that randomises lipid distribution bidirectionally. Which enzyme is activated?","o":["Flippase","Floppase","Kinase","Phospholipase","Scramblase"],"a":4,"p":"Scramblase Ca²⁺-ACTIVATED → bidirectional → PS externalised → macrophage recognition → apoptotic cell cleared. Flippase Ca²⁺-INACTIVATED. Both effects ensure rapid PS externalisation."},{"id":63,"s":"Cell Membrane","t":"Membrane Proteins","u":"Peripheral Proteins","q":"A biochemist treats a membrane preparation with high salt solution removing only one class of proteins. Which class was removed?","o":["GPI-anchored","Integral","Lipid-anchored","Peripheral","Transmembrane"],"a":3,"p":"Peripheral proteins attached via ionic bonds and H-bonds → disrupted by high salt or extreme pH. Integral proteins need detergents. GPI-anchored need phospholipase C cleavage."},{"id":64,"s":"Cell Membrane","t":"Membrane Proteins","u":"GPI-Anchored","q":"A researcher isolates a protein from the outer leaflet of the plasma membrane released by phospholipase C but not by detergent or salt. Which anchor type should she look for?","o":["Farnesyl","Geranylgeranyl","GPI","Myristoyl","Palmitoyl"],"a":2,"p":"GPI-anchored proteins cleaved by phospholipase C. Not removed by salt (not ionic) or detergent alone. Reside in lipid rafts. Examples: CD55, CD59, 5'-nucleotidase, folate receptor, prion protein (PrP)."},{"id":65,"s":"Cell Membrane","t":"Membrane Lipids","u":"Cholesterol","q":"A cell biologist notes that cholesterol stabilises membrane fluidity. How is cholesterol distributed between the two leaflets?","o":["Equally","Inner leaflet only","Outer leaflet only","Randomly","Unevenly"],"a":0,"p":"Cholesterol distributed EQUALLY in both leaflets. Fluidity buffer: at high temperature restricts movement; at low temperature prevents tight packing. Concentrated in lipid rafts."},{"id":66,"s":"Cell Membrane","t":"Membrane Lipids","u":"Lipid Rafts","q":"Which lipid together with cholesterol forms membrane microdomains called lipid rafts?","o":["Glycosphingolipid","Phosphatidylcholine","Phosphatidylinositol","Phosphatidylserine","Triacylglycerol"],"a":0,"p":"Lipid rafts = cholesterol + glycosphingolipids + GPI-anchored proteins. More ordered and less fluid. Platforms for signal transduction, endocytosis, and pathogen entry."},{"id":67,"s":"Cell Membrane","t":"Signal Transduction","u":"ICAM-1","q":"A 7-year-old child develops runny nose and nasal congestion after contact with a classmate. The responsible virus attaches to nasal epithelial cells via ICAM-1. ICAM-1 belongs to which protein superfamily?","o":["Cadherin family","Ficolin family","Immunoglobulin superfamily","Integrin family","Selectin family"],"a":2,"p":"ICAM-1 belongs to the immunoglobulin superfamily. Rhinovirus binds ICAM-1 on nasal epithelial cells via its canyon region. ~90% of rhinovirus serotypes use ICAM-1 (major group)."},{"id":68,"s":"Cell Membrane","t":"Signal Transduction","u":"Rhodopsin","q":"Light strikes a rod photoreceptor and activates rhodopsin. Rhodopsin belongs to which receptor class?","o":["Enzyme-linked receptor","G protein-coupled receptor","Ligand-gated ion channel","Nuclear receptor","Receptor tyrosine kinase"],"a":1,"p":"Rhodopsin = GPCR (7 transmembrane helices). 11-cis retinal → all-trans retinal → activates transducin (Gt) → cGMP-PDE → ↓cGMP → Na⁺ channels close → hyperpolarisation → visual signal."},{"id":69,"s":"Cell Membrane","t":"Signal Transduction","u":"Transducin","q":"During phototransduction in a rod cell, which G protein is specifically activated by light-stimulated rhodopsin?","o":["G12","Gi","Gq","Gs","Gt"],"a":4,"p":"Transducin (Gt) unique to photoreceptors. Activated rhodopsin → Gt α-subunit GDP→GTP → activates cGMP-PDE → ↓cGMP → Na⁺ channels close → rod hyperpolarises → ↓ glutamate."},{"id":70,"s":"Cell Membrane","t":"Signal Transduction","u":"Phototransduction","q":"A man is walking in the dark. A watchman suddenly shines a torch beam on him. Which of the following best describes the activity generated in his rod photoreceptors due to the light beam?","o":["Depolarisation","Hyperpolarisation","Inactivation","Oscillation","Repolarisation"],"a":1,"p":"Dark: cGMP-gated Na⁺ channels OPEN → depolarised → glutamate released. Light → PDE → ↓cGMP → channels CLOSE → HYPERPOLARISES → ↓ glutamate. Rods unique — stimulus causes hyperpolarisation."},{"id":71,"s":"Cell Membrane","t":"Signal Transduction","u":"Glucagon Pathway","q":"A marathon runner has not eaten for 14 hours. Glucagon is released to prevent hypoglycaemia. The glucagon receptor signals via which G protein α-subunit?","o":["Gα12","Gαi","Gαo","Gαq","Gαs"],"a":4,"p":"Glucagon → GPCR → Gαs → adenylyl cyclase → cAMP → PKA → CREB → PEPCK → gluconeogenesis. Gαs = stimulatory (↑cAMP). Gαi = inhibitory (↓cAMP). Gαq = PLC (↑IP3+DAG)."},{"id":72,"s":"Cell Membrane","t":"Signal Transduction","u":"CREB","q":"A man fasting for Ramadan is waiting for the azan to break his fast. He has not eaten for 16 hours and glycogen stores are depleted. Which second messenger is helping glucagon maintain his blood glucose?","o":["Ca²⁺","cAMP","cGMP","Diacylglycerol","Nitric oxide"],"a":1,"p":"Glucagon → Gαs → adenylyl cyclase → cAMP (second messenger) → PKA → phosphorylates CREB → CREB binds CRE in nucleus → ↑ PEPCK → gluconeogenesis. The man is NOT hypoglycaemic — gluconeogenesis is maintaining normal glucose."},{"id":73,"s":"Cell Membrane","t":"GPI Anchors — Clinical","u":"PNH","q":"Paroxysmal Nocturnal Haemoglobinuria is suspected in a patient with dark morning urine and anaemia. Flow cytometry shows absence of a specific protein class on her RBC membranes. Which protein class is likely absent?","o":["GPI-anchored","Integral","Lipid-anchored cytosolic","Peripheral","Transmembrane"],"a":0,"p":"PNH = PIGA gene mutation → defective GPI anchor biosynthesis → ALL GPI-anchored proteins absent (CD55, CD59) → uncontrolled complement → intravascular haemolysis → haemoglobinuria. Treat with eculizumab."},{"id":74,"s":"Cell Membrane","t":"GPI Anchors — Clinical","u":"PNH","q":"A patient with Paroxysmal Nocturnal Haemoglobinuria lacks CD55 and CD59 on her red blood cells. CD55 and CD59 protect RBCs by inhibiting the action of which of the following?","o":["Apoptosis cascade","Clotting cascade","Complement cascade","MAP kinase cascade","Phagocytosis cascade"],"a":2,"p":"CD55 (DAF) accelerates decay of C3 convertase. CD59 (protectin) blocks MAC formation. Both GPI-anchored. Without them → complement cascade destroys RBC membrane → intravascular haemolysis."},{"id":75,"s":"Cell Membrane","t":"Signal Transduction","u":"Digoxin","q":"A heart failure patient is prescribed digoxin to improve cardiac contractility. Digoxin improves contractility by inhibiting which of the following?","o":["Adenylyl cyclase","Ca²⁺-ATPase","H⁺/K⁺-ATPase","Mg²⁺-ATPase","Na⁺/K⁺-ATPase"],"a":4,"p":"Digoxin inhibits Na⁺/K⁺ ATPase → ↑ intracellular Na⁺ → ↓ Na⁺/Ca²⁺ exchanger → ↑ Ca²⁺ → stronger contraction (positive inotropy). Same mechanism as ouabain."},{"id":76,"s":"Cell Membrane","t":"Signal Transduction","u":"Ouabain","q":"A researcher blocks Na⁺/K⁺ ATPase in cardiac myocytes using a plant-derived toxin causing ↑ intracellular Na⁺ then ↑ Ca²⁺. Which toxin is being used?","o":["Colchicine","Ibotenic acid","Muscarine","Ouabain","Saxitoxin"],"a":3,"p":"Ouabain (Strophanthus plant, cardiac glycoside) inhibits Na⁺/K⁺ ATPase like digoxin. Tetrodotoxin and saxitoxin block voltage-gated Na⁺ channels. Phalloidin locks F-actin. Colchicine blocks tubulin polymerisation."},{"id":77,"s":"Cell Membrane","t":"Active Transport — Pump Classes","u":"H⁺/K⁺ ATPase","q":"A 45-year-old woman with chronic GERD is prescribed omeprazole. It irreversibly inhibits which enzyme in gastric parietal cells?","o":["Ca²⁺-ATPase","H⁺/K⁺-ATPase","H⁺/Na⁺-ATPase","Mg²⁺-ATPase","Na⁺/K⁺-ATPase"],"a":1,"p":"H⁺/K⁺ ATPase = P-class pump on gastric parietal cell apical membrane. PPIs are prodrugs → activated by acid → irreversible covalent bond with cysteine → blocks acid secretion. Side effects: ↓ B12, ↓ Mg²⁺, ↑ C. difficile."},{"id":78,"s":"Cell Membrane","t":"Membrane Transport","u":"CFTR","q":"A 6-year-old boy with recurrent chest infections, steatorrhoea, sweat Cl⁻ 90 mEq/L. The defective CFTR belongs to which membrane protein class?","o":["Cytoskeletal","GPI-anchored","Integral","Lipid-anchored","Peripheral"],"a":2,"p":"CFTR = integral transmembrane protein, cAMP-regulated Cl⁻ channel, ABC transporter family. ΔF508 → misfolds in ER → proteasomal degradation → absent from membrane."},{"id":79,"s":"Cell Membrane","t":"Membrane Transport","u":"CFTR","q":"A 3-year-old with homozygous ΔF508 mutation. CFTR protein fails to reach the plasma membrane. What is its molecular fate?","o":["Exocytosed","Glycosylated in Golgi","Inserted but inactive","Proteasomal degradation","Secreted extracellularly"],"a":3,"p":"ΔF508 → CFTR misfolds in ER → Hsp70/calnexin recognition → ubiquitinated → proteasomal degradation → absent from plasma membrane. Ivacaftor (potentiator) + lumacaftor/tezacaftor (correctors) rescue this."},{"id":80,"s":"Cell Membrane","t":"Membrane Transport","u":"Cholera","q":"An aid worker develops rice-water diarrhoea after drinking contaminated water. Vibrio cholerae is cultured. Massive fluid loss is due to constitutive activation of which transporter?","o":["Aquaporin","CFTR","Na⁺/K⁺ ATPase","SGLT1","SGLT2"],"a":1,"p":"Cholera toxin → locks Gαs active → ↑↑ cAMP → PKA → CFTR constitutively open → massive Cl⁻ secretion → water follows osmotically (up to 20 L/day). ORS uses SGLT1 (still functional) to drive Na⁺ and water absorption."},{"id":81,"s":"Cell Membrane","t":"Active Transport — Pump Classes","u":"P Class","q":"Which ATP-driven pumps transport cations such as Na⁺, K⁺, H⁺, Ca²⁺ and form a phosphorylated intermediate during transport?","o":["ABC class","F class","Ion channel","P class","V class"],"a":3,"p":"P-class: 'P' = Phosphorylated aspartate intermediate. Transport cations ONLY. Examples: Na⁺/K⁺ ATPase, H⁺/K⁺ ATPase, Ca²⁺ ATPase (SERCA). All = primary active transport."},{"id":82,"s":"Cell Membrane","t":"Active Transport — Pump Classes","u":"P Class","q":"A subunit of which transport protein is phosphorylated during its transport cycle?","o":["ABC class","F class","Ion channel","P class","V class"],"a":3,"p":"Defining feature of P-class: aspartate residue on α-subunit transiently phosphorylated each cycle. This drives conformational change that moves the ion across the membrane."},{"id":83,"s":"Cell Membrane","t":"Active Transport — Pump Classes","u":"P Class","q":"Calcium ATPase (SERCA) which pumps Ca²⁺ back into the sarcoplasmic reticulum after muscle contraction is classified as:","o":["ABC class","F class","Ion channel","P class","V class"],"a":3,"p":"SERCA (Sarcoplasmic Endoplasmic Reticulum Ca²⁺ ATPase) = P-class pump. Pumps Ca²⁺ back into SR after contraction → muscle relaxation. Inhibited by thapsigargin."},{"id":84,"s":"Cell Membrane","t":"Membrane Transport","u":"Band 3","q":"A researcher treats a red blood cell membrane preparation with a detergent. Which of the following integral membrane proteins is removed?","o":["Actin","Ankyrin","Band 3","Dystrophin","Spectrin"],"a":2,"p":"Band 3 (AE1 — Anion Exchanger 1) is an integral membrane protein spanning the RBC lipid bilayer — exchanges HCO₃⁻ for Cl⁻ (chloride shift) and anchors the cytoskeleton via ankyrin. Integral proteins require detergents to remove. Spectrin, ankyrin, actin = peripheral → removed by salt."},{"id":85,"s":"Cell Membrane","t":"Membrane Transport","u":"Aquaporins","q":"A patient develops polyuria of 8 litres per day. Investigation shows kidney collecting duct cells cannot insert water channels into the apical membrane. Which protein is deficient?","o":["Aquaporin","GLUT","Ion channel","Na⁺/K⁺ ATPase","SGLT"],"a":0,"p":"Aquaporins are water-specific channels inserted into the apical membrane of collecting duct cells → water reabsorption. Without aquaporins → excessive dilute urine → Diabetes Insipidus."},{"id":86,"s":"Cell Membrane","t":"Membrane Transport","u":"LDL Receptor","q":"A patient cannot internalise cholesterol-carrying lipoproteins into his liver cells due to a defective membrane receptor. Which membrane process is defective?","o":["Diffusion","Endocytosis","Exocytosis","Phagocytosis","Pinocytosis"],"a":1,"p":"LDL binds LDLR in clathrin-coated pits → receptor-mediated endocytosis → lysosomal degradation releases cholesterol → ↓ HMG-CoA reductase. Familial Hypercholesterolaemia = LDLR mutation. Statins ↑ LDLR expression."},{"id":87,"s":"Cell Membrane","t":"Active Transport — Pump Classes","u":"Primary vs Secondary","q":"Glucose absorption via SGLT1 in intestinal cells is powered by the Na⁺ gradient created by Na⁺/K⁺ ATPase. SGLT1 transport is therefore classified as:","o":["Active transport","Facilitated diffusion","Primary active transport","Secondary active transport","Simple diffusion"],"a":3,"p":"Primary active transport = directly uses ATP (Na⁺/K⁺ ATPase, H⁺/K⁺ ATPase, ABC transporters). Secondary = uses electrochemical gradient from primary transport (SGLT1/2, Na⁺/amino acid cotransporters). Na⁺ gradient = stored energy from primary transport."},{"id":88,"s":"Cell Membrane","t":"Signal Transduction","u":"G Proteins","q":"A student learns that different G protein α-subunits activate different downstream effectors. Which α-subunit when activated inhibits adenylyl cyclase decreasing cAMP?","o":["Gα12","Gαi","Gαo","Gαq","Gαs"],"a":1,"p":"Gαi (inhibitory) inhibits adenylyl cyclase → ↓cAMP → ↓PKA. Receptors using Gαi: M2 muscarinic, α2 adrenergic, opioid, somatostatin. Pertussis toxin ADP-ribosylates Gαi → locks it inactive → ↑ cAMP."},{"id":89,"s":"Cell Membrane","t":"Signal Transduction","u":"Cholera Toxin","q":"A pharmacologist studies Vibrio cholerae toxin that permanently activates adenylyl cyclase. The toxin acts by permanently activating which G protein α-subunit?","o":["Gα12","Gαi","Gαo","Gαq","Gαs"],"a":4,"p":"Cholera toxin → ADP-ribosylates Gαs (Arg201) → prevents GTPase → permanently active → adenylyl cyclase constitutively ON → ↑↑ cAMP → CFTR open → Cl⁻ and water secretion. Compare: Pertussis toxin ADP-ribosylates Gαi."},{"id":90,"s":"Cell Membrane","t":"Signal Transduction","u":"PLC Pathway","q":"A biochemist notes that activation of phospholipase C produces two second messengers simultaneously. Which pair of molecules are produced?","o":["cAMP and cGMP","cAMP and DAG","cGMP and IP3","DAG and IP3","IP3 and NO"],"a":3,"p":"Gαq → PLC → cleaves PIP2 → DAG + IP3. DAG stays in membrane → activates PKC. IP3 → ER → Ca²⁺ release → calmodulin. The two second messengers work in parallel."},{"id":91,"s":"Cell Membrane","t":"Signal Transduction","u":"cGMP","q":"In darkness rod cells are depolarised and continuously release glutamate. The molecule keeping Na⁺ channels open in darkness is:","o":["Ca²⁺","cAMP","cGMP","DAG","IP3"],"a":2,"p":"cGMP = second messenger in rods. High cGMP → cGMP-gated Na⁺/Ca²⁺ channels OPEN → dark current. Light → PDE → ↓cGMP → channels close → hyperpolarisation. cAMP = glucagon second messenger."},{"id":92,"s":"Cell Membrane","t":"Signal Transduction","u":"PKA","q":"A fasting patient has glucagon activate adenylyl cyclase producing cAMP. cAMP directly activates which enzyme?","o":["Adenylyl cyclase","Phosphodiesterase","Phospholipase C","Protein kinase A","Protein kinase C"],"a":3,"p":"cAMP → binds regulatory subunits of PKA → releases catalytic subunits → active PKA → phosphorylates CREB → CREB binds CRE → ↑ PEPCK → gluconeogenesis. PDE degrades cAMP → terminates signal."},{"id":94,"s":"Cytoskeleton","t":"DMD / Dystrophin","u":"Dystrophin — Mechanism","q":"A 9-year-old boy with progressive proximal muscle weakness has a confirmed dystrophin mutation. His muscle fibres are damaged during contraction because dystrophin normally:","o":["Links actin to basal lamina","Links actin to myosin","Links myosin to sarcolemma","Links titin to Z-disc","Links tropomyosin to troponin"],"a":0,"p":"Dystrophin = mechanical linker: F-actin (N-terminal) → rod domain → dystroglycan complex (C-terminal) → laminin in basal lamina. Distributes contractile forces. Without it every contraction tears the membrane."},{"id":95,"s":"Cytoskeleton","t":"DMD / Dystrophin","u":"Dystrophin — ECM","q":"A muscle physiologist studies dystrophin in normal muscle. Dystrophin ultimately connects the intracellular cytoskeleton to which extracellular matrix protein?","o":["Collagen I","Collagen IV","Fibronectin","Laminin","Vitronectin"],"a":3,"p":"Dystrophin → β-dystroglycan → α-dystroglycan → Laminin-2 (merosin) in basal lamina. This chain distributes forces laterally preventing sarcolemmal damage. Laminin-2 deficiency itself causes congenital muscular dystrophy (MDC1A)."},{"id":96,"s":"Cytoskeleton","t":"DMD / Dystrophin","u":"Pseudohypertrophy","q":"A 10-year-old boy with DMD has pseudohypertrophy of his calf muscles. This occurs because degenerating muscle fibres are replaced by:","o":["Bone and cartilage","Fat and fibrous tissue","Granulation tissue","Haematopoietic tissue","Smooth muscle"],"a":1,"p":"In DMD, repeated cycles of necrosis and failed regeneration → progressive replacement by fat and fibrous tissue → calves appear large (pseudohypertrophy) but functionally weak. Eventually fibrofatty tissue dominates → wheelchair by age 12."},{"id":97,"s":"Cytoskeleton","t":"DMD / Dystrophin","u":"Gower's Sign","q":"An 11-year-old boy with DMD cannot rise from the floor without using his hands to climb up his own thighs. Gower's sign reflects weakness of which muscle group?","o":["Distal muscles","Extraocular muscles","Pharyngeal muscles","Proximal muscles","Respiratory muscles"],"a":3,"p":"Gower's sign reflects proximal muscle weakness — gluteus maximus, quadriceps, and erector spinae. Child rolls prone → pushes up → walks hands up thighs → stands. Classic in DMD but also seen in SMA and Becker MD."},{"id":98,"s":"Cytoskeleton","t":"DMD / Dystrophin","u":"Healthy Muscle","q":"In a healthy muscle cell, dystrophin prevents myocyte membrane damage during contraction by acting as a mechanical stabiliser linking:","o":["Actin to basal lamina","Actin to myosin","Myosin to sarcolemma","Titin to Z-disc","Tropomyosin to troponin"],"a":0,"p":"Dystrophin = mechanical shock absorber in healthy muscle: F-actin → rod domain → dystroglycan complex → laminin in basal lamina. Transmits contractile forces to ECM. Without dystrophin → sarcolemma tears every contraction → Ca²⁺ influx → fibre necrosis."},{"id":99,"s":"Cytoskeleton","t":"Hereditary Spherocytosis","u":"Spectrin Deficiency","q":"A 6-year-old girl presents with haemolytic anaemia, splenomegaly, and spherocytes on blood smear. Deficiency of which cytoskeletal protein is responsible for loss of the normal biconcave RBC shape?","o":["Actin","Dystrophin","Fibrillin","Myosin","Spectrin"],"a":4,"p":"Spectrin = major peripheral protein of RBC cytoskeleton. Spectrin dimers → tetramers crosslinked by actin → linked to membrane via ankyrin and band 3. Deficiency → loss of cytoskeletal integrity → spherocyte → haemolysis."},{"id":100,"s":"Cytoskeleton","t":"Hereditary Spherocytosis","u":"MCHC","q":"In a hereditary spherocytosis patient, the characteristic abnormality in RBC indices is:","o":["Decreased MCH","Decreased MCHC","Decreased MCV","Increased MCHC","Increased MCV"],"a":3,"p":"HS: membrane loss without haemoglobin loss → same haemoglobin in smaller volume → ↑ MCHC (often >36 g/dL). MCV usually normal or mildly reduced. ↑ MCHC + spherocytes + negative direct Coombs + family history = classic HS."},{"id":101,"s":"Cytoskeleton","t":"Hereditary Spherocytosis","u":"Scleral Icterus","q":"An HS patient develops scleral icterus. The pigment responsible for yellow discolouration of the sclera is:","o":["Biliverdin","Conjugated bilirubin","Stercobilin","Unconjugated bilirubin","Urobilin"],"a":3,"p":"Haemolytic anaemia → ↑ RBC destruction → haem → ↑ unconjugated bilirubin → lipid-soluble → deposits in sclera → yellow. Unconjugated bilirubin not in urine (acoluric jaundice). Conjugated bilirubin causes dark urine — not in haemolytic jaundice."},{"id":102,"s":"Cytoskeleton","t":"Hereditary Spherocytosis","u":"SA:Volume Ratio","q":"A 6-year-old girl with hereditary spherocytosis has spherical RBCs instead of the normal biconcave disc shape. Compared to a normal biconcave RBC, the spherocyte has:","o":["Decreased surface area to volume ratio","Decreased volume only","Increased surface area to volume ratio","Increased volume only","Unchanged surface area to volume ratio"],"a":0,"p":"Biconcave disc = maximum surface area to volume ratio. In HS: spectrin deficiency → membrane lost → cell rounds into sphere. Sphere = LOWEST surface area to volume ratio → reduced gas exchange + reduced deformability → trapped and destroyed in spleen."},{"id":103,"s":"Cytoskeleton","t":"Hereditary Spherocytosis","u":"SA:Volume — Spleen","q":"In hereditary spherocytosis, spherocytes are selectively destroyed in the spleen because their decreased surface area to volume ratio causes:","o":["Complement activation","Decreased deformability","Increased antibody binding","Increased haemoglobin loss","Increased osmotic fragility"],"a":1,"p":"Normal RBCs deform through splenic sinusoids (3 µm) because high surface area to volume ratio → membrane excess → flexibility. Spherocytes: minimum membrane excess → cannot deform → trapped → macrophage destruction → extravascular haemolysis. Splenectomy = curative in most HS patients."},{"id":104,"s":"Cytoskeleton","t":"Hereditary Spherocytosis","u":"MCHC Mechanism","q":"In hereditary spherocytosis, RBCs progressively lose membrane lipids through repeated splenic passage while retaining their haemoglobin content. This disproportionate loss causes which characteristic change in RBC indices?","o":["Decreased MCH","Decreased MCHC","Decreased MCV","Increased MCHC","Increased MCV"],"a":3,"p":"Membrane loss without haemoglobin loss → same haemoglobin concentrated in smaller volume → ↑ MCHC (>36 g/dL). Most characteristic laboratory finding in HS — more specific than spherocytes on blood smear."},{"id":105,"s":"Cytoskeleton","t":"Hereditary Spherocytosis","u":"Band 3","q":"A researcher treats an RBC membrane with detergent. Which of the following integral membrane proteins is removed?","o":["Actin","Ankyrin","Band 3","Fibronectin","Spectrin"],"a":2,"p":"Band 3 (AE1) = integral RBC membrane protein — exchanges HCO₃⁻ for Cl⁻ (chloride shift) and anchors cytoskeleton via ankyrin. Integral proteins require detergents to remove. Spectrin, ankyrin, actin are peripheral — removed by salt."},{"id":106,"s":"Cytoskeleton","t":"Hereditary Spherocytosis","u":"SA:Volume — Physical","q":"A student studying RBC physiology notes that the biconcave disc shape gives red blood cells a mechanical advantage. This advantage relates to which physical property?","o":["Density to volume ratio","Mass to volume ratio","Pressure to volume ratio","Surface area to volume ratio","Viscosity to volume ratio"],"a":3,"p":"Biconcave disc = maximum surface area to volume ratio → maximum gas exchange surface + membrane excess for deformation. Spherocyte = minimum surface area to volume ratio → no membrane excess → cannot deform → trapped in spleen."},{"id":107,"s":"Cytoskeleton","t":"Hereditary Spherocytosis","u":"Deformability","q":"In hereditary spherocytosis, the spleen selectively destroys spherocytes because spherocytes cannot pass through splenic sinusoids due to:","o":["Complement activation","Decreased deformability","Increased adhesion","Increased osmotic pressure","Reduced haemoglobin"],"a":1,"p":"Normal RBCs deform through splenic sinusoids (3 µm) due to membrane excess from high SA:Vol ratio. Spherocytes: minimum membrane excess → cannot deform → trapped in splenic cords → macrophage destruction → extravascular haemolysis → splenomegaly."},{"id":108,"s":"Cytoskeleton","t":"Cytoskeletal Poisons","u":"Phalloidin","q":"A biochemist studying actin dynamics adds a toxin that binds F-actin and prevents its depolymerisation. Which toxin is this?","o":["Colchicine","Cytochalasin","Latrunculin","Phalloidin","Taxol"],"a":3,"p":"Phalloidin (Amanita phalloides) binds F-actin → stabilises filaments → prevents depolymerisation → actin locked in polymerised state. Cytochalasin → caps barbed ends. Latrunculin → sequesters G-actin. Colchicine/Taxol act on tubulin not actin."},{"id":109,"s":"Cytoskeleton","t":"Cytoskeletal Poisons","u":"Colchicine","q":"A cancer patient is prescribed a drug that binds free tubulin dimers preventing their addition to growing microtubules. The mitotic spindle collapses. Which drug is this?","o":["Colchicine","Cytochalasin","Latrunculin","Phalloidin","Taxol"],"a":0,"p":"Colchicine binds FREE tubulin dimers (G-tubulin, non-polymerised) → prevents polymerisation → spindle collapses → metaphase arrest. Used in gout (prevents neutrophil migration). Taxol: OPPOSITE — binds polymerised tubulin → stabilises → prevents depolymerisation."},{"id":110,"s":"Cytoskeleton","t":"Cytoskeletal Poisons","u":"Taxol","q":"A second cancer patient is prescribed a drug that binds to polymerised microtubules stabilising them and preventing depolymerisation. Which drug is this?","o":["Colchicine","Cytochalasin","Latrunculin","Phalloidin","Taxol"],"a":4,"p":"Taxol (paclitaxel) binds polymerised tubulin → stabilises microtubules → prevents depolymerisation → chromosome separation fails → mitotic arrest. Used in breast, ovarian, lung cancer. Compare: Colchicine binds G-tubulin (opposite mechanism, same end result)."},{"id":111,"s":"Cytoskeleton","t":"Cytoskeletal Poisons","u":"Death Cap","q":"A 20-year-old woman develops liver failure four days after eating wild mushrooms from a death cap. The actin-disrupting toxin responsible is:","o":["Alpha-amanitin","Cytochalasin","Latrunculin","Muscarine","Phalloidin"],"a":4,"p":"Phalloidin (Amanita phalloides/death cap) binds F-actin → prevents depolymerisation → hepatocyte death. Alpha-amanitin (same mushroom) inhibits RNA polymerase II → also hepatotoxic. Classic honeymoon phase: GI (12h) → apparent recovery → liver failure (day 4)."},{"id":112,"s":"Cytoskeleton","t":"Cytoskeletal Poisons","u":"Colchicine vs Taxol","q":"A student compares colchicine and taxol. Both cause mitotic arrest by opposite mechanisms. Colchicine prevents spindle formation by binding:","o":["F-actin polymers","F-tubulin polymers","G-actin monomers","G-tubulin monomers","Intermediate filaments"],"a":3,"p":"Colchicine binds G-tubulin (free monomers) → blocks polymerisation → depolymerisation → spindle collapse. Taxol binds F-tubulin (polymerised) → stabilises → prevents depolymerisation. Both → mitotic arrest. Vinca alkaloids (vincristine, vinblastine) also bind G-tubulin like colchicine."},{"id":113,"s":"Cytoskeleton","t":"Cytoskeleton — Intermediate Filaments","u":"Nuclear Lamins","q":"A cell biology student learns that the nuclear lamina is composed of a specific class of intermediate filaments found exclusively inside the nucleus. Which of the following best describes these filaments?","o":["Desmin filaments","Glial filaments","Keratin filaments","Neurofilaments","Nuclear lamins"],"a":4,"p":"Nuclear lamins (Type V) — Lamin A, B1, B2, C — found in ALL nucleated cells forming the nuclear lamina. Keratin (Type I/II) — epithelial. Vimentin/desmin/GFAP (Type III) — mesenchymal/muscle/astrocytes. Neurofilaments (Type IV) — neurons. Lamins ONLY intermediate filaments inside nucleus."},{"id":114,"s":"Cytoskeleton","t":"Cytoskeleton — Intermediate Filaments","u":"Progeria — Lamin","q":"A 15-year-old girl with features of accelerated ageing is diagnosed with Hutchinson-Gilford Progeria Syndrome. The mutated gene encodes:","o":["Fibrillin","Integrin","Lamin","Laminin","Progerin"],"a":2,"p":"LMNA gene encodes Lamin — major structural protein of nuclear lamina. Maintains nuclear shape, DNA replication, transcription, chromatin organisation. Progerin is the ABNORMAL product of mutated LMNA. Laminin is an ECM protein — completely different."},{"id":115,"s":"Cytoskeleton","t":"Cytoskeleton — Intermediate Filaments","u":"Progeria — Farnesylation","q":"A molecular biologist studies processing of prelamin A. Without cleavage of a specific post-translational modification, the protein remains permanently anchored to the inner nuclear membrane. Which modification is retained in progerin?","o":["Acetyl group","Farnesyl group","Methyl group","Phosphate group","Ubiquitin tag"],"a":1,"p":"Prelamin A → farnesylated at CAAX → endoprotease cleaves farnesylcysteine → soluble mature lamin A. Progerin: 50 aa deletion removes cleavage site → farnesyl permanently retained → membrane-anchored → nuclear blebbing → premature senescence."},{"id":116,"s":"Cytoskeleton","t":"Cytoskeleton — Intermediate Filaments","u":"Progeria — Splice Site","q":"A genetics student learns that a silent point mutation in the LMNA gene causes progeria despite not changing the amino acid sequence. The mutation causes disease by acting as a:","o":["Deletion mutation","Frameshift mutation","Missense mutation","Nonsense mutation","Splice site mutation"],"a":4,"p":"C1824T is silent (Gly608Gly) but simultaneously activates a cryptic internal splice site in exon 11 → 150 bp deleted from mRNA → 50 aa deletion from C-terminus of lamin A → progerin. Classic example of synonymous mutation causing disease through aberrant splicing."},{"id":117,"s":"Cytoskeleton","t":"Cytoskeleton — Intermediate Filaments","u":"Progeria — Endoprotease","q":"Comparing lamin A processing in a normal individual with progerin in a progeria patient, the endoprotease cannot act on progerin because progerin lacks the:","o":["CAAX motif","Cleavage site","Farnesyl group","Nuclear signal","Phosphorylation site"],"a":1,"p":"Normal: prelamin A → farnesylated → endoprotease cleaves last 15 aa including farnesylcysteine → soluble lamin A. Progeria: 50 aa deletion removes the CLEAVAGE SITE → progerin remains farnesylated → permanently membrane-bound → accumulates → nuclear stiffness → accelerated ageing."},{"id":118,"s":"Cytoskeleton","t":"Cytoskeleton — Intermediate Filaments","u":"Progeria — Farnesylation Type","q":"The functional group that anchors progerin to the inner nuclear membrane is attached to a cysteine residue via a thioether bond. This type of post-translational modification is classified as:","o":["Acetylation","Farnesylation","Glycosylation","Phosphorylation","Ubiquitination"],"a":1,"p":"Farnesylation = attachment of 15-carbon isoprenoid (farnesyl) to cysteine via thioether bond at CAAX motif. Lipophilic group inserts into inner nuclear membrane. In normal lamin A, endoprotease removes this anchor. In progerin, removal never happens → permanent anchoring."},{"id":119,"s":"Extracellular Matrix","t":"Collagen Structure","u":"Vitamin C","q":"A biochemist studies collagen biosynthesis noting that newly synthesised procollagen chains must be hydroxylated before forming a stable triple helix. Which vitamin is required as a cofactor?","o":["Vitamin A","Vitamin B12","Vitamin C","Vitamin D","Vitamin K"],"a":2,"p":"Vitamin C = essential cofactor for prolyl hydroxylase and lysyl hydroxylase (iron-containing) AND lysyl oxidase (copper-containing). Pro → Hyp (stabilises triple helix). Lys → Hyl (crosslink sites). Without Vit C → unstable triple helix AND poor crosslinking → scurvy."},{"id":120,"s":"Extracellular Matrix","t":"Collagen Structure","u":"Prolyl Hydroxylase","q":"A 70-year-old woman on a tea-and-toast diet presents with bleeding gums and poor wound healing. Vitamin C deficiency impairs collagen triple helix formation. Which of the following best describes the enzyme inactivated by Vitamin C deficiency that is directly involved in triple helix formation?","o":["Copper-containing crosslinker","Copper-containing hydroxylase","Iron-containing crosslinker","Iron-containing hydroxylase","Zinc-containing hydroxylase"],"a":3,"p":"Prolyl hydroxylase = iron-containing hydroxylase. Vit C keeps iron as Fe²⁺ (active) → Pro → Hyp → stabilises triple helix. Also: Lysyl hydroxylase (iron-containing, Vit C-dependent) → Lys → Hyl → crosslink sites. Lysyl oxidase (copper-containing, Vit C-dependent) → extracellular crosslinking of mature fibrils."},{"id":121,"s":"Extracellular Matrix","t":"Collagen Structure","u":"Scurvy — Iron Absorption","q":"A scurvy patient presents with pallor alongside bleeding gums. The pallor is primarily due to Vitamin C deficiency impairing:","o":["Erythropoietin synthesis","Folate metabolism","Globin synthesis","Haem synthesis","Iron absorption"],"a":4,"p":"Vitamin C (via DCYTB enzyme) reduces Fe³⁺ (ferric, non-absorbable) → Fe²⁺ (ferrous, absorbable) in gut lumen → absorbed by DMT1 transporter. In scurvy: ↓ iron absorption → iron deficiency anaemia → pallor. Combined with blood loss from fragile vessels → worsened anaemia."},{"id":122,"s":"Extracellular Matrix","t":"Collagen Structure","u":"Scurvy — Dentition","q":"A 70-year-old edentulous woman develops scurvy while her 80-year-old husband living in the same house has no symptoms. The most likely reason the husband is protected is his:","o":["Ability to absorb","Ability to chew","Ability to digest","Ability to metabolise","Ability to synthesise"],"a":1,"p":"Wife: edentulous → cannot chew → tea and toast only → no Vitamin C → defective hydroxylation → unstable collagen → scurvy. Husband: intact teeth → can eat fresh vegetables → adequate Vitamin C → normal collagen synthesis."},{"id":123,"s":"Extracellular Matrix","t":"Collagen Structure","u":"Lysyl Oxidase","q":"A 20-year-old woman with progressive skin thickening has overactive extracellular collagen crosslinking. Which enzyme crosslinks collagen fibrils in the ECM?","o":["Lysyl hydroxylase","Lysyl oxidase","Prolyl hydroxylase","Prolyl isomerase","Transglutaminase"],"a":1,"p":"Lysyl oxidase (copper-dependent, extracellular) oxidises lysine/hydroxylysine → aldehydes → spontaneous crosslinks → insoluble mature collagen fibrils. Vit C also required. β-aminopropionitrile (BAPN, from sweet peas) inhibits lysyl oxidase → lathyrism."},{"id":124,"s":"Extracellular Matrix","t":"Collagen Structure","u":"Propeptides","q":"A biochemist studying collagen notes that procollagen is synthesised as a precursor with extra peptides at both ends. These extra peptides are called:","o":["Propeptides","Signal peptides","Telopeptides","Tripeptides","Zymopeptides"],"a":0,"p":"Procollagen = soluble precursor with N and C-terminal propeptides. Propeptides: (1) prevent intracellular fibril assembly, (2) guide correct chain registration. After secretion: procollagen N/C-proteinase cleavage → tropocollagen → fibril assembly → crosslinking by lysyl oxidase → mature collagen."},{"id":125,"s":"Extracellular Matrix","t":"Collagen Disorders","u":"OI — Glycine Substitution","q":"A 15-year-old boy with recurrent fractures, blue sclerae, and hearing loss is diagnosed with Osteogenesis Imperfecta type I. The causative mutation involves replacement of glycine in the Gly-X-Y repeat by which of the following?","o":["Alanine","Asparagine","Glutamine","Proline","Tryptophan"],"a":4,"p":"Glycine (smallest amino acid) at every third position inside the triple helix. Any bulky amino acid physically blocks triple helix formation. Tryptophan has the largest bulky side chain. Even one abnormal α-chain poisons the entire trimer — dominant negative effect."},{"id":126,"s":"Extracellular Matrix","t":"Collagen Disorders","u":"OI — Blue Sclerae","q":"In the OI patient, blue sclerae occur because thin scleral collagen allows the underlying structure to be visible. Which underlying structure causes the blue colour?","o":["Aqueous humour","Choroid vasculature","Iris","Phenylalanine deposits","Vitreous humour"],"a":1,"p":"Normal sclera: white and opaque — dense type I collagen. OI: thin translucent sclera → underlying dark choroid vasculature shows through → blue appearance. Blue sclerae + recurrent fractures + hearing loss = classic OI triad."},{"id":127,"s":"Extracellular Matrix","t":"Collagen Disorders","u":"OI — Hearing Loss","q":"Hearing loss in the OI patient primarily occurs due to defective type I collagen in:","o":["Auditory nerve","Cochlear hair cells","Eustachian tube","Middle ear ossicles","Tympanic membrane"],"a":3,"p":"Middle ear ossicles (malleus, incus, stapes) = smallest bones in body — composed of type I collagen matrix. In OI: defective collagen → brittle ossicles → impaired vibration conduction → conductive hearing loss."},{"id":128,"s":"Extracellular Matrix","t":"Collagen Disorders","u":"OI — Dominant Negative","q":"A 15-year-old OI boy has a mutation causing replacement of glycine by a bulky amino acid in type I collagen. This mutation acts by which mechanism?","o":["Dominant gain of function","Dominant negative","Haploinsufficiency","Loss of function","Recessive loss"],"a":1,"p":"OI collagen mutations act by dominant negative: one abnormal α-chain incorporated into heterotrimer (2α1+1α2) → bulky side chain blocks triple helix formation → entire molecule abnormal even though two of three chains may be normal. One mutant allele poisons products of both alleles."},{"id":129,"s":"Extracellular Matrix","t":"Collagen Disorders","u":"OI — Triple Helix","q":"Comparing the OI patient's type I collagen with his normal sibling, the OI patient's procollagen triple helix fails to form because of:","o":["Glycine substitution","Failed hydroxylation","Failed propeptide cleavage","Failed secretion","Failed crosslinking"],"a":0,"p":"Normal: Gly-X-Y repeat → three α-chains wind C→N terminus. OI: bulky amino acid replaces Gly → blocks triple helix zippering → all upstream folding stalls. One abnormal chain in heterotrimer disrupts entire molecule."},{"id":130,"s":"Extracellular Matrix","t":"Collagen Disorders","u":"OI — Least Disruptive","q":"A 15-year-old OI patient has a mutation replacing glycine in the Gly-X-Y repeat of type I collagen. Which type of side chain replacing glycine would be LEAST likely to disrupt the triple helix?","o":["Acidic side chain","Bulky side chain","Non-polar side chain","Polar side chain","Small side chain"],"a":4,"p":"Glycine has the smallest possible side chain (single H atom) essential to fit inside the cramped triple helix centre. Replacement with any bulky side chain blocks triple helix formation. A small side chain replacement causes least disruption. Severity of OI correlates with bulkiness of substituting amino acid."},{"id":131,"s":"Extracellular Matrix","t":"Collagen Disorders","u":"EDS — Type V Collagen","q":"A 4-year-old boy with skin hyperextensibility, joint hypermobility, and easy bruising has a collagen type V mutation. Type V collagen is important because it regulates fibrillogenesis of:","o":["Collagen type I and III","Collagen type II and IV","Collagen type II and VI","Collagen type III and IV","Collagen type IV and VII"],"a":0,"p":"Type V collagen nucleates and regulates fibril diameter of type I and III collagen. Collagen V mutation → disorganised type I/III fibrils → weakened skin (hyperextensibility) and joint capsules/tendons (hypermobility) = Ehlers-Danlos Syndrome classic type."},{"id":132,"s":"Extracellular Matrix","t":"Collagen Disorders","u":"EDS — Joints","q":"A boy with Ehlers-Danlos Syndrome has a collagen type V mutation. Joint hypermobility occurs because disorganised type I collagen fibrils weaken which structures?","o":["Articular cartilage","Periosteum","Subchondral bone","Synovial membrane","Tendons and ligaments"],"a":4,"p":"Type I collagen in tendons, ligaments, and joint capsules stabilises joints. COL5A1 mutation → disorganised type I fibrils → reduced tensile strength → excessive range of motion. Skin hyperextensibility reflects same mechanism in dermis."},{"id":133,"s":"Extracellular Matrix","t":"Collagen Disorders","u":"EDS — Fibril Diameter","q":"A 25-year-old man with hyperextensible skin and hypermobile joints has a collagen type V mutation. Which property of his skin collagen fibrils is most likely abnormal?","o":["Composition","Crosslinking","Diameter","Hydroxylation","Solubility"],"a":2,"p":"Type V collagen regulates fibril DIAMETER of type I and III collagen. COL5A1/COL5A2 mutation → absent type V collagen at fibril core → type I/III fibrils form with abnormal (larger, irregular) diameter → disorganised fibrils → reduced tensile strength."},{"id":134,"s":"Extracellular Matrix","t":"Collagen Disorders","u":"Collagen Aging","q":"A 50-year-old woman develops facial wrinkles. The primary biochemical mechanism involves free radicals causing which change to dermal collagen?","o":["Denaturation","Demethylation","Glycation","Hydroxylation failure","Oxidative crosslinking"],"a":4,"p":"Free radicals from UV → aberrant oxidative crosslinking of collagen → thick rigid fibres → skin loses elasticity → wrinkles. Collagen synthesis also slows with age → less collagen. Elastin simultaneously damaged → loss of skin snap-back."},{"id":135,"s":"Extracellular Matrix","t":"Collagen Disorders","u":"Retinoids","q":"A dermatologist prescribes retinoid cream for wrinkle reduction. Retinoids reduce wrinkles by inhibiting collagenases AND stimulating:","o":["Elastin degradation","Keratinocyte apoptosis","Melanin production","New collagen synthesis","Sebum production"],"a":3,"p":"Retinoids (retinoic acid) bind nuclear RAR/RXR receptors: (1) Inhibit collagenase (MMP) synthesis → less collagen degradation. (2) Stimulate fibroblasts to produce new collagen → increased dermal collagen content."},{"id":136,"s":"Extracellular Matrix","t":"Glycosaminoglycans","u":"GAG Class","q":"A 55-year-old obese woman with bilateral knee pain takes glucosamine sulfate and chondroitin sulfate over the counter for osteoarthritis. These compounds belong to which class of biomolecules?","o":["Glycolipids","Glycoproteins","Glycosaminoglycans","Mucoproteins","Proteoglycans"],"a":2,"p":"Glucosamine sulfate and chondroitin sulfate are GAGs — long unbranched heteropolysaccharide chains. Highly negatively charged → attract water → hydrated shells → resist compression → ideal for cartilage and synovial fluid."},{"id":137,"s":"Extracellular Matrix","t":"Glycosaminoglycans","u":"GAG Compression","q":"A 60-year-old man with osteoarthritis takes chondroitin sulfate over the counter. The benefit in joint lubrication comes from GAGs ability to:","o":["Activate fibroblasts","Chelate calcium","Inhibit prostaglandins","Resist compression","Stimulate collagen"],"a":3,"p":"GAGs carry many negative charges → repel each other → attract water → form hydrated shells → resist compression. When compressed water squeezed out; when released negative charge repulsion restores hydrated volume → resilience of synovial fluid and cartilage."},{"id":138,"s":"Extracellular Matrix","t":"Glycosaminoglycans","u":"GAG Negative Charges","q":"A 50-year-old woman with stiff painful finger joints has lost cartilage ability to resist compression. Which molecular property of cartilage glycosaminoglycans is responsible for this resistance?","o":["Covalent bonds","Hydrophobic core","Negative charges","Peptide bonds","Positive charges"],"a":2,"p":"GAGs (chondroitin sulfate, glucosamine sulfate) carry large numbers of negative charges from sulfate and carboxylate groups → repel each other → attract water → form hydrated shells → resilience. Loss of GAGs in OA → cartilage loses water-binding capacity → brittle."},{"id":139,"s":"Extracellular Matrix","t":"Glycosaminoglycans","u":"Chondroitin Sulfate","q":"A researcher studying proteoglycans notes that aggrecan in articular cartilage is decorated with many glycosaminoglycan chains. Which glycosaminoglycan is most abundant in cartilage?","o":["Chondroitin sulfate","Dermatan sulfate","Heparan sulfate","Hyaluronic acid","Keratan sulfate"],"a":0,"p":"Chondroitin sulfate is the most abundant GAG in articular cartilage. Aggrecan monomers attach to hyaluronic acid via link proteins → large proteoglycan aggregates → trap water → resist compression."},{"id":140,"s":"Extracellular Matrix","t":"ECM Proteins","u":"Fibronectin","q":"A biochemist studying the extracellular matrix isolates a glycoprotein that bridges collagen fibres to integrin receptors on the cell surface. Which glycoprotein is this?","o":["Aggrecan","Decorin","Fibronectin","Laminin","Versican"],"a":2,"p":"Fibronectin = ECM bridge: collagen/fibrin ↔ RGD sequence binds integrin ↔ intracellular actin. Disrupted in cancer metastasis. Laminin bridges basement membrane to integrins."},{"id":141,"s":"Extracellular Matrix","t":"ECM Proteins","u":"Type I Collagen","q":"A 30-year-old man with recurrent fractures has a mutation in his type I collagen gene. Type I collagen is a heterotrimer composed of which chains?","o":["One α1 and two α2","Three α1","Three α2","Two α1 and one α2","Two α2 and one α3"],"a":3,"p":"Type I collagen heterotrimer = 2α1(I) + 1α2(I) chains. Each chain has Gly-X-Y repeating sequence. In OI: mutation in COL1A1 (α1) or COL1A2 (α2) → dominant negative → poisons entire heterotrimer."},{"id":142,"s":"Extracellular Matrix","t":"AAT Deficiency","u":"AAT — Deficiency","q":"A 40-year-old woman presents with progressive shortness of breath and jaundice. She has never smoked. Her father and paternal uncle both died of liver disease in their forties. Chest X-ray shows hyperinflated lungs. Which protein is most likely deficient in her blood?","o":["Alpha-1 antitrypsin","Alpha-2 macroglobulin","Antithrombin III","Ceruloplasmin","Haptoglobin"],"a":0,"p":"AAT deficiency = serine protease inhibitor deficiency → unchecked neutrophil elastase → alveolar wall destruction → emphysema. Liver disease from misfolded AAT accumulation in hepatocyte RER. Strong family history of early liver disease = key clue in non-smokers."},{"id":143,"s":"Extracellular Matrix","t":"AAT Deficiency","u":"AAT — Methionine","q":"A 35-year-old man who smokes heavily develops rapidly progressive emphysema despite being young. Smoking accelerates lung destruction by oxidising which residue in the AAT active site?","o":["Cysteine","Histidine","Methionine","Serine","Tryptophan"],"a":2,"p":"Active site methionine of AAT → oxidised by cigarette smoke ROS → methionine sulfoxide → AAT cannot inhibit neutrophil elastase → elastase degrades alveolar walls. Even normal amounts of AAT are inactivated by smoking → accelerated emphysema."},{"id":144,"s":"Extracellular Matrix","t":"AAT Deficiency","u":"AAT — Liver","q":"An AAT-deficient patient develops liver cirrhosis. The primary mechanism is accumulation of misfolded AAT in:","o":["Bile canaliculi","Hepatic sinusoids","Hepatocyte RER","Kupffer cells","Portal tracts"],"a":2,"p":"Z-variant AAT misfolds in hepatocyte RER → polymerises → accumulates as PAS-positive insoluble aggregates → ER stress → fibrosis → cirrhosis. Gain-of-toxic-function (accumulation damages liver) separate from loss-of-function in lungs."},{"id":145,"s":"Extracellular Matrix","t":"AAT Deficiency","u":"AAT — Synthesis Site","q":"The primary site of AAT synthesis in the body is:","o":["Alveolar macrophages","Enterocytes","Hepatocytes","Neutrophils","Renal tubular cells"],"a":2,"p":"~90% of circulating AAT synthesised and secreted by hepatocytes. Liver transplant corrects both liver and lung disease simultaneously — replaces defective hepatocytes with normal AAT-secreting cells."},{"id":146,"s":"Extracellular Matrix","t":"AAT Deficiency","u":"AAT — Augmentation","q":"A 55-year-old woman with progressive shortness of breath and obstructive spirometry pattern has a family history of early liver disease and has never smoked. Augmentation therapy is started. Intravenous AAT reaches which compartment to inhibit neutrophil elastase?","o":["Alveolar macrophages","Bronchial mucosa","Lung epithelial lining fluid","Pleural space","Pulmonary capillaries"],"a":2,"p":"IV AAT → enters bloodstream → diffuses into epithelial lining fluid (ELF) of lung → therapeutic antiprotease levels → inhibits neutrophil elastase → slows alveolar destruction. Does NOT repair existing emphysema."},{"id":147,"s":"Extracellular Matrix","t":"AAT Deficiency","u":"AAT — PAS Granules","q":"A histopathology report describes PAS-positive intracellular granules in periportal hepatocytes of a patient with emphysema and cirrhosis. These granules consist of:","o":["Alpha-1 antitrypsin","Alpha-fetoprotein","Caeruloplasmin","Ferritin","Transferrin"],"a":0,"p":"PAS-positive granules in periportal hepatocytes = accumulated Z-variant AAT polyaggregates. Compare: Wilson disease = copper deposits (rhodanine stain), haemochromatosis = iron (Prussian blue), glycogen storage disease = PAS-positive but diffuse not periportal."},{"id":148,"s":"Cell Adhesion","t":"Cell Junctions","u":"Tight Junctions","q":"A gastroenterologist explains that in coeliac disease the intestinal epithelium becomes leaky allowing antigens to cross into the lamina propria. Which junction is disrupted?","o":["Adherens junction","Desmosome","Gap junction","Hemidesmosome","Tight junction"],"a":4,"p":"Tight junctions (zonula occludens) = claudins + occludins + ZO proteins seal paracellular space. Disrupted in coeliac disease and IBD → leaky gut → antigen entry → inflammation. Also establish apical/basolateral epithelial polarity."},{"id":149,"s":"Cell Adhesion","t":"Cell Junctions","u":"Tight Junctions — Epithelium","q":"A student is asked which junction seals the paracellular space between adjacent epithelial cells preventing movement of ions and molecules between cells. Which junction does this?","o":["Adherens junction","Desmosome","Gap junction","Hemidesmosome","Tight junction"],"a":4,"p":"Tight junctions = claudins + occludins + ZO-1/2/3 proteins. Seal paracellular space → create epithelial barrier. Also prevent lateral diffusion of membrane proteins between apical and basolateral domains."},{"id":150,"s":"Cell Adhesion","t":"Cell Junctions","u":"Gap Junctions — Cardiac","q":"A student is asked which cell junction allows direct electrical communication between adjacent cardiac muscle cells enabling them to contract as a syncytium. Which junction is this?","o":["Adherens junction","Desmosome","Gap junction","Hemidesmosome","Tight junction"],"a":2,"p":"Gap junctions = connexin proteins (6 per cell) form connexons → two connexons from adjacent cells align → aqueous channel → ions and molecules <1 kDa pass directly → electrical coupling → coordinated cardiac contraction. Connexin 43 dominant in heart."},{"id":151,"s":"Cell Adhesion","t":"Cell Junctions","u":"Gap Junctions — Dye Transfer","q":"A researcher applies a 500 dalton dye to one cardiac myocyte. The dye rapidly appears in adjacent myocytes. Which junction allows this direct intercellular transfer?","o":["Adherens junction","Desmosome","Gap junction","Hemidesmosome","Tight junction"],"a":2,"p":"Gap junctions allow molecules <1 kDa to pass between cells. Connexin 43 = dominant in heart. Connexin 26 = cochlea (mutation = DFNB1 hereditary deafness). Connexin 32 = peripheral myelin (mutation = Charcot-Marie-Tooth X-linked)."},{"id":152,"s":"Cell Adhesion","t":"Cell Junctions","u":"Connexin Structure","q":"A molecular biology student studies connexin structure. Each connexin protein spans the membrane how many times?","o":["Once","Twice","Three times","Four times","Seven times"],"a":3,"p":"Each connexin monomer has 4 transmembrane domains (TM1-TM4) with both N and C termini in cytoplasm. Six connexins → connexon (hemichannel). Two connexons from adjacent cells dock → complete gap junction channel. Pore gating regulated by pH, Ca²⁺, voltage, and phosphorylation."},{"id":153,"s":"Cell Adhesion","t":"Pemphigus Vulgaris","u":"PV — Junction","q":"A 50-year-old woman presents with painful oral ulcers and erosions. Biopsy shows loss of intercellular attachments between keratinocytes with intraepidermal blistering. IgG autoantibodies against keratinocyte surface molecules are detected. Which cell junction is targeted?","o":["Adherens junctions","Desmosomes","Gap junctions","Hemidesmosomes","Tight junctions"],"a":1,"p":"Pemphigus vulgaris = IgG autoantibodies target desmoglein proteins on desmosomes → loss of desmosomal adhesion → acantholysis → intraepidermal blister. Hemidesmosomes anchor basal keratinocytes to basement membrane — not targeted in PV."},{"id":154,"s":"Cell Adhesion","t":"Pemphigus Vulgaris","u":"PV — Tombstone","q":"A biopsy from a patient with oral blistering shows basal keratinocytes separated from each other but remaining attached to the basement membrane forming a tombstone pattern. The basal cells retain their attachment to the basement membrane via:","o":["Desmosomes","Focal adhesions","Gap junctions","Hemidesmosomes","Tight junctions"],"a":3,"p":"In PV, IgG targets desmoglein on desmosomes → basal keratinocytes lose desmosomal adhesion to each other (acantholysis) but RETAIN attachment to basement membrane via hemidesmosomes → tombstone pattern. Hemidesmosomes (α6β4 integrin + BP180) not targeted in PV."},{"id":155,"s":"Cell Adhesion","t":"Pemphigus Vulgaris","u":"PV — Acantholysis","q":"The process by which pemphigus IgG antibodies cause loss of cell-to-cell adhesion between keratinocytes is termed:","o":["Acantholysis","Apoptosis","Dyskeratosis","Parakeratosis","Spongiosis"],"a":0,"p":"Acantholysis = loss of cohesion between epidermal cells due to breakdown of intercellular bridges (desmosomes). Anti-desmoglein antibodies block desmosomal adhesion AND trigger p38 MAPK/PKC → cytoskeletal retraction → keratinocyte rounding. Antibody alone (without complement) sufficient to cause acantholysis."},{"id":156,"s":"Cell Adhesion","t":"Pemphigus Vulgaris","u":"PV — Desmosome","q":"A histology student examines a skin biopsy stained with anti-desmoplakin antibody. Desmoplakin is a component of which cell junction?","o":["Adherens junction","Desmosome","Gap junction","Hemidesmosome","Tight junction"],"a":1,"p":"Desmosome (macula adherens) components: extracellular — desmoglein + desmocollin (cadherin family); intracellular plaque — desmoplakin + plakophilin + plakoglobin; linked to keratin intermediate filaments. Provides tensile strength to epithelium."},{"id":157,"s":"Cell Adhesion","t":"Pemphigus Vulgaris","u":"PV — Hemidesmosome","q":"A pathologist examines skin from a patient with pemphigus vulgaris. Basal keratinocytes remain attached to the basement membrane but are separated from each other. The basal cells retain their anchor to the basement membrane via:","o":["Adherens junction","Desmosome","Gap junction","Hemidesmosome","Tight junction"],"a":3,"p":"Hemidesmosomes anchor the base of keratinocytes to the basement membrane. Components: α6β4 integrin + BP180 (type XVII collagen) + BP230 + plectin → linked to keratin. Bullous pemphigoid = IgG targets BP180/BP230 on hemidesmosomes → subepidermal blister (different from PV)."},{"id":158,"s":"Cell Adhesion","t":"Cell Adhesion Molecules","u":"Scramblase — Apoptosis","q":"A researcher adds calcium ions to a cell suspension. Within minutes phosphatidylserine appears on the outer leaflet. Which enzyme is activated by calcium to cause this?","o":["Flippase","Floppase","Kinase","Phospholipase","Scramblase"],"a":4,"p":"Scramblase Ca²⁺-ACTIVATED → bidirectional → PS externalised → 'eat me' signal → macrophage recognition → phagocytosis (efferocytosis). Ca²⁺ simultaneously inactivates flippase. Both mechanisms together ensure rapid PS externalisation in apoptosis."},{"id":159,"s":"Cell Adhesion","t":"Cell Adhesion Molecules","u":"Fibronectin","q":"A biochemist isolates a glycoprotein from the extracellular matrix that bridges collagen to integrin receptors using an RGD sequence. Which glycoprotein is this?","o":["Aggrecan","Decorin","Fibronectin","Laminin","Vitronectin"],"a":2,"p":"Fibronectin = ECM bridge glycoprotein: collagen/fibrin ↔ RGD sequence binds integrin receptor ↔ intracellular actin. Disrupted in cancer metastasis — cancer cells downregulate fibronectin → loss of anchorage → increased motility."},{"id":160,"s":"Cell Adhesion","t":"Cell Adhesion Molecules","u":"Cholesterol Fluidity","q":"A cell biologist studies how membrane lipid composition affects fluidity. Incorporating which specific sterol makes unsaturated membranes more rigid and saturated membranes more fluid?","o":["Ceramide","Cholesterol","Diacylglycerol","Phosphatidylcholine","Sphingomyelin"],"a":1,"p":"Cholesterol = membrane fluidity buffer. With UNSATURATED fatty acids → restricts movement → more rigid. With SATURATED fatty acids → prevents tight packing → more fluid. Net: optimal fluidity across temperature range."},{"id":161,"s":"Cell Membrane","t":"Signal Transduction — Cholera and Pertussis","u":"Cholera — G protein alpha-s","q":"A 5-year-old boy in a flood-affected area presents with profuse rice-water diarrhoea and severe dehydration. Vibrio cholerae is identified. Cholera toxin adenosine diphosphate-ribosylates which G protein subunit?","o":["G protein alpha-12","G protein alpha-i","G protein alpha-o","G protein alpha-q","G protein alpha-s"],"a":4,"p":"Cholera toxin B subunit binds ganglioside GM1 on enterocyte surface → A subunit enters cytosol → adenosine diphosphate-ribosylates G protein alpha-s at Arg201 → prevents GTPase activity → G protein alpha-s permanently active → adenylyl cyclase constitutively ON → cyclic adenosine monophosphate rises dramatically → protein kinase A → CFTR open → chloride secretion → water follows osmotically → rice-water diarrhoea → dehydration.\n\n📹 [VIDEO LINK]"},{"id":162,"s":"Cell Membrane","t":"Signal Transduction — Cholera and Pertussis","u":"Cholera — cAMP","q":"A child in a flood-affected area is diagnosed with cholera. Constitutive activation of adenylyl cyclase in his intestinal epithelial cells leads to elevation of which second messenger?","o":["Calcium","Cyclic adenosine monophosphate","Cyclic guanosine monophosphate","Diacylglycerol","Inositol trisphosphate"],"a":1,"p":"Cholera toxin → permanently active G protein alpha-s → adenylyl cyclase constitutively ON → ATP → cyclic adenosine monophosphate (cAMP) → activates protein kinase A → phosphorylates CFTR chloride channel → constitutive chloride secretion → water follows osmotically → massive fluid loss → dehydration. Cyclic guanosine monophosphate is the second messenger for nitric oxide and nitroglycerin — a completely different pathway.\n\n📹 [VIDEO LINK]"},{"id":163,"s":"Cell Membrane","t":"Signal Transduction — Cholera and Pertussis","u":"Cholera — CFTR","q":"A 5-year-old boy with cholera develops severe dehydration due to massive fluid loss. The activated CFTR channel in intestinal epithelial cells causes secretion of which ion into the gut lumen?","o":["Bicarbonate","Chloride","Phosphate","Potassium","Sodium"],"a":1,"p":"Protein kinase A phosphorylates CFTR (chloride channel, ABC transporter family) → constitutively open → chloride secreted into gut lumen → osmotic gradient created → water follows chloride into lumen → rice-water diarrhoea → dehydration. Oral rehydration therapy uses SGLT1 (sodium-glucose cotransporter — remains functional) to drive sodium and water absorption back.\n\n📹 [VIDEO LINK]"},{"id":164,"s":"Cell Membrane","t":"Signal Transduction — Cholera and Pertussis","u":"Cholera — ORS","q":"A 5-year-old cholera patient is given oral rehydration therapy. ORS works by using which transporter to drive sodium and water absorption despite active CFTR-mediated secretion?","o":["GLUT1","GLUT4","Na⁺/K⁺ ATPase","SGLT1","SGLT2"],"a":3,"p":"SGLT1 (sodium-glucose cotransporter 1) on the apical membrane of intestinal enterocytes remains functional during cholera. Glucose in ORS drives sodium cotransport into enterocytes → osmotic gradient draws water in → effective rehydration despite ongoing CFTR-mediated chloride secretion. This is the biochemical rationale for glucose in ORS — not for calories.\n\n📹 [VIDEO LINK]"},{"id":165,"s":"Cell Membrane","t":"Signal Transduction — Cholera and Pertussis","u":"Pertussis — Organism","q":"A 1-year-old unvaccinated boy presents with paroxysmal whooping cough and post-tussive vomiting. The causative organism is:","o":["Bordetella bronchiseptica","Bordetella parapertussis","Bordetella pertussis","Haemophilus influenzae","Streptococcus pneumoniae"],"a":2,"p":"Bordetella pertussis causes whooping cough. Attaches to respiratory epithelium → produces pertussis toxin → adenosine diphosphate-ribosylates G protein alpha-i → G protein alpha-i cannot exchange GDP for GTP → remains inactive → cannot inhibit adenylyl cyclase → adenylyl cyclase runs unopposed → cyclic adenosine monophosphate rises → excessive mucus secretion → characteristic whoop on inhalation.\n\n📹 [VIDEO LINK]"},{"id":166,"s":"Cell Membrane","t":"Signal Transduction — Cholera and Pertussis","u":"Pertussis — G protein alpha-i","q":"Pertussis toxin causes excess mucus production by adenosine diphosphate-ribosylating which G protein subunit, preventing it from inhibiting adenylyl cyclase?","o":["G protein alpha-12","G protein alpha-i","G protein alpha-o","G protein alpha-q","G protein alpha-s"],"a":1,"p":"Pertussis toxin adenosine diphosphate-ribosylates G protein alpha-i (at Cys351) → G protein alpha-i CANNOT exchange GDP for GTP → remains in inactive GDP-bound state → cannot inhibit adenylyl cyclase → adenylyl cyclase runs unopposed → cyclic adenosine monophosphate rises → protein kinase A → excessive mucus secretion. Compare: Cholera toxin adenosine diphosphate-ribosylates G protein alpha-s (ACTIVATING it). Both raise cyclic adenosine monophosphate but by opposite mechanisms.\n\n📹 [VIDEO LINK]"},{"id":167,"s":"Cell Membrane","t":"Signal Transduction — Cholera and Pertussis","u":"Cholera vs Pertussis","q":"Both cholera toxin and pertussis toxin raise intracellular cyclic adenosine monophosphate but by different mechanisms. The key difference is that cholera toxin acts on G protein alpha-s while pertussis toxin acts on:","o":["G protein alpha-12","G protein alpha-i","G protein alpha-o","G protein alpha-q","G protein alpha-t"],"a":1,"p":"Cholera toxin: adenosine diphosphate-ribosylates G protein alpha-s → locks it ACTIVE → adenylyl cyclase permanently ON → cyclic adenosine monophosphate rises. Pertussis toxin: adenosine diphosphate-ribosylates G protein alpha-i → locks it INACTIVE → cannot inhibit adenylyl cyclase → cyclic adenosine monophosphate rises. Both use adenosine diphosphate-ribosylation. Both raise cyclic adenosine monophosphate. Different subunits. Different mechanisms. Same end result.\n\n📹 [VIDEO LINK]"},{"id":168,"s":"Cell Membrane","t":"Steroid Hormone Signalling","u":"Tamoxifen — Antagonist","q":"A woman with oestrogen receptor-positive breast cancer is prescribed tamoxifen for 5 years after surgery. Tamoxifen reduces cancer recurrence by acting as which type of agent at the oestrogen receptor?","o":["Agonist","Antagonist","Inverse agonist","Partial agonist","Superagonist"],"a":1,"p":"Tamoxifen competitively binds oestrogen receptor → displaces oestrogen → conformational change at helix 12 → co-activators cannot bind → oestrogen-responsive genes not transcribed → no cell proliferation signal → cytostatic effect → prevents recurrence of oestrogen receptor-positive breast cancer. Classified as a Selective Oestrogen Receptor Modulator — antagonist in breast, partial agonist in bone and uterus.\n\n📹 [VIDEO LINK]"},{"id":169,"s":"Cell Membrane","t":"Steroid Hormone Signalling","u":"Tamoxifen — Gene Transcription","q":"A woman with breast cancer takes tamoxifen regularly for 5 years and remains cancer-free. A non-compliant woman develops cancer recurrence. The difference in outcome is because tamoxifen blocks which cellular process that oestrogen normally drives in breast cancer cells?","o":["Apoptosis","Differentiation","Gene transcription","Membrane transport","Protein degradation"],"a":2,"p":"Oestrogen normally binds oestrogen receptor → receptor dimerises → binds oestrogen response elements in DNA → co-activators recruited → gene transcription begins → cell proliferation. Tamoxifen: occupies oestrogen receptor → wrong conformational change at helix 12 → co-activators CANNOT bind → gene transcription blocked → no proliferation signal → tumour growth halted. Cytostatic not cytotoxic. Non-compliant patients lose this blockade → cancer resumes growth.\n\n📹 [VIDEO LINK]"},{"id":170,"s":"Cell Membrane","t":"Steroid Hormone Signalling","u":"Tamoxifen — Nuclear Receptor","q":"Tamoxifen is lipid-soluble and enters the breast cancer cell. It then competes with oestrogen at the oestrogen receptor. The oestrogen receptor belongs to which class of receptors based on its location?","o":["Cell surface receptor","Endoplasmic reticulum receptor","Mitochondrial receptor","Nuclear receptor","Plasma membrane receptor"],"a":3,"p":"Oestrogen receptors are nuclear receptors — present in cytosol (unliganded, associated with chaperones) and nucleus. Being lipid-soluble, oestrogen crosses the plasma membrane directly → binds cytosolic receptor → complex translocates to nucleus → binds oestrogen response elements → gene transcription. Tamoxifen follows the same route → reaches nucleus → competes with oestrogen → blocks transcription. All steroid hormones, thyroid hormones, and vitamin D use nuclear receptors.\n\n📹 [VIDEO LINK]"},{"id":171,"s":"Extracellular Matrix","t":"Peroxisomal Disorders","u":"Zellweger — Organelle","q":"A 6-week-old girl presents with seizures, hypotonia, hepatomegaly, and dysmorphic facial features. Lab results show elevated very-long-chain fatty acids and phytanic acid. The defective organelle in this condition is:","o":["Endoplasmic reticulum","Golgi apparatus","Lysosome","Mitochondria","Peroxisome"],"a":4,"p":"Zellweger syndrome = defective peroxisome biogenesis due to PEX gene mutations → defective peroxin proteins → matrix proteins cannot be imported into peroxisomes → no functional peroxisomes → ALL peroxisomal functions lost → very-long-chain fatty acids accumulate (cannot be beta-oxidised) and phytanic acid accumulates (cannot be alpha-oxidised). Presenting in neonates with hypotonia, seizures, hepatomegaly, dysmorphic features.\n\n📹 [VIDEO LINK]"},{"id":172,"s":"Extracellular Matrix","t":"Peroxisomal Disorders","u":"Zellweger — VLCFA","q":"A newborn with Zellweger syndrome has elevated very-long-chain fatty acids in plasma. In which organelle is very-long-chain fatty acid beta-oxidation normally carried out?","o":["Cytosol","Endoplasmic reticulum","Lysosome","Mitochondria","Peroxisome"],"a":4,"p":"Very-long-chain fatty acids (longer than C22) are beta-oxidised EXCLUSIVELY in peroxisomes. Medium and long chain fatty acids are oxidised in mitochondria. Peroxisomal beta-oxidation shortens very-long-chain fatty acids to medium chain fatty acids → transferred to mitochondria for complete oxidation. In Zellweger: no functional peroxisomes → very-long-chain fatty acids accumulate → toxic to central nervous system neurons → neuronal injury → demyelination → seizures.\n\n📹 [VIDEO LINK]"},{"id":173,"s":"Extracellular Matrix","t":"Peroxisomal Disorders","u":"ALD — ABC Transporter","q":"A boy with X-linked Adrenoleukodystrophy has progressive neurological deterioration due to very-long-chain fatty acid accumulation. The protein responsible for importing very-long-chain fatty acids into peroxisomes belongs to which transporter family?","o":["ABC transporters","Aquaporins","GLUT family","P-class pumps","SGLT family"],"a":0,"p":"ALDP (adrenoleukodystrophy protein) encoded by ABCD1 gene = ABC transporter in peroxisomal membrane → imports very-long-chain fatty acid-CoA esters into peroxisomes for beta-oxidation. Mutation → very-long-chain fatty acids cannot enter peroxisome → accumulate in cytosol → elongated further by ELOVL1 elongase → toxic accumulation in myelin → demyelination → neurological symptoms.\n\n📹 [VIDEO LINK]"},{"id":174,"s":"Extracellular Matrix","t":"Peroxisomal Disorders","u":"ALD — Lorenzo's Oil","q":"A boy with X-linked Adrenoleukodystrophy is given Lorenzo's oil. Lorenzo's oil reduces plasma very-long-chain fatty acid levels by competitively inhibiting which enzyme?","o":["ELOVL1 elongase","Fatty acid synthase","HMG-CoA reductase","Lipoprotein lipase","Stearoyl-CoA desaturase"],"a":0,"p":"Lorenzo's oil = 4:1 mixture of glyceryl trioleate (oleic acid C18:1) and glyceryl trierucate (erucic acid C22:1). Oleic and erucic acids competitively inhibit ELOVL1 elongase → cannot elongate C22 to C24 to C26 → reduced endogenous very-long-chain fatty acid synthesis → reduced plasma very-long-chain fatty acid levels. Does not repair the underlying ABCD1 mutation. Most effective if started before neurological symptoms appear.\n\n📹 [VIDEO LINK]"},{"id":175,"s":"Extracellular Matrix","t":"Peroxisomal Disorders","u":"Zellweger — Peroxins","q":"A 6-week-old girl with Zellweger syndrome has absent functional peroxisomes. This is caused by mutations in PEX genes which encode which proteins required for peroxisome biogenesis?","o":["Caveolins","Claudins","Connexins","Peroxins","Porins"],"a":3,"p":"PEX genes encode PEROXINS — proteins required for peroxisome membrane assembly and import of peroxisomal matrix proteins via peroxisomal targeting sequences PTS1 and PTS2. Mutations in any of more than 14 PEX genes → defective peroxins → cannot import matrix enzymes → empty peroxisomal ghosts → all peroxisomal functions lost → Zellweger spectrum disorder.\n\n📹 [VIDEO LINK]"},{"id":176,"s":"Cell Membrane","t":"Membrane Transport — Endocytosis","u":"Botulinum — SNAP-25","q":"A 20-year-old woman develops flaccid paralysis after eating commercially canned meat. The toxin responsible prevents acetylcholine release at the neuromuscular junction by cleaving which protein?","o":["Acetylcholinesterase","Myosin","SNAP-25","Spectrin","Synapsin"],"a":2,"p":"Botulinum toxin heavy chain binds ganglioside on presynaptic membrane → receptor-mediated endocytosis → acidic endosome → disulfide bond reduced → light chain translocates to cytosol → cleaves SNAP-25 → SNARE complex cannot form → synaptic vesicles cannot fuse → NO acetylcholine release → flaccid paralysis. Toxin acts PRE-synaptically — does NOT block acetylcholine receptors.\n\n📹 [VIDEO LINK]"},{"id":177,"s":"Cell Membrane","t":"Membrane Transport — Endocytosis","u":"Botulinum — SNARE","q":"The botulinum toxin light chain enters the cytosol and cleaves SNAP-25. SNAP-25 is a component of which protein complex responsible for vesicle-membrane fusion?","o":["Clathrin complex","Dynein complex","Kinesin complex","SNARE complex","Tubulin complex"],"a":3,"p":"SNARE complex = three proteins: SNAP-25 (target membrane) + Syntaxin (target membrane) + Synaptobrevin/VAMP (vesicle membrane). These wind around each other → pull vesicle and plasma membrane into close apposition → membrane fusion → neurotransmitter release. Botulinum toxin type A and E cleave SNAP-25. Type B, D, F, G cleave synaptobrevin. Type C cleaves syntaxin. All prevent SNARE complex formation → no acetylcholine release.\n\n📹 [VIDEO LINK]"},{"id":178,"s":"Cell Membrane","t":"Membrane Transport — Endocytosis","u":"Botulinum — ACh","q":"A woman with botulism develops respiratory failure requiring mechanical ventilation. Botulinum toxin causes flaccid paralysis because it prevents release of which neurotransmitter at the neuromuscular junction?","o":["Acetylcholine","Dopamine","Glutamate","Noradrenaline","Serotonin"],"a":0,"p":"Neuromuscular junction uses acetylcholine as its neurotransmitter. Botulinum toxin cleaves SNARE proteins → synaptic vesicles containing acetylcholine cannot fuse with presynaptic membrane → no acetylcholine released → no nicotinic acetylcholine receptor activation → no muscle action potential → flaccid paralysis → respiratory muscles affected → respiratory failure. Management: mechanical ventilation + botulinum antitoxin.\n\n📹 [VIDEO LINK]"},{"id":179,"s":"Cell Membrane","t":"Membrane Transport — Endocytosis","u":"Botulinum — Non-exposure","q":"The mother of the botulism patient did not eat the canned meat and has no symptoms. The reason she is protected from botulinum toxicity is:","o":["Immunity","Metabolism","Non-exposure","Nutrition","Vaccination"],"a":2,"p":"Botulinum toxin has NO taste and NO smell — cannot be detected organoleptically. The mother was protected entirely by non-exposure. Bulging of the can is caused by gas produced by Clostridium botulinum metabolism — a warning sign of contamination. Toxin is heat-labile — proper cooking at 85 degrees Celsius for 5 or more minutes destroys it.\n\n📹 [VIDEO LINK]"},{"id":180,"s":"Cell Organelles","t":"Cell Organelles — LHON","u":"LHON — Organelle","q":"A 17-year-old male presents with progressive painless bilateral visual loss. Genetic testing shows a point mutation in the mitochondrial gene encoding NADH dehydrogenase. Which organelle carries this mutated gene?","o":["Endoplasmic reticulum","Golgi apparatus","Lysosome","Mitochondria","Nucleus"],"a":3,"p":"Mitochondria contain their own circular DNA encoding 13 proteins of the oxidative phosphorylation machinery including NADH dehydrogenase subunits, 22 transfer RNAs, and 2 ribosomal RNAs. Leber Hereditary Optic Neuropathy = point mutation in mitochondrial DNA encoding NADH dehydrogenase (Complex I) → defective oxidative phosphorylation → reduced ATP → retinal ganglion cell apoptosis → optic nerve degeneration → visual loss.\n\n📹 [VIDEO LINK]"},{"id":181,"s":"Cell Organelles","t":"Cell Organelles — LHON","u":"LHON — ETC","q":"A 17-year-old with Leber Hereditary Optic Neuropathy has a mutation in the gene encoding NADH dehydrogenase. Deficiency of this enzyme reduces ATP production causing retinal ganglion cell death. NADH dehydrogenase is a component of which metabolic pathway?","o":["Beta oxidation","Citric acid cycle","Electron transport chain","Glycolysis","Urea cycle"],"a":2,"p":"NADH dehydrogenase = Complex I of the electron transport chain on the inner mitochondrial membrane. Accepts electrons from NADH → passes them to ubiquinone → simultaneously pumps hydrogen ions into intermembrane space building the proton gradient used by ATP synthase. Mutation → Complex I dysfunctional → reduced ATP → retinal ganglion cells (high energy demand, no collateral blood supply) most vulnerable → apoptosis → optic atrophy → visual loss.\n\n📹 [VIDEO LINK]"},{"id":182,"s":"Cell Membrane","t":"Membrane Transport","u":"Osmotic Diuresis — CKD","q":"An 80-year-old male with chronic kidney disease has progressive accumulation of urea and creatinine in his blood causing hyperosmolarity. This hyperosmolarity triggers which of the following membrane transport mechanisms in his kidneys causing increased urine output?","o":["Active secretion","Facilitated diffusion","Osmotic diuresis","Primary active transport","Simple diffusion"],"a":2,"p":"Accumulated waste solutes (urea, creatinine) in blood and tubular lumen create an osmotic gradient → water cannot be reabsorbed from kidney tubules → osmotic diuresis → increased urine output → progressive fluid and electrolyte loss → worsening kidney function. The same mechanism explains why uncontrolled diabetes causes excessive urination — glucose in the tubular lumen drives water out osmotically.\n\n📹 [VIDEO LINK]"},{"id":183,"s":"Cell Organelles","t":"Cell Organelles — MCAD","u":"MCAD — Enzyme","q":"A 6-month-old boy presents with lethargy and hypoglycaemia after several days of poor feeding. He is hypoketotic. A mutation in the ACADM gene is identified. Which enzyme is likely deficient?","o":["Isovaleryl-CoA dehydrogenase","Long-chain acyl-CoA dehydrogenase","Medium-chain acyl-CoA dehydrogenase","Short-chain acyl-CoA dehydrogenase","Very-long-chain acyl-CoA dehydrogenase"],"a":2,"p":"MCAD (Medium-Chain Acyl-CoA Dehydrogenase) = first enzyme in mitochondrial beta-oxidation of medium-chain fatty acids (C6 to C12). Deficiency → cannot beta-oxidise medium-chain fatty acids → no acetyl-CoA from fat → no ketone bodies (hypoketonaemia) → cannot spare glucose → hypoglycaemia especially during fasting. All options are real acyl-CoA dehydrogenases each handling a different chain length. MCAD deficiency is the most common fatty acid oxidation disorder.\n\n📹 [VIDEO LINK]"},{"id":184,"s":"Cell Organelles","t":"Cell Organelles — MCAD","u":"MCAD — Acetyl-CoA","q":"The MCAD-deficient boy is hypoketotic. This occurs because he cannot generate which molecule from medium-chain fatty acids that is required for ketone body synthesis?","o":["Acetyl-CoA","Citrate","Malate","Oxaloacetate","Pyruvate"],"a":0,"p":"Normal beta-oxidation: fatty acids → acetyl-CoA → in liver → HMG-CoA → ketone bodies (acetoacetate, beta-hydroxybutyrate). In MCAD deficiency: medium-chain fatty acids cannot be beta-oxidised → no acetyl-CoA generated from fat → no substrate for ketone body synthesis → hypoketonaemia → brain cannot use ketones as alternative fuel during fasting → relies entirely on glucose → rapid glucose depletion → hypoglycaemia → lethargy and seizures.\n\n📹 [VIDEO LINK]"},{"id":185,"s":"Cell Organelles","t":"Cell Organelles — MCAD","u":"MCAD — Coupling","q":"A 6-month-old with MCAD deficiency develops hypoglycaemia during fasting. Unlike normal fasting physiology, this patient is simultaneously hypoketotic. Which molecule cannot be generated from medium-chain fatty acids due to the enzyme deficiency?","o":["Acetyl-CoA","Citrate","Malate","Oxaloacetate","Pyruvate"],"a":0,"p":"IMPORTANT CONCEPT — Normal fasting: hypoglycaemia → fat breakdown → beta-oxidation → acetyl-CoA → ketone bodies → hyperketonaemia. Hypoglycaemia and hyperketonaemia are NORMALLY coupled — ketones form precisely when glucose is low.\n\nIn MCAD deficiency this coupling is BROKEN: medium-chain fatty acids cannot be beta-oxidised → no acetyl-CoA from fat → no ketone bodies → hypoketonaemia despite hypoglycaemia. Any cause of food unavailability (illness, vomiting, prolonged fasting) precipitates a crisis. The diagnostic hallmark: HYPOGLYCAEMIA + HYPOKETONAEMIA occurring together — pathological uncoupling.\n\n📹 [VIDEO LINK]"},{"id":186,"s":"Cell Organelles","t":"Cell Organelles — MCAD","u":"MCAD — Organelle","q":"A 6-month-old boy with MCAD deficiency cannot beta-oxidise medium-chain fatty acids. Beta-oxidation of medium and long-chain fatty acids normally occurs in which organelle?","o":["Cytosol","Endoplasmic reticulum","Golgi apparatus","Mitochondria","Peroxisome"],"a":3,"p":"Medium and long chain fatty acid beta-oxidation occurs in the MITOCHONDRIAL MATRIX. Fatty acids enter mitochondria via the carnitine shuttle (carnitine acyltransferase I and II). MCAD deficiency → medium-chain acyl-CoA dehydrogenase absent in mitochondrial matrix → cannot perform first step of beta-oxidation of medium-chain fatty acids → acetyl-CoA not generated → no ATP from fat → no ketone bodies → hypoglycaemia and hypoketonaemia. Very-long-chain fatty acid beta-oxidation = peroxisomes (different organelle).\n\n📹 [VIDEO LINK]"},{"id":187,"s":"Cell Organelles","t":"Cell Organelles — Gaucher","u":"Gaucher — Organelle","q":"A 6-year-old boy is diagnosed with Gaucher disease due to deficiency of glucocerebrosidase. Glucocerebrosides accumulate in which organelle of macrophages in the liver and spleen?","o":["Endoplasmic reticulum","Golgi apparatus","Lysosome","Mitochondria","Nucleus"],"a":2,"p":"Glucocerebrosidase is a lysosomal enzyme. Its deficiency → glucocerebrosides cannot be broken down in lysosomes → accumulate in lysosomes of macrophages → macrophages become engorged → Gaucher cells (crumpled tissue paper cytoplasm) → accumulate in liver, spleen, bone marrow → hepatosplenomegaly, anaemia, bone pain. Treatment: enzyme replacement therapy (imiglucerase).\n\n📹 [VIDEO LINK]"},{"id":188,"s":"Cell Organelles","t":"Cell Organelles — Gaucher","u":"Gaucher — Hepatosplenomegaly","q":"A 6-year-old boy has bone marrow aspiration showing cells with crumpled tissue paper appearance of the cytoplasm (Gaucher cells). These are macrophages engorged with glucocerebrosides due to glucocerebrosidase deficiency. Which of the following is the MOST LIKELY finding on abdominal examination of this patient?","o":["Cardiomegaly","Hepatomegaly","Hepatosplenomegaly","Nephromegaly","Splenomegaly"],"a":2,"p":"Gaucher cells accumulate in BOTH liver (hepatomegaly) AND spleen (splenomegaly) → hepatosplenomegaly. The spleen is typically more dramatically enlarged than the liver. Also accumulates in bone marrow → anaemia, thrombocytopenia, bone pain. Gaucher disease = most common lysosomal storage disease. Type 1 (non-neuronopathic) most common — no central nervous system involvement.\n\n📹 [VIDEO LINK]"},{"id":189,"s":"Cell Organelles","t":"Cell Organelles — Gaucher","u":"Gaucher — ERT","q":"A 6-year-old with Gaucher disease is started on enzyme replacement therapy. The drug administered provides a functional version of which deficient enzyme?","o":["Alpha-galactosidase A","Glucocerebrosidase","Hexosaminidase A","Sphingomyelinase","Sulphatase"],"a":1,"p":"Imiglucerase (Cerezyme) = recombinant glucocerebrosidase given intravenously every 2 weeks → taken up by macrophages via mannose receptors → delivered to lysosomes → breaks down accumulated glucocerebrosides → reduces organ size and bone complications. Most effective for Type 1 (non-neuronopathic) Gaucher. Does not cross blood-brain barrier → ineffective for Type 2 and 3 (neuronopathic forms).\n\n📹 [VIDEO LINK]"},{"id":190,"s":"Cell Membrane","t":"Membrane Transport","u":"Haemodialysis","q":"A 20-year-old man with kidney failure undergoes haemodialysis. Haemodialysis removes metabolic waste products across which type of membrane?","o":["Ion exchange membrane","Lipid bilayer membrane","Nuclear membrane","Permeable membrane","Semipermeable membrane"],"a":4,"p":"Haemodialysis uses a dialyzer containing a semipermeable membrane — allows small molecules (urea, creatinine, electrolytes, excess water) to cross by diffusion and osmosis while blocking large molecules (proteins, blood cells). A fully permeable membrane would allow everything through including proteins and blood cells — not useful. Counter-current flow maximises the concentration gradient across the semipermeable membrane → maximises waste removal.\n\n📹 [VIDEO LINK]"},{"id":191,"s":"Cell Membrane","t":"Membrane Transport","u":"Reverse Osmosis","q":"Filter water is purified by pushing water under pressure through a semipermeable membrane removing contaminants. This process is called:","o":["Dialysis","Filtration","Osmosis","Reverse osmosis","Ultrafiltration"],"a":3,"p":"Reverse osmosis = application of pressure greater than osmotic pressure to force water through a semipermeable membrane AGAINST its osmotic gradient → water passes through → contaminants (ions, bacteria, organic molecules) cannot → purified water collected on the other side. Compare: Normal osmosis = water moves from low to high solute concentration spontaneously without applied pressure.\n\n📹 [VIDEO LINK]"},{"id":192,"s":"Cell Membrane","t":"Membrane Transport","u":"Simple vs Active Transport","q":"A student is studying how substances move across cell membranes. She notes that some substances move from an area of high concentration to low concentration without using energy. Which of the following is an example of this type of transport?","o":["Glucose absorption in gut","Na⁺/K⁺ ATPase pumping","Oxygen crossing alveolar membrane","SGLT1 cotransport","Sodium reabsorption in kidney"],"a":2,"p":"Oxygen is a small nonpolar gas → crosses the plasma membrane directly through the phospholipid bilayer from high concentration (alveoli) to low concentration (blood) → simple diffusion → no protein, no energy required. This is passive transport. Compare: Na⁺/K⁺ ATPase = primary active transport (uses ATP directly). SGLT1 = secondary active transport (uses sodium gradient). Glucose absorption in gut = secondary active transport via SGLT1.\n\n📹 [VIDEO LINK]"},{"id":193,"s":"Cell Membrane","t":"Membrane Transport","u":"HHS — Dehydration","q":"A 50-year-old woman with type 2 diabetes mellitus develops severe dehydration. High blood glucose causes water to be lost from the body through the kidneys. This process is called:","o":["Active secretion","Facilitated diffusion","Osmosis","Osmotic diuresis","Simple diffusion"],"a":3,"p":"When blood glucose is very high, glucose appears in the urine (exceeds the renal threshold of 180 milligrams per decilitre). Glucose in the kidney tubule creates an osmotic gradient → water cannot be reabsorbed → large volumes of water and salt lost in urine → severe dehydration. This is osmotic diuresis. It is the reason diabetic patients urinate excessively and become severely dehydrated.\n\n📹 [VIDEO LINK]"},{"id":194,"s":"Cell Membrane","t":"Membrane Transport","u":"HHS — Osmotic Diuresis","q":"A 50-year-old woman with type 2 diabetes mellitus develops Hyperosmolar Hyperglycaemic State. Her blood glucose is 900 milligrams per decilitre. Glucose accumulates in her kidney tubules exceeding the renal threshold. Which membrane transport mechanism in her kidneys is actively driving massive water loss into urine?","o":["Active secretion","Facilitated diffusion","Osmotic diuresis","Primary active transport","Simple diffusion"],"a":2,"p":"When blood glucose exceeds 180 milligrams per decilitre (renal threshold), glucose appears in the tubular lumen. Glucose in the tubular lumen creates an osmotic gradient → water is actively drawn OUT of the tubular cells into the lumen → osmotic diuresis → massive water and electrolyte loss in urine → severe dehydration → hyperosmolarity worsens. Osmotic diuresis is not a failure — it is a functional membrane transport mechanism actively working, but driven by an abnormal osmotic gradient created by excess glucose.\n\n📹 [VIDEO LINK]"},{"id":195,"s":"Cell Membrane","t":"Membrane Transport","u":"HHS — Stress Hormones","q":"A 50-year-old woman with type 2 diabetes mellitus develops Hyperosmolar Hyperglycaemic State after a urinary tract infection. Infections worsen blood glucose by causing release of which hormones?","o":["Cortisol and glucagon","Insulin and leptin","Oestrogen and progesterone","Parathyroid hormone and calcitonin","Thyroid hormone and thyroxine"],"a":0,"p":"Infection → physiological stress → cortisol and glucagon released → increased gluconeogenesis and glycogenolysis → blood glucose rises further → worsens the condition. This is why infections commonly trigger diabetic emergencies. In type 2 diabetes patients enough insulin remains to suppress ketone body production — so ketones stay low and the condition is Hyperosmolar Hyperglycaemic State rather than diabetic ketoacidosis.\n\n📹 [VIDEO LINK]"},{"id":196,"s":"Cell Adhesion","t":"Cell Adhesion Molecules","u":"E-Cadherin — Family","q":"A patient with invasive breast cancer has lost cell-to-cell adhesion allowing tumour cells to detach and spread. The transmembrane glycoprotein responsible for epithelial cell-cell adhesion that is lost in most epithelial cancers belongs to which protein family?","o":["Cadherin family","Immunoglobulin superfamily","Integrin family","Selectin family","Siglec family"],"a":0,"p":"E-cadherin = epithelial cadherin — transmembrane glycoprotein requiring calcium for homophilic binding. Intracellularly linked to beta-catenin → alpha-catenin → actin cytoskeleton. Loss of E-cadherin → loss of cell-cell adhesion → epithelial-mesenchymal transition → cells become motile → invasion and metastasis. E-cadherin is a tumour suppressor — its loss is a hallmark of cancer progression.\n\n📹 [VIDEO LINK]"},{"id":197,"s":"Cell Adhesion","t":"Cell Adhesion Molecules","u":"E-Cadherin — Catenins","q":"Loss of E-cadherin function in cancer cells promotes metastasis. E-cadherin intracellularly links to the actin cytoskeleton via which family of proteins?","o":["Ankyrin family","Catenin family","Claudin family","Connexin family","Spectrin family"],"a":1,"p":"E-cadherin intracellular domain → binds beta-catenin → alpha-catenin → actin cytoskeleton. This cadherin-catenin complex links adjacent cells cytoskeletons together. Loss of E-cadherin → beta-catenin released → translocates to nucleus → activates Wnt target genes → promotes proliferation. Dual role: structural (cell adhesion) and signalling (Wnt pathway).\n\n📹 [VIDEO LINK]"},{"id":198,"s":"Cell Adhesion","t":"Cell Adhesion Molecules","u":"E-Cadherin — Calcium","q":"A cancer patient has altered E-cadherin expression. E-cadherin-mediated cell-to-cell adhesion requires which ion for cadherin binding to adjacent cadherins?","o":["Calcium","Chloride","Magnesium","Potassium","Sodium"],"a":0,"p":"Calcium binds to extracellular cadherin domains → causes conformational rigidity → enables homophilic binding between E-cadherins on adjacent cells. Without calcium → cadherins are flexible → cannot bind adjacent cadherins → cell-cell adhesion lost. EDTA (calcium chelator) disrupts cadherin-mediated adhesion in vitro. Calcium is essential for maintaining epithelial tissue integrity.\n\n📹 [VIDEO LINK]"},{"id":199,"s":"Cell Adhesion","t":"Extravasation","u":"Rolling — Selectins","q":"A 16-year-old boy with acute tonsillitis has elevated white blood cells on blood count. Leukocytes migrate to the site of inflammation through a 4-step process. The first step of rolling is mediated by which adhesion molecules?","o":["Cadherins","Integrins","Laminins","Selectins","Syndecans"],"a":3,"p":"Extravasation steps: (1) Rolling — selectins (P-selectin and E-selectin on endothelium bind sialyl-Lewis X on leukocytes). (2) Activation — chemokines activate leukocyte integrins. (3) Firm adhesion — activated integrins bind ICAM-1 on endothelium → leukocyte stops rolling. (4) Transmigration (diapedesis) — leukocyte squeezes through endothelial junctions → reaches site of inflammation.\n\n📹 [VIDEO LINK]"},{"id":200,"s":"Cell Adhesion","t":"Extravasation","u":"Firm Adhesion — ICAM-1","q":"After rolling, leukocytes firmly adhere to the endothelium. This firm adhesion step involves activated leukocyte integrins binding to which endothelial adhesion molecules?","o":["Cadherins","E-selectins","ICAM-1","P-selectins","Syndecans"],"a":2,"p":"Firm adhesion: activated integrins (LFA-1 = CD11a:CD18 = alpha-L-beta-2) on leukocytes bind ICAM-1 on endothelium → firm arrest → leukocyte stops rolling → undergoes diapedesis. In Leukocyte Adhesion Deficiency: mutation in beta-2 integrin subunit → integrins cannot bind ICAM-1 → leukocytes cannot firmly adhere → cannot extravasate → recurrent bacterial infections despite elevated white blood cell count.\n\n📹 [VIDEO LINK]"},{"id":201,"s":"Cell Adhesion","t":"Leukocyte Adhesion Deficiency","u":"LAD — Beta-2 Integrin","q":"A 10-month-old patient with Leukocyte Adhesion Deficiency type I develops recurrent bacterial infections. This condition is caused by a mutation in which protein?","o":["Beta-1 integrin","Beta-2 integrin","E-selectin","ICAM-1","P-selectin"],"a":1,"p":"Leukocyte Adhesion Deficiency type I = autosomal recessive mutation in beta-2 integrin subunit (CD18) → all beta-2 integrins non-functional (LFA-1, Mac-1, p150,95) → leukocytes cannot firmly adhere to endothelium → cannot extravasate to infection sites → recurrent life-threatening bacterial infections → delayed wound healing. Paradoxically: very HIGH white blood cell count in blood (cells cannot leave circulation).\n\n📹 [VIDEO LINK]"},{"id":202,"s":"Cell Adhesion","t":"Atherosclerosis","u":"Atherosclerosis — VCAM-1","q":"A 45-year-old obese smoker develops atherosclerosis. Oxidised LDL injures the endothelium causing upregulation of adhesion molecules. Circulating monocytes adhere to the injured endothelium via which adhesion molecule expressed on the endothelial surface?","o":["Cadherin","Connexin","E-selectin","Fibronectin","VCAM-1"],"a":4,"p":"Oxidised LDL injures endothelium → Vascular Cell Adhesion Molecule-1 (VCAM-1) and ICAM-1 expressed on endothelial surface → circulating monocytes express VLA-4 (alpha-4-beta-1 integrin) which binds VCAM-1 → monocytes adhere → roll → firmly arrest → diapedesis into subendothelium → differentiate into macrophages → engulf oxidised LDL via scavenger receptors → foam cells → fatty streak → atherosclerotic plaque.\n\n📹 [VIDEO LINK]"},{"id":203,"s":"Cell Adhesion","t":"Atherosclerosis","u":"Atherosclerosis — Foam Cells","q":"In atherosclerosis, macrophages in the subendothelium engulf oxidised LDL via scavenger receptors. Unlike the LDL receptor which is downregulated by intracellular cholesterol, the scavenger receptor is not downregulated. The macrophage continues to engulf oxidised LDL until it becomes a:","o":["Foam cell","Giant cell","Kupffer cell","Mast cell","Reed-Sternberg cell"],"a":0,"p":"Scavenger receptor (SR-A, CD36) NOT downregulated → macrophage continues engulfing oxidised LDL → cytoplasm fills with lipid droplets → foam cell. Foam cells accumulate in subendothelium → fatty streak → atherosclerotic plaque → rupture → thrombosis → myocardial infarction. Foam cells are the pathological hallmark of early atherosclerosis.\n\n📹 [VIDEO LINK]"},{"id":204,"s":"Cell Adhesion","t":"Atherosclerosis","u":"Atherosclerosis — VCAM-1 Binding","q":"In the atherosclerosis patient, monocytes adhere to the injured endothelium because endothelial injury causes upregulation of which adhesion molecule that binds monocyte integrins?","o":["Cadherin","Connexin","Fibronectin","Spectrin","VCAM-1"],"a":4,"p":"Oxidised LDL injures endothelium → endothelial cells express Vascular Cell Adhesion Molecule-1 (VCAM-1) → circulating monocytes express VLA-4 (alpha-4-beta-1 integrin) which binds VCAM-1 → monocytes adhere → roll → firmly arrest → diapedesis into subendothelium → differentiate into macrophages → engulf oxidised LDL → foam cells → fatty streak.\n\n📹 [VIDEO LINK]"},{"id":205,"s":"Cell Adhesion","t":"Asthma","u":"Asthma — ICAM-1","q":"A 6-year-old boy with seasonal asthma has increased migration of leukocytes to his airways. In asthma, which adhesion molecule is overexpressed on the bronchial endothelium causing excessive leukocyte migration?","o":["E-cadherin","ICAM-1","N-cadherin","P-selectin","Spectrin"],"a":1,"p":"In asthma, allergen exposure → mast cell degranulation → interleukins released → ICAM-1 overexpressed on bronchial endothelium → increased eosinophil and leukocyte adhesion via integrin-ICAM-1 interaction → increased migration into airways → increased inflammation → airway oedema, bronchospasm, mucus → wheezing and cough.\n\n📹 [VIDEO LINK]"},{"id":206,"s":"Cell Adhesion","t":"Rheumatoid Arthritis","u":"RA — ICAM-2","q":"A 35-year-old woman with rheumatoid arthritis has synovial inflammation due to increased leukocyte adhesion. Which adhesion molecule is overexpressed on the synovial membrane facilitating leukocyte binding?","o":["ICAM-1","ICAM-2","ICAM-3","VCAM-1","VCAM-2"],"a":1,"p":"In rheumatoid arthritis: synovial membrane overexpresses ICAM-2 combined with increased LFA-1 integrin on leukocytes → increased leukocyte adhesion and migration into synovial membrane → increased inflammation → synovitis → cartilage destruction → bone erosion → joint deformity. Tumour necrosis factor alpha and interleukin-1 drive this adhesion upregulation. Biologics targeting tumour necrosis factor alpha reduce this inflammatory cascade.\n\n📹 [VIDEO LINK]"},{"id":207,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"I-Cell — Diagnosis","q":"A newborn presents with coarse facial features, corneal clouding, claw hand deformity, and kyphoscoliosis. Blood testing shows markedly elevated plasma levels of ALL lysosomal enzymes simultaneously. This simultaneous elevation of ALL lysosomal enzymes in plasma points to a defect in which enzyme responsible for tagging lysosomal enzymes for delivery to lysosomes?","o":["Alpha-galactosidase A","Beta-glucocerebrosidase","GlcNAc-1-phosphotransferase","Hexosaminidase A","Sphingomyelinase"],"a":2,"p":"The diagnostic hallmark of I-cell disease is elevation of ALL lysosomal enzymes in plasma simultaneously — this points immediately to a defect in the TAGGING system, not any individual enzyme. GlcNAc-1-phosphotransferase (in the cis-Golgi) adds the mannose-6-phosphate tag to ALL lysosomal enzymes. Without this tag → ALL enzymes secreted extracellularly → ALL substrates accumulate → inclusion bodies (I-cells). If only ONE enzyme were deficient only ONE substrate accumulates — Gaucher (glucocerebrosidase), Tay-Sachs (hexosaminidase A). Elevation of ALL lysosomal enzymes = I-cell disease.\n\n📹 [VIDEO LINK]"},{"id":208,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"I-Cell — M6P","q":"In I-cell disease, lysosomal enzymes are secreted extracellularly instead of being delivered to lysosomes because the enzymes lack which targeting signal?","o":["GPI anchor","Mannose-6-phosphate","N-acetylgalactosamine","Signal peptide","Ubiquitin tag"],"a":1,"p":"Normal lysosomal enzyme targeting: GlcNAc-1-phosphotransferase adds mannose-6-phosphate tag to lysosomal enzymes in the Golgi → mannose-6-phosphate receptors recognise the tag → package enzymes into vesicles → deliver to lysosomes. In I-cell disease: no mannose-6-phosphate tag → enzymes not recognised → secreted extracellularly → lysosomes lack all hydrolases → substrates accumulate → inclusion bodies.\n\n📹 [VIDEO LINK]"},{"id":209,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"I-Cell — Golgi","q":"In I-cell disease, the defective enzyme GlcNAc-1-phosphotransferase catalyses the first step in synthesising the mannose-6-phosphate marker. This enzyme is located in which organelle?","o":["Endoplasmic reticulum","Golgi apparatus","Lysosome","Mitochondria","Nucleus"],"a":1,"p":"GlcNAc-1-phosphotransferase is located in the cis-Golgi apparatus. It transfers N-acetylglucosamine-1-phosphate to mannose residues on lysosomal enzyme precursors → forms GlcNAc-1-phosphomannose → a second enzyme removes GlcNAc → leaves mannose-6-phosphate. This mannose-6-phosphate tag is the universal signal for lysosomal targeting in mammalian cells except hepatocytes, Kupffer cells, and leukocytes which use an alternative pathway.\n\n📹 [VIDEO LINK]"},{"id":210,"s":"Cell Membrane","t":"Membrane Lipids","u":"Membrane Lipids — Subclass","q":"A student is studying membrane lipid composition. Phospholipids are the most abundant lipids in the plasma membrane. Which of the following is a subclass of membrane phospholipids?","o":["Ceramide","Cholesterol","Glycerophospholipid","Glycosphingolipid","Triacylglycerol"],"a":2,"p":"Membrane phospholipids are divided into two main subclasses: (1) Glycerophospholipids (glycerol backbone + 2 fatty acids + phosphate head group) — include phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, phosphatidylinositol. (2) Sphingophospholipids (sphingosine backbone) — sphingomyelin is the main example. Glycolipids are a separate class (no phosphate). Cholesterol is a sterol — separate class. Triacylglycerol = storage fat, not a membrane lipid.\n\n📹 [VIDEO LINK]"},{"id":211,"s":"Cell Membrane","t":"Membrane Lipids","u":"Glycolipids — Antigenic","q":"Glycolipids in the plasma membrane are found exclusively on the outer leaflet where they contribute to the glycocalyx. Which property do glycolipids confer on the cell surface?","o":["Antigenic properties","Electrical conduction","Hydrophobic barrier","Mechanical rigidity","Water impermeability"],"a":0,"p":"Glycolipids on the outer leaflet — their sugar chains project into the extracellular space → form part of the glycocalyx → confer antigenic properties (ABO blood groups determined by glycolipid sugar sequences) → cell-cell recognition → tissue-specific identity. The major glycolipids in animal tissues are glycosphingolipids (ceramide + one or more sugars).\n\n📹 [VIDEO LINK]"},{"id":212,"s":"Cell Membrane","t":"Membrane Lipids","u":"Cholesterol — Paradoxical","q":"A student is asked about the paradoxical effect of cholesterol on membrane fluidity. At physiological temperature in a membrane with saturated fatty acids, cholesterol acts to:","o":["Decrease fluidity","Decrease permeability","Increase fluidity","Increase rigidity","Maintain asymmetry"],"a":2,"p":"Cholesterol paradoxical (buffering) effect: In membranes with SATURATED fatty acids (normally rigid, tightly packed) → cholesterol inserts between tails → prevents tight packing → INCREASES fluidity. In membranes with UNSATURATED fatty acids (normally fluid) → cholesterol restricts movement → DECREASES fluidity. Net result: optimal membrane fluidity maintained across temperature range.\n\n📹 [VIDEO LINK]"},{"id":213,"s":"Cell Membrane","t":"Membrane Transport","u":"Glucose Transport — Classification","q":"A 45-year-old obese woman adds glucose to her tea. Glucose is absorbed from the gut lumen into intestinal cells against its concentration gradient. This absorption is classified as:","o":["Facilitated diffusion","Osmosis","Primary active transport","Secondary active transport","Simple diffusion"],"a":3,"p":"SGLT1 uses the sodium gradient (created by Na⁺/K⁺ ATPase using ATP) to drive glucose uptake against its concentration gradient — secondary active transport. Energy comes indirectly from ATP via the sodium gradient not directly from ATP hydrolysis by SGLT1. GLUT transporters on basolateral membrane transfer glucose to blood by facilitated diffusion (down concentration gradient, no energy).\n\n📹 [VIDEO LINK]"},{"id":214,"s":"Cell Membrane","t":"Membrane Transport","u":"Glucose Transport — SGLT1","q":"The transporter responsible for glucose absorption from the gut lumen into enterocytes is:","o":["GLUT1","GLUT2","GLUT4","GLUT5","SGLT1"],"a":4,"p":"SGLT1 on the apical membrane of intestinal enterocytes cotransports sodium and glucose into the cell (symport, secondary active transport). GLUT2 on the basolateral membrane transfers glucose from enterocyte to portal blood (facilitated diffusion). GLUT5 transports fructose only. GLUT1 = ubiquitous. GLUT4 = insulin-dependent (muscle and adipose). The gut absorption route: lumen → SGLT1 → enterocyte → GLUT2 → blood.\n\n📹 [VIDEO LINK]"},{"id":215,"s":"Cell Membrane","t":"Membrane Transport","u":"SGLT2 Inhibitors","q":"A 45-year-old obese woman with type 2 diabetes is started on an SGLT2 inhibitor. This drug blocks glucose reabsorption in the kidney. SGLT2 transports glucose into kidney tubular cells using which energy source?","o":["ATP directly","Calcium gradient","Potassium gradient","Proton gradient","Sodium gradient"],"a":4,"p":"SGLT2 (sodium-glucose cotransporter 2) on the proximal tubule apical membrane uses the sodium gradient as its energy source — secondary active transport. Sodium flows into the tubular cell down its gradient, dragging glucose with it. The sodium gradient is maintained by Na⁺/K⁺ ATPase on the basolateral membrane using ATP. SGLT2 inhibitors (empagliflozin, dapagliflozin) block this → glucose stays in tubule lumen → glucosuria → reduced blood glucose → cardioprotective and nephroprotective effects.\n\n📹 [VIDEO LINK]"},{"id":216,"s":"Cell Organelles","t":"Cell Organelles — Mitochondria","u":"Mitochondria — Oxidative Phosphorylation","q":"A 20-year-old presents with progressive visual deterioration due to retinal ganglion cell degeneration. Genetic testing shows a mutation in the mitochondrial gene encoding NADH dehydrogenase. Which mitochondrial function is primarily compromised?","o":["Beta oxidation","DNA replication","Oxidative phosphorylation","Protein synthesis","Urea synthesis"],"a":2,"p":"NADH dehydrogenase (Complex I) is the entry point for NADH into the electron transport chain — essential for oxidative phosphorylation and ATP synthesis. Mutation → reduced Complex I activity → reduced ATP production → retinal ganglion cells (high energy demand, end-artery blood supply) most vulnerable → apoptosis → optic nerve degeneration → visual loss (Leber Hereditary Optic Neuropathy).\n\n📹 [VIDEO LINK]"},{"id":217,"s":"Cell Organelles","t":"Cell Organelles — Mitochondria","u":"Mitochondria — Exclusive Functions","q":"A student lists the functions of mitochondria. Which of the following is a function that takes place exclusively within mitochondria?","o":["Fatty acid synthesis","Gluconeogenesis","Glycolysis","Oxidative phosphorylation","Protein synthesis"],"a":3,"p":"Oxidative phosphorylation (electron transport chain + ATP synthase) occurs EXCLUSIVELY on the inner mitochondrial membrane. Other mitochondrial functions: citric acid cycle (matrix), beta-oxidation (matrix), part of urea cycle (matrix), haem synthesis (matrix), programmed cell death (cytochrome c release). Glycolysis = cytosol. Fatty acid synthesis = cytosol and smooth endoplasmic reticulum. Gluconeogenesis = cytosol and mitochondria (shared).\n\n📹 [VIDEO LINK]"},{"id":218,"s":"Cell Organelles","t":"Cell Organelles — Mitochondria","u":"Mitochondria — Apoptosis","q":"Mitochondria initiate programmed cell death by releasing which molecule into the cytosol?","o":["Caspase-8","Cytochrome c","Cytochrome P450","NADH","Ubiquitin"],"a":1,"p":"Intrinsic apoptosis pathway: cellular stress → Bax/Bak (pro-apoptotic) → mitochondrial outer membrane permeabilisation → cytochrome c released into cytosol → forms apoptosome with Apaf-1 and procaspase-9 → activates caspase-9 → activates executioner caspases (3, 6, 7) → cell death. Bcl-2 (anti-apoptotic) prevents mitochondrial outer membrane permeabilisation. Cytochrome c normally functions in the electron transport chain between Complex III and Complex IV.\n\n📹 [VIDEO LINK]"},{"id":219,"s":"Cell Membrane","t":"Signal Transduction — Nitroglycerin","u":"Nitroglycerin — cGMP","q":"A 65-year-old man with angina is given sublingual nitroglycerin which relieves chest pain through vasodilation. Nitroglycerin is metabolised to nitric oxide in vascular smooth muscle cells. Which second messenger does nitric oxide produce to cause smooth muscle relaxation?","o":["Calcium","Cyclic adenosine monophosphate","Cyclic guanosine monophosphate","Diacylglycerol","Inositol trisphosphate"],"a":2,"p":"Nitroglycerin → nitric oxide → activates soluble guanylyl cyclase → GTP → cyclic guanosine monophosphate → activates protein kinase G → phosphorylates myosin light chain kinase → myosin light chain kinase inactivated → smooth muscle relaxation → vasodilation → reduced cardiac preload and afterload → reduced myocardial oxygen demand → angina relieved. Phosphodiesterase 5 inhibitors (sildenafil) also raise cyclic guanosine monophosphate by preventing its degradation — hence contraindicated with nitrates.\n\n📹 [VIDEO LINK]"},{"id":220,"s":"Cytoskeleton","t":"Hereditary Spherocytosis","u":"HS — Splenectomy","q":"A 42-year-old woman with hereditary spherocytosis underwent splenectomy. The primary reason for splenectomy in hereditary spherocytosis is to remove the site of:","o":["Antibody production","Complement activation","Erythrocyte destruction","Erythropoiesis","Iron storage"],"a":2,"p":"In hereditary spherocytosis, spectrin-deficient spherocytes cannot deform through splenic sinusoids (3 micrometres diameter) → trapped in splenic cords → destroyed by resident macrophages → extravascular haemolysis → anaemia and jaundice. Splenectomy removes this destruction site → spherocytes survive longer in circulation → haemolysis decreases dramatically → anaemia improves.\n\n📹 [VIDEO LINK]"},{"id":221,"s":"Cytoskeleton","t":"Hereditary Spherocytosis","u":"HS — Jaundice","q":"A 2.5-year-old girl with hereditary spherocytosis develops jaundice during a viral illness. The scleral and skin yellow discolouration is caused by accumulation of which pigment in tissues?","o":["Biliverdin","Conjugated bilirubin","Stercobilin","Unconjugated bilirubin","Urobilin"],"a":3,"p":"Increased haemolysis → haem released → unconjugated bilirubin formed → lipid-soluble → deposits in sclera and skin → jaundice. Why NOT urobilin: urobilin is produced in the GUT from urobilinogen (derived from bilirubin) and is responsible for the yellow colour of URINE — not scleral or skin jaundice. Urobilin is water-soluble and excreted in urine. It does not deposit in sclera. Unconjugated bilirubin is lipid-soluble — it deposits in tissues including sclera causing the yellow discolouration seen in jaundice.\n\n📹 [VIDEO LINK]"},{"id":222,"s":"Cytoskeleton","t":"Hereditary Spherocytosis","u":"HS — Gallstones","q":"A 5-year-old girl with hereditary spherocytosis develops gallstones. These gallstones are composed of which substance due to chronic haemolysis?","o":["Calcium oxalate","Calcium phosphate","Cholesterol","Bilirubin","Uric acid"],"a":3,"p":"Chronic haemolysis → persistently elevated unconjugated bilirubin → liver conjugates it → elevated conjugated bilirubin in bile → exceeds solubility → precipitates as bilirubin (pigment) gallstones. This is why cholecystectomy is often performed alongside splenectomy in hereditary spherocytosis patients. Compare: cholesterol gallstones are most common in the general population (female, fat, forty, fertile) but bilirubin stones are the hallmark of haemolytic conditions.\n\n📹 [VIDEO LINK]"},{"id":223,"s":"Cytoskeleton","t":"Hereditary Spherocytosis","u":"HS — Spectrin Scaffold","q":"In hereditary spherocytosis, the erythrocytes have mutated spectrin. Spectrin deficiency causes spherocytosis because spectrin normally:","o":["Carries haemoglobin","Forms the RBC cytoskeletal scaffold","Pumps ions across the membrane","Regulates RBC enzyme activity","Transports oxygen"],"a":1,"p":"Spectrin = major peripheral protein of RBC cytoskeleton forming a flexible hexagonal lattice on the cytosolic face. Spectrin dimers → tetramers → crosslinked at junctional complexes (actin and protein 4.1) → anchored to membrane via ankyrin-band 3 linkage → network maintains biconcave disc shape and membrane flexibility. Spectrin deficiency → loss of lattice → membrane fragility → sphere → trapped in spleen → haemolysis.\n\n📹 [VIDEO LINK]"},{"id":224,"s":"Cytoskeleton","t":"Cell Cycle","u":"Cell Cycle — Interphase","q":"A bone marrow stem cell is in interphase of the cell cycle. Which of the following may be observed in this cell?","o":["Breakdown of nucleolus","Disassembly of nuclear envelope","Migration of chromatids to poles","Spindle fibre formation","Synthesis of nuclear DNA"],"a":4,"p":"Interphase consists of G1 (growth, organelle duplication), S phase (DNA synthesis), and G2 (growth, preparation for mitosis). DNA synthesis occurs in S phase of interphase. During interphase: nuclear envelope intact, nucleolus visible, chromosomes decondensed. Breakdown of nucleolus, nuclear envelope disassembly, spindle formation, and chromatid migration all occur during MITOSIS.\n\n📹 [VIDEO LINK]"},{"id":225,"s":"Cytoskeleton","t":"Cell Cycle","u":"Cell Cycle — G1","q":"A hepatocyte actively participating in the cell cycle increases in size and duplicates its organelles during a particular phase. In which phase does this hepatocyte reside?","o":["Anaphase","G1 phase","G2 phase","Metaphase","S phase"],"a":1,"p":"G1 phase (first growth phase): cell increases in size, duplicates organelles (mitochondria, ribosomes, endoplasmic reticulum), synthesises proteins needed for DNA replication. Cell commits to division at the Restriction Point in late G1 — controlled by Rb protein and CDK4/6-cyclin D. If Restriction Point not passed → cell exits into G0 (quiescent). G2 phase = second growth phase AFTER DNA synthesis (S phase).\n\n📹 [VIDEO LINK]"},{"id":226,"s":"Cytoskeleton","t":"Cell Cycle","u":"Cell Cycle — Telophase","q":"A mitotic cell is in telophase. Which of the following may be observed in this cell?","o":["Alignment of chromosomes at equator","Chromatin decondensation","Chromosome condensation","Mitotic spindle formation","Sister chromatid separation"],"a":1,"p":"Telophase: nuclear envelopes reform around each set of chromosomes → chromosomes DECONDENSE (unwind) → nucleolus reappears → spindle fibres disassemble → two daughter nuclei formed. Cytokinesis begins (cleavage furrow in animal cells). Chromosome alignment = metaphase. Chromosome condensation = prophase. Sister chromatid separation = anaphase. Spindle formation = prometaphase.\n\n📹 [VIDEO LINK]"},{"id":227,"s":"Cytoskeleton","t":"Cell Cycle","u":"Cell Cycle — Anaphase","q":"An actively dividing lymphocyte shows separation of paired kinetochores and migration of sister chromatids to opposite poles. This cell currently resides in:","o":["Anaphase","G2 phase","Metaphase","Prophase","Telophase"],"a":0,"p":"Anaphase: cohesin proteins (holding sister chromatids together) cleaved by separase → sister chromatids pulled to opposite poles by shortening kinetochore microtubules → simultaneously polar microtubules push poles apart → cell elongates. Spindle assembly checkpoint must be satisfied at all kinetochores before anaphase begins. Anaphase A = chromatids move to poles. Anaphase B = poles move apart.\n\n📹 [VIDEO LINK]"},{"id":228,"s":"Cytoskeleton","t":"Cell Cycle","u":"Cell Cycle — Aurora A","q":"A cell that is stimulated to divide lacks Aurora A kinase. Aurora A is required for centrosome maturation and mitotic spindle assembly. Without Aurora A, this cell will be arrested in:","o":["Anaphase","G1 phase","G2 phase","Metaphase","S phase"],"a":3,"p":"Aurora A kinase is essential for centrosome maturation and bipolar spindle assembly. Without Aurora A → centrosomes cannot mature → bipolar spindle cannot form → spindle assembly checkpoint detects unattached kinetochores → arrests cell in METAPHASE (spindle assembly checkpoint prevents anaphase onset until all chromosomes are correctly attached to bipolar spindle). Aurora A overexpression is found in many cancers → chromosomal instability.\n\n📹 [VIDEO LINK]"},{"id":229,"s":"Cell Organelles","t":"Cell Organelles — Lysosomal Storage","u":"Niemann-Pick — Sphingomyelinase","q":"A 6-month-old infant with hepatosplenomegaly, cherry-red spot on fundoscopy, and foam cells on bone marrow aspiration. Niemann-Pick disease is suspected. Which enzyme is likely deficient?","o":["Alpha-galactosidase A","Glucocerebrosidase","Hexosaminidase A","Sphingomyelinase","Sulphatase"],"a":3,"p":"Niemann-Pick disease Type A and B: sphingomyelinase deficiency → sphingomyelin accumulates in lysosomes of macrophages → foam cells in liver, spleen, bone marrow → hepatosplenomegaly. Type A (severe): central nervous system involvement + cherry-red macula (ganglion cell death around fovea). Compare: Gaucher (glucocerebrosidase, no cherry-red spot). Tay-Sachs (hexosaminidase A, no hepatosplenomegaly, only neurological).\n\n📹 [VIDEO LINK]"},{"id":230,"s":"Cell Organelles","t":"Cell Organelles — Lysosomal Storage","u":"Tay-Sachs — Hexosaminidase A","q":"A 6-month-old infant of Ashkenazi Jewish descent presents with exaggerated startle response, progressive neurodegeneration, and a cherry-red spot on fundoscopy. Tay-Sachs disease is suspected. Which enzyme is likely deficient?","o":["Alpha-galactosidase A","Glucocerebrosidase","Hexosaminidase A","Sphingomyelinase","Sulphatase"],"a":2,"p":"Tay-Sachs disease: hexosaminidase A deficiency → GM2 ganglioside accumulates in lysosomes of neurons → neuronal death → progressive neurodegeneration. Cherry-red spot: ganglion cell death in retina (appears white due to lipid accumulation) except fovea (no ganglion cells) → fovea appears red by contrast. No hepatosplenomegaly (unlike Gaucher and Niemann-Pick). Fatal by age 2 to 4. Carrier screening in Ashkenazi Jewish populations.\n\n📹 [VIDEO LINK]"},{"id":231,"s":"Cell Organelles","t":"Cell Organelles — Lysosomal Storage","u":"Fabry — Alpha-galactosidase A","q":"A 25-year-old man presents with painful burning sensations in his hands and feet, angiokeratomas on his skin, and progressive renal failure. Fabry disease is suspected. His maternal uncle had similar symptoms. Which enzyme is likely deficient?","o":["Alpha-galactosidase A","Glucocerebrosidase","Hexosaminidase A","Sphingomyelinase","Sulphatase"],"a":0,"p":"Fabry disease: alpha-galactosidase A deficiency → globotriaosylceramide accumulates in lysosomes of vascular endothelial cells, kidney, heart, neurons → painful crises (acroparesthesias), angiokeratomas, corneal opacities, progressive renal failure, cardiomyopathy, stroke. X-linked → affects males primarily, females are carriers. Maternal uncle affected = consistent with X-linked inheritance. Treatment: agalsidase alfa or beta (enzyme replacement).\n\n📹 [VIDEO LINK]"},{"id":232,"s":"Cell Organelles","t":"Cell Organelles — Lysosomal Storage","u":"Pompe — Acid Glucosidase","q":"A 3-month-old infant presents with severe hypotonia, cardiomegaly, and macroglossia. Pompe disease is suspected. Muscle biopsy shows massive glycogen accumulation in lysosomes. Which enzyme is likely deficient?","o":["Debranching enzyme","Glucose-6-phosphatase","Glycogen phosphorylase","Lysosomal acid alpha-glucosidase","Phosphofructokinase"],"a":3,"p":"Pompe disease (Glycogen Storage Disease Type II): lysosomal acid alpha-glucosidase (acid maltase) deficiency → glycogen accumulates in LYSOSOMES (not cytosol — key distinction from other glycogen storage diseases) → lysosomal dysfunction → especially severe in heart and skeletal muscle → cardiomegaly, hypotonia, macroglossia. Infantile form fatal by 1 year without treatment. Treatment: alglucosidase alfa (Myozyme).\n\n📹 [VIDEO LINK]"},{"id":233,"s":"Extracellular Matrix","t":"Peroxisomal Disorders","u":"Refsum — Phytanic Acid","q":"A 20-year-old man presents with retinitis pigmentosa, peripheral neuropathy, and cerebellar ataxia. Plasma phytanic acid levels are markedly elevated. Phytanic acid is derived from phytol — the side chain of chlorophyll — and requires alpha-oxidation in which organelle?","o":["Endoplasmic reticulum","Golgi apparatus","Lysosome","Mitochondria","Peroxisome"],"a":4,"p":"Refsum disease: deficiency of phytanoyl-CoA hydroxylase (peroxisomal enzyme) → cannot perform alpha-oxidation of phytanic acid. Phytanic acid is derived from PHYTOL — the alcohol side chain of chlorophyll (not from chlorophyll itself — chlorophyll is a pigment like haem, it is phytol that gives rise to phytanic acid). Phytol released during chlorophyll digestion → absorbed → converted to phytanic acid → normally alpha-oxidised in peroxisomes. Accumulation → toxic to myelin → retinitis pigmentosa, cerebellar ataxia, peripheral neuropathy. Treatment: dietary restriction of phytanic acid.\n\n📹 [VIDEO LINK]"},{"id":234,"s":"Cell Membrane","t":"Membrane Transport — Endocytosis","u":"SNARE — Components","q":"A researcher studying vesicle fusion at the neuromuscular junction identifies the three proteins forming the SNARE complex. Which of the following is a component of the SNARE complex located on the target (plasma) membrane?","o":["Rab GTPase","SNAP-25","Synapsin","Synaptobrevin","Synaptotagmin"],"a":1,"p":"SNARE complex = three proteins: (1) SNAP-25 (target membrane). (2) Syntaxin (target membrane). (3) Synaptobrevin/VAMP (vesicle membrane). These wind around each other → pull vesicle and plasma membrane into close apposition → membrane fusion → neurotransmitter release. Botulinum toxin type A and E cleave SNAP-25. Type B, D, F, G cleave synaptobrevin. Type C cleaves syntaxin. Tetanus toxin also cleaves synaptobrevin.\n\n📹 [VIDEO LINK]"},{"id":235,"s":"Cell Organelles","t":"Cell Organelles — LHON","u":"LHON — Maternal Inheritance","q":"A woman has Leber Hereditary Optic Neuropathy. She has three children — two sons and one daughter. Which of her children are at risk of inheriting this condition?","o":["Both sons and daughters","Daughter only","Neither son nor daughter","Sons more than daughters","Sons only"],"a":0,"p":"Mitochondrial inheritance = MATERNAL. An affected MOTHER transmits to ALL children (sons AND daughters) because mitochondria are inherited from the egg not sperm. However: affected FATHER transmits to NONE (paternal mitochondria are destroyed after fertilisation). All children of an affected mother are at risk regardless of sex. This distinguishes mitochondrial inheritance from X-linked inheritance where only sons are affected by X-linked recessive diseases.\n\n📹 [VIDEO LINK]"},{"id":236,"s":"Cytoskeleton","t":"Cell Cycle","u":"Cell Cycle — G2/M Checkpoint","q":"A cell has successfully completed DNA replication in S phase. Before entering mitosis, it must pass the G2/M checkpoint. Which kinase complex is responsible for driving the cell from G2 into mitosis?","o":["CDK1 and cyclin A","CDK1 and cyclin B","CDK2 and cyclin D","CDK4 and cyclin D","CDK6 and cyclin E"],"a":1,"p":"G2/M checkpoint: CDK1 and cyclin B form mitosis-promoting factor → phosphorylates nuclear lamins (causes nuclear envelope breakdown), condensin (chromosome condensation), and other mitotic substrates → entry into mitosis. Cyclin B accumulates during G2 → CDK1 activated → mitosis begins. CDK4 and CDK6 with cyclin D = G1 phase (phosphorylates Rb protein). CDK2 and cyclin E = G1/S transition. Aurora A activates CDK1-cyclin B at centrosomes.\n\n📹 [VIDEO LINK]"},{"id":237,"s":"Cell Membrane","t":"Membrane Lipids","u":"Caveolin","q":"A researcher studying membrane microdomains identifies flask-shaped invaginations of the plasma membrane enriched in cholesterol and sphingolipids. Which protein stabilises these flask-shaped lipid rafts?","o":["Actin","Caveolin","Fibronectin","Integrin","Spectrin"],"a":1,"p":"Caveolae = flask-shaped specialised lipid rafts stabilised by caveolin proteins. Caveolin is an integral hairpin protein (not GPI-anchored) that inserts into the inner leaflet of the plasma membrane. Functions: endocytosis, signal transduction, cholesterol transport, mechanosensing. Distinct from flat lipid rafts which lack caveolin.\n\n📹 [VIDEO LINK]"},{"id":238,"s":"Cell Membrane","t":"Membrane Transport","u":"Glucose Hydrophilic Barrier","q":"A biochemistry student examines two cell types — one expressing GLUT1 on its surface and one with no transport proteins. When extracellular glucose is raised to 5 mM, only the GLUT1-expressing cell takes up glucose. The cell without transport proteins cannot take up glucose despite the favourable concentration gradient. The most likely reason is that glucose is:","o":["Charged","Hydrophilic","Large","Nonpolar","Volatile"],"a":1,"p":"Glucose is HYDROPHILIC (polar, uncharged) → cannot dissolve in the hydrophobic fatty acid core of the lipid bilayer → requires transport proteins. Substances crossing freely by simple diffusion: small nonpolar molecules (O₂, CO₂, steroid hormones, ethanol, glycerol). Glucose is not charged — charge is not the barrier; hydrophilicity is.\n\n📹 [VIDEO LINK]"},{"id":239,"s":"Cell Membrane","t":"Membrane Transport","u":"Chloride Shift Antiport","q":"During the chloride shift in a red blood cell passing through peripheral tissues, bicarbonate exits the cell while chloride enters. Which type of membrane transport is this?","o":["Antiport","Cotransport","Primary active transport","Simple diffusion","Symport"],"a":0,"p":"Antiport = two molecules move in OPPOSITE directions through the same transporter. Band 3 (AE1) exchanges HCO₃⁻ OUT for Cl⁻ IN (chloride shift) → increases CO₂ carrying capacity of blood. Other antiports: Na⁺/Ca²⁺ exchanger, Na⁺/H⁺ exchanger. Symport = same direction (SGLT1). Uniport = one molecule one direction (GLUT).\n\n📹 [VIDEO LINK]"},{"id":240,"s":"Cell Membrane","t":"Membrane Transport","u":"Symport Na/Cl","q":"A 35-year-old woman with hypertension is prescribed hydrochlorothiazide which blocks the Na⁺/Cl⁻ cotransporter in the kidney distal tubule. Na⁺/Cl⁻ cotransport is an example of which type of membrane transport?","o":["Antiport","Facilitated diffusion","Primary active transport","Simple diffusion","Symport"],"a":4,"p":"Symport = two molecules move in the SAME direction through the same transporter. Na⁺/Cl⁻ cotransporter: both Na⁺ and Cl⁻ move INTO the tubular cell together. Other symporters: SGLT1 (Na⁺ + glucose), Na⁺/K⁺/2Cl⁻ (furosemide target). All symporters use the Na⁺ gradient — secondary active transport.\n\n📹 [VIDEO LINK]"},{"id":241,"s":"Cell Membrane","t":"Membrane Transport","u":"Antiport Na/Ca","q":"A 60-year-old man with heart failure has elevated intracellular Na⁺ in his cardiac myocytes. Na⁺ exits the cell in exchange for Ca²⁺ entering via the Na⁺/Ca²⁺ exchanger. The Na⁺/Ca²⁺ exchanger is an example of which type of membrane transport?","o":["Antiport","Facilitated diffusion","Primary active transport","Simple diffusion","Symport"],"a":0,"p":"Na⁺/Ca²⁺ exchanger (NCX) = antiport: Na⁺ moves IN while Ca²⁺ moves OUT (3 Na⁺ in : 1 Ca²⁺ out). Uses Na⁺ gradient to extrude Ca²⁺ — secondary active transport. In heart failure with digoxin: ↑ intracellular Na⁺ → ↓ NCX activity → ↑ intracellular Ca²⁺ → positive inotropy.\n\n📹 [VIDEO LINK]"},{"id":242,"s":"Cell Membrane","t":"Signal Transduction","u":"Phosphodiesterase cAMP","q":"The enzyme phosphodiesterase inactivates which of the following second messengers?","o":["Calcium","Cyclic adenosine monophosphate","Diacylglycerol","Inositol trisphosphate","Nitric oxide"],"a":1,"p":"Phosphodiesterase (PDE) hydrolyses cAMP → 5'-AMP (inactive) → terminates the cAMP signal. Also hydrolyses cGMP. Caffeine inhibits PDE → cAMP accumulates → prolonged PKA activation → alertness. Sildenafil inhibits PDE5 (cGMP-specific) → ↑ cGMP → vasodilation.\n\n📹 [VIDEO LINK]"},{"id":243,"s":"Cell Membrane","t":"Signal Transduction","u":"Caffeine Phosphodiesterase","q":"A first-year medical student preparing for her biochemistry exam orders a cappuccino to stay alert. Which enzyme does caffeine inhibit to increase intracellular cyclic adenosine monophosphate and promote alertness?","o":["Adenylyl cyclase","Phosphodiesterase","Phospholipase A","Protein kinase A","Tyrosine kinase"],"a":1,"p":"Caffeine inhibits phosphodiesterase → cAMP cannot be degraded → cAMP accumulates → prolonged PKA activation → increased neuronal excitability → alertness. Phosphodiesterase inhibitors: caffeine, theophylline (asthma), sildenafil (erectile dysfunction), milrinone (heart failure).\n\n📹 [VIDEO LINK]"},{"id":244,"s":"Cell Membrane","t":"Signal Transduction","u":"PKA Activated by cAMP","q":"Which of the following enzymes is directly activated by cyclic adenosine monophosphate?","o":["Mitogen kinase","Phospholipase C","Protein kinase A","Protein kinase C","Tyrosine kinase"],"a":2,"p":"cAMP → binds regulatory subunits of Protein Kinase A (PKA) → releases catalytic subunits → active PKA → phosphorylates serine/threonine residues on target proteins. PKC is activated by DAG and Ca²⁺ (Gαq pathway). Tyrosine kinases activated by growth factors at receptor tyrosine kinases.\n\n📹 [VIDEO LINK]"},{"id":245,"s":"Cell Membrane","t":"Signal Transduction — Cholera and Pertussis","u":"Adenylyl Cyclase Cholera","q":"Which of the following enzymes is permanently active in a cholera patient causing rice-water diarrhoea?","o":["Adenylyl cyclase","Phosphodiesterase","Phospholipase C","Protein kinase C","Tyrosine kinase"],"a":0,"p":"Cholera toxin → ADP-ribosylates G protein alpha-s → Gαs locked active → adenylyl cyclase permanently ON → cAMP rises continuously → PKA → CFTR constitutively open → massive Cl⁻ secretion → water follows osmotically → rice-water diarrhoea. Adenylyl cyclase is the permanently active enzyme — PKA is activated secondarily.\n\n📹 [VIDEO LINK]"},{"id":246,"s":"Cell Membrane","t":"Signal Transduction — Cholera and Pertussis","u":"Adenylyl Cyclase Pertussis","q":"Which of the following enzymes is permanently active in a whooping cough patient causing excess mucus secretion?","o":["Adenylyl cyclase","Phosphodiesterase","Phospholipase C","Protein kinase C","Tyrosine kinase"],"a":0,"p":"Pertussis toxin → ADP-ribosylates Gαi → locked INACTIVE → cannot inhibit adenylyl cyclase → adenylyl cyclase runs unopposed → cAMP rises → PKA → excessive mucus secretion → characteristic whoop. Compare: cholera locks Gαs ACTIVE; pertussis locks Gαi INACTIVE. Both result in adenylyl cyclase permanently active.\n\n📹 [VIDEO LINK]"},{"id":247,"s":"Cell Membrane","t":"Signal Transduction","u":"IP3 Calcium Channels","q":"Inositol trisphosphate (IP3) acts as a ligand for which of the following channels in the endoplasmic reticulum membrane?","o":["Calcium channels","Chloride channels","Potassium channels","Sodium channels","Sodium-potassium channels"],"a":0,"p":"Gαq → PLC → PIP2 → DAG + IP3. IP3 diffuses to ER → binds IP3-gated Ca²⁺ channels on ER membrane → Ca²⁺ released into cytosol → activates calmodulin. DAG remains in membrane → activates PKC. The two second messengers work simultaneously.\n\n📹 [VIDEO LINK]"},{"id":248,"s":"Cell Membrane","t":"Signal Transduction","u":"Cyclic Nucleotide Gated Channels","q":"A 28-year-old man is involved in a road traffic accident at night and reports abnormal rod photoreceptor responses on electroretinography. The rod photoreceptor ion channels that are open in darkness and closed by light are gated by intracellular cyclic nucleotides. These channels are classified as:","o":["Cyclic nucleotide-gated channels","Ligand-gated channels","Mechanically-gated channels","Stretch-activated channels","Voltage-gated channels"],"a":0,"p":"Cyclic Nucleotide-Gated (CNG) channels in rod photoreceptors opened by cGMP in darkness → Na⁺ and Ca²⁺ enter → dark current → rod depolarised. Light → rhodopsin → transducin → cGMP-PDE → cGMP falls → CNG channels close → hyperpolarisation → visual signal. CNG channels respond to intracellular cyclic nucleotides — NOT extracellular ligands.\n\n📹 [VIDEO LINK]"},{"id":249,"s":"Cell Membrane","t":"Signal Transduction","u":"Nicotinic ACh Sodium","q":"A 25-year-old man with descending paralysis is diagnosed with botulism after eating home-canned food. If acetylcholine were released normally at the neuromuscular junction, it would bind a receptor and open channels allowing which ion to flow in and depolarise the muscle cell?","o":["Calcium","Chloride","Potassium","Sodium","Sodium and potassium"],"a":3,"p":"Nicotinic acetylcholine receptors at the neuromuscular junction are ligand-gated ion channels. Acetylcholine binding → channel opens → Na⁺ flows IN (primary current) → membrane depolarises → muscle action potential → contraction. Botulinum toxin prevents ACh release → no channel opening → flaccid paralysis.\n\n📹 [VIDEO LINK]"},{"id":250,"s":"Cell Membrane","t":"Signal Transduction","u":"Beta-Gamma Dimer Potassium","q":"A 68-year-old man with hypertension is prescribed a drug that activates M2 muscarinic receptors on his cardiac pacemaker cells causing bradycardia. Electrophysiology shows SA node hyperpolarisation. The beta-gamma dimer of the activated G protein directly gates which type of channel to cause this hyperpolarisation?","o":["Inward rectifier calcium channels","Inward rectifier chloride channels","Inward rectifier potassium channels","Inward rectifier sodium channels","Inward rectifier sodium-potassium channels"],"a":2,"p":"M2 muscarinic receptor → Gαi/βγ activated → beta-gamma dimer directly opens GIRK (G protein-coupled Inward Rectifier Potassium) channels → K⁺ flows OUT → membrane hyperpolarises → SA node slows → bradycardia. The beta-gamma dimer (not the Gα subunit) gates these channels directly.\n\n📹 [VIDEO LINK]"},{"id":251,"s":"Cell Membrane","t":"Signal Transduction","u":"Ras Monomeric G Protein","q":"A 55-year-old man with pancreatic cancer undergoes genetic analysis. A point mutation is found in a small monomeric G protein that cannot hydrolyse GTP, keeping it permanently active and driving uncontrolled cell proliferation. Which protein is mutated?","o":["G protein alpha-i","G protein alpha-q","G protein alpha-s","G protein alpha-t","G protein Ras"],"a":4,"p":"G protein Ras is a MONOMERIC G protein (single subunit ~21 kDa) — distinct from heterotrimeric G proteins (αβγ). Mutant Ras: GTPase impaired → permanently GTP-bound → permanently active → Raf → MEK → ERK → uncontrolled proliferation → cancer. Found in pancreatic cancer (~90%), colorectal (~40%), lung adenocarcinoma (~30%).\n\n📹 [VIDEO LINK]"},{"id":252,"s":"Cell Membrane","t":"Signal Transduction","u":"Ras MAPK Cascade","q":"A 55-year-old man with pancreatic cancer has a mutation in a G protein that permanently activates the MAP kinase cascade. Which G protein is mutated?","o":["G protein alpha-i","G protein alpha-q","G protein alpha-s","G protein alpha-t","G protein Ras"],"a":4,"p":"G protein Ras → activates Raf (MAPKKK) → MEK (MAPKK) → ERK (MAPK) → nucleus → transcription factors (c-Fos, c-Jun, c-Myc) → cell growth and proliferation. Mutant Ras permanently active → uncontrolled proliferation → cancer. Ras is NOT a heterotrimeric G protein.\n\n📹 [VIDEO LINK]"},{"id":253,"s":"Cell Membrane","t":"Signal Transduction","u":"G Proteins Lipid Anchored","q":"G proteins belong to which category of membrane proteins?","o":["GPI-anchored","Integral","Lipid-anchored","Peripheral","Transmembrane"],"a":2,"p":"G proteins are lipid-anchored membrane proteins — the Gα subunit is modified by myristoylation (Gαi, Gαo, Gαt) or palmitoylation (Gαs, Gαq) anchoring them to the cytosolic face of the plasma membrane. The Gγ subunit is prenylated (geranylgeranylated). They attach to the inner leaflet without any transmembrane domain.\n\n📹 [VIDEO LINK]"},{"id":254,"s":"Cell Membrane","t":"Signal Transduction","u":"Autocrine Signalling","q":"A cell releases a signalling molecule that acts on the same cell that secreted it. This type of signalling is called:","o":["Autocrine","Endocrine","Juxtacrine","Paracrine","Synaptic"],"a":0,"p":"Autocrine signalling: cell releases ligand → ligand binds receptors on the SAME cell → self-stimulation. Example: cancer cells secreting growth factors that stimulate their own proliferation — a hallmark of cancer autonomy. Compare: Paracrine = nearby cells. Endocrine = distant cells via blood.\n\n📹 [VIDEO LINK]"},{"id":255,"s":"Cell Membrane","t":"Signal Transduction","u":"Paracrine Signalling","q":"A cell releases a signalling molecule that diffuses locally and acts on neighbouring cells. This type of signalling is called:","o":["Autocrine","Endocrine","Juxtacrine","Paracrine","Synaptic"],"a":3,"p":"Paracrine signalling: cell releases ligand → diffuses short distances → acts on nearby cells. Examples: prostaglandins, growth factors, cytokines, histamine. Effective only over short distances because ligand is rapidly degraded. Compare: Autocrine = acts on self. Endocrine = travels through blood to distant organs.\n\n📹 [VIDEO LINK]"},{"id":256,"s":"Cell Membrane","t":"Signal Transduction","u":"Endocrine Signalling","q":"A hormone is released from a gland, travels through the bloodstream, and acts on a distant target organ. This type of signalling is called:","o":["Autocrine","Endocrine","Juxtacrine","Paracrine","Synaptic"],"a":1,"p":"Endocrine signalling: hormone secreted by endocrine gland → enters bloodstream → travels to distant target organ → binds specific receptors → response. Examples: insulin (pancreas → liver/muscle/adipose), cortisol (adrenal cortex → multiple organs). Slower onset than paracrine/synaptic but longer duration.\n\n📹 [VIDEO LINK]"},{"id":257,"s":"Cell Membrane","t":"Steroid Hormone Signalling","u":"Zinc Finger DNA Binding","q":"A molecular biologist studies the DNA-binding domain of the glucocorticoid receptor. The receptor binds to glucocorticoid response elements in DNA through a specific structural motif. Which of the following structural motifs forms the DNA-binding domain of steroid hormone nuclear receptors?","o":["Alpha helix","Beta barrel","Beta meander","Leucine zipper","Zinc finger"],"a":4,"p":"Steroid hormone nuclear receptors contain ZINC FINGER motifs in their DNA-binding domain. Two C4 zinc fingers — four cysteine residues coordinate one zinc ion each — recognise specific hormone response elements (HREs). Carboxyl terminus = ligand-binding domain. Amino terminus = transcriptional activation domain (AF-1).\n\n[Diagram: Nuclear receptor domain N→C: AF-1 → DBD (two zinc fingers each with 4 cysteine residues coordinating Zn²⁺) → Hinge → LBD. Enlarged zinc finger: C-C-C-C coordinating Zn²⁺ → finger loop inserts into DNA major groove → reads HRE sequence.]\n\n📹 [VIDEO LINK]"},{"id":258,"s":"Cell Membrane","t":"Steroid Hormone Signalling","u":"Type 1 Receptor Glucocorticoids","q":"A 62-year-old woman with severe rheumatoid arthritis is prescribed prednisolone. Prednisolone exerts its anti-inflammatory effects by binding to which type of receptor?","o":["Cytoplasmic kinase receptor","Ion-gated channel receptor","Membrane-bound GPCR","Receptor tyrosine kinase","Type 1 nuclear receptor"],"a":4,"p":"Glucocorticoids (prednisolone, cortisol) bind Type 1 nuclear receptors — cytoplasmic receptors that translocate to nucleus when ligand binds. Type 1 receptors: glucocorticoid (GR), mineralocorticoid (MR), androgen (AR), oestrogen (ER), progesterone (PR). Anti-inflammatory: inhibit NF-κB → ↓ cytokine genes.\n\n📹 [VIDEO LINK]"},{"id":259,"s":"Cell Membrane","t":"Steroid Hormone Signalling","u":"Type 2 Receptor Thyroid Hormone","q":"A 45-year-old woman with hypothyroidism is prescribed levothyroxine. Thyroid hormone exerts its metabolic effects by binding to which type of receptor?","o":["Cytoplasmic kinase receptor","Ion-gated channel receptor","Membrane-bound GPCR","Receptor tyrosine kinase","Type 2 nuclear receptor"],"a":4,"p":"Thyroid hormones (T3, T4) bind Type 2 nuclear receptors — thyroid hormone receptors (TR) already in the nucleus (bound to DNA as repressors when unliganded). Type 2 receptors: TR, retinoic acid receptor (RAR), retinoid X receptor (RXR), vitamin D receptor (VDR). They form heterodimers with RXR.\n\n📹 [VIDEO LINK]"},{"id":260,"s":"Cell Membrane","t":"Steroid Hormone Signalling","u":"Type 2 Receptor Retinoic Acid","q":"A 20-year-old woman with acne vulgaris is prescribed retinoic acid cream. Retinoic acid exerts its effects on the epithelium by binding to which type of receptor?","o":["Cytoplasmic kinase receptor","Ion-gated channel receptor","Membrane-bound GPCR","Receptor tyrosine kinase","Type 2 nuclear receptor"],"a":4,"p":"Retinoic acid (vitamin A derivative) binds Type 2 nuclear receptors — Retinoic Acid Receptors (RAR) and Retinoid X Receptors (RXR). RAR/RXR heterodimers bind retinoic acid response elements (RAREs) → regulate genes controlling cell differentiation, proliferation, apoptosis. In skin: promotes keratinocyte differentiation → reduces sebum → treats acne. Also teratogenic in pregnancy.\n\n📹 [VIDEO LINK]"},{"id":261,"s":"Cell Membrane","t":"Steroid Hormone Signalling","u":"Type 1 Receptor Tamoxifen","q":"A post-menopausal woman with oestrogen receptor-positive breast cancer is prescribed tamoxifen. Tamoxifen inhibits which type of receptor?","o":["Ion-gated channel receptor","Membrane-bound GPCR","Receptor tyrosine kinase","Type 1 nuclear receptor","Type 2 nuclear receptor"],"a":3,"p":"Oestrogen receptor (ER) = Type 1 nuclear receptor. Tamoxifen competitively binds ER → blocks oestrogen binding → conformational change → co-activators cannot bind → oestrogen-responsive genes not transcribed → no cell proliferation → cytostatic effect. ER-positive tumours (70% of breast cancers) respond to tamoxifen and aromatase inhibitors.\n\n📹 [VIDEO LINK]"},{"id":262,"s":"Cell Organelles","t":"Cell Organelles — Mitochondria","u":"Marker Inner Mitochondrial Membrane","q":"A cell biology laboratory performs subcellular fractionation of a liver homogenate. The fraction sedimenting at 15000g centrifugation has a tightly folded inner membrane visible on electron microscopy. Which marker enzyme identifies this fraction?","o":["Acid phosphatase","Catalase","Cytochrome c oxidase","Galactosyl transferase","Lactate dehydrogenase"],"a":2,"p":"Cytochrome c oxidase (Complex IV) = marker enzyme for the INNER MITOCHONDRIAL MEMBRANE. Marker enzymes per fraction: 600g pellet = nuclei. 15000g = mitochondria (cytochrome c oxidase, succinate dehydrogenase). 100000g = microsomes/ER (glucose-6-phosphatase). Supernatant = cytosol (LDH).\n\n📹 [VIDEO LINK]"},{"id":263,"s":"Cell Organelles","t":"Cell Organelles — Mitochondria","u":"Marker Succinate Dehydrogenase","q":"A patient with mitochondrial myopathy has reduced activity of an enzyme embedded in the inner mitochondrial membrane that directly links the TCA cycle to the electron transport chain. Which enzyme is deficient?","o":["Fumarase","Isocitrate dehydrogenase","Malate dehydrogenase","Succinate dehydrogenase","Succinyl-CoA synthetase"],"a":3,"p":"Succinate dehydrogenase (Complex II) is the only TCA cycle enzyme embedded in the INNER MITOCHONDRIAL MEMBRANE. Directly links TCA cycle to ETC — feeds electrons from succinate to ubiquinone (CoQ). Mutations cause hereditary paraganglioma/phaeochromocytoma and some renal cell carcinomas.\n\n📹 [VIDEO LINK]"},{"id":264,"s":"Cell Organelles","t":"Cell Organelles — Mitochondria","u":"Marker Mitochondrial Matrix","q":"A 25-year-old woman with fumarate hydratase deficiency develops uterine fibroids and renal cell carcinoma. Accumulating fumarate causes pseudohypoxia. In which cellular compartment does fumarate hydratase normally function?","o":["Cytosol","Golgi lumen","Lysosomal lumen","Mitochondrial matrix","Peroxisomal matrix"],"a":3,"p":"Fumarase (fumarate hydratase) = TCA cycle enzyme in the MITOCHONDRIAL MATRIX. Catalyses fumarate → malate. All TCA cycle enzymes are in the matrix EXCEPT succinate dehydrogenase (inner mitochondrial membrane). Fumarate hydratase mutations cause HLRCC — fumarate accumulates → pseudohypoxia → tumour growth.\n\n📹 [VIDEO LINK]"},{"id":265,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Marker Golgi","q":"A newborn with I-cell disease has elevated plasma levels of all lysosomal enzymes. The enzyme responsible for tagging lysosomal enzymes with mannose-6-phosphate is located in which organelle?","o":["Endoplasmic reticulum","Golgi apparatus","Lysosome","Mitochondria","Nucleus"],"a":1,"p":"Galactosyl transferase is the marker enzyme for the Golgi apparatus. GlcNAc-1-phosphotransferase (which tags lysosomal enzymes with M6P) is also a Golgi enzyme. The Golgi performs: glycosylation, sulfation, M6P tagging, proteolytic processing, and sorting. Without M6P tagging → all lysosomal enzymes secreted extracellularly → inclusion bodies form.\n\n📹 [VIDEO LINK]"},{"id":266,"s":"Cell Organelles","t":"Cell Organelles — Lysosomal Storage","u":"Marker Lysosome","q":"A 35-year-old man with Tay-Sachs disease has progressive neurodegeneration. His lysosomes cannot break down sphingolipids because specific hydrolytic enzymes are absent. Elevated activity of which enzyme in his serum would best confirm lysosomal membrane leakage?","o":["Acid phosphatase","Catalase","Fumarase","Galactosyl transferase","Lactate dehydrogenase"],"a":0,"p":"Acid phosphatase = marker enzyme for LYSOSOMES. Functions at acidic pH (~4.5). Lysosomal membrane leakage → acid phosphatase released into serum → elevated serum levels. Tartrate-resistant acid phosphatase (TRAP) = marker for osteoclasts and hairy cell leukaemia.\n\n📹 [VIDEO LINK]"},{"id":267,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Marker Endoplasmic Reticulum","q":"A 2-year-old boy with Von Gierke disease has deficiency of glucose-6-phosphatase causing severe fasting hypoglycaemia despite abundant hepatic glycogen. Glucose-6-phosphatase is the marker enzyme for which organelle?","o":["Cytosol","Golgi apparatus","Lysosome","Mitochondria","Smooth endoplasmic reticulum"],"a":4,"p":"Glucose-6-phosphatase = marker enzyme for the SMOOTH ENDOPLASMIC RETICULUM. Catalyses glucose-6-phosphate → free glucose + Pi — the final step releasing glucose into blood during gluconeogenesis and glycogenolysis. Von Gierke disease (GSD Type Ia) = glucose-6-phosphatase deficiency → hypoglycaemia + glycogen accumulation → enlarged liver and kidneys.\n\n📹 [VIDEO LINK]"},{"id":268,"s":"Cell Organelles","t":"Cell Organelles — Mitochondria","u":"Marker Cytosol","q":"A 70-year-old man with alcoholic liver cirrhosis has extensive hepatocyte necrosis on biopsy. His hepatocytes are rupturing and releasing cytoplasmic contents into the bloodstream. Which cytosolic marker enzyme would be most elevated in his serum?","o":["Acid phosphatase","Catalase","Cytochrome c oxidase","Galactosyl transferase","Lactate dehydrogenase"],"a":4,"p":"Lactate dehydrogenase (LDH) = marker enzyme for the CYTOSOL. Hepatocyte necrosis → cytoplasmic contents including LDH leak into bloodstream → elevated serum LDH. LDH-5 (liver + skeletal muscle subtype) specifically elevated in liver disease. LDH-1 predominates in MI and haemolysis.\n\n📹 [VIDEO LINK]"},{"id":269,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Marker Peroxisome","q":"A 6-week-old girl with seizures, hypotonia, and elevated plasma very-long-chain fatty acids undergoes liver biopsy. Subcellular fractionation shows that catalase is dispersed throughout the cytosol rather than concentrated in its usual compartment. Which organelle is absent or dysfunctional in this girl?","o":["Endoplasmic reticulum","Golgi apparatus","Lysosome","Mitochondria","Peroxisome"],"a":4,"p":"Catalase = marker enzyme for PEROXISOMES. In Zellweger syndrome (PEX gene mutations): peroxisomes fail to form → catalase synthesised on free ribosomes but cannot be imported → mislocalised to cytosol. This cytosolic displacement of normally peroxisomal catalase is diagnostic of peroxisome biogenesis disorders.\n\n📹 [VIDEO LINK]"},{"id":270,"s":"Cell Organelles","t":"Cell Organelles — Mitochondria","u":"Fatty Acid Synthesis Cytosol","q":"A medical student eats a laccha paratha for breakfast. Excess starch is digested to glucose, absorbed, and the resulting surplus is diverted towards de novo fatty acid synthesis in the liver. This synthesis occurs in which cellular compartment?","o":["Cytosol","Golgi apparatus","Lysosome","Mitochondrial matrix","Peroxisome"],"a":0,"p":"Fatty acid synthase (FAS) complex operates EXCLUSIVELY IN THE CYTOSOL. Excess dietary carbohydrate → glucose → pyruvate → acetyl-CoA (mitochondria) → exported as citrate via citrate shuttle → cleaved back to acetyl-CoA in cytosol → fatty acid synthesis begins. Builds fatty acids two carbons at a time up to palmitate (C16). Spatial separation of synthesis (cytosol) and degradation (mitochondria) prevents futile cycling.\n\n📹 [VIDEO LINK]"},{"id":271,"s":"Cell Organelles","t":"Cell Organelles — Mitochondria","u":"Urea Cycle Both Compartments","q":"A 3-day-old male neonate presents with poor feeding, lethargy, and seizures. Blood ammonia is markedly elevated. A birth defect in a metabolic pathway requiring enzymes in both the mitochondria and the cytosol is suspected. Which pathway is defective?","o":["Beta oxidation","Fatty acid synthesis","Glycolysis","Ketone body synthesis","Urea cycle"],"a":4,"p":"Urea cycle: MITOCHONDRIA — CPS-I (carbamoyl phosphate) → OTC (citrulline formed — most common urea cycle defect, X-linked). CYTOSOL — argininosuccinate synthetase → argininosuccinate lyase → arginase (urea released, ornithine regenerated). Neonatal hyperammonaemia within first days of life = classic presentation of severe urea cycle defects. Note: elevated orotic acid in OTC deficiency is a SECONDARY consequence — accumulated carbamoyl phosphate overflows into cytosol → pyrimidine synthesis → orotic acid (a completely separate pathway).\n\n📹 [VIDEO LINK]"},{"id":272,"s":"Cell Organelles","t":"Cell Organelles — Mitochondria","u":"Steroid Hormone Mitochondria SER","q":"Which of the following processes occurs partly in both the mitochondria and the smooth endoplasmic reticulum?","o":["Beta oxidation","Cholesterol synthesis","Glycolysis","Ketone body synthesis","Steroid hormone synthesis"],"a":4,"p":"Steroid hormone synthesis: MITOCHONDRIA — cholesterol → pregnenolone via CYP11A1 (rate-limiting step). SMOOTH ER — subsequent conversion steps for most steroid hormones. Cytochrome P450 enzymes in both mitochondria and SER drive steroid biosynthesis. Compare: Cholesterol synthesis = cytosol + SER (HMG-CoA reductase in SER membrane).\n\n📹 [VIDEO LINK]"},{"id":273,"s":"Cell Organelles","t":"Cell Organelles — Mitochondria","u":"Desaturation Smooth ER","q":"Desaturation of fatty acids occurs in which cellular organelle?","o":["Cytosol","Golgi apparatus","Mitochondria","Peroxisome","Smooth endoplasmic reticulum"],"a":4,"p":"Fatty acid desaturation (introduction of double bonds) performed by desaturases on the SMOOTH ENDOPLASMIC RETICULUM membrane using cytochrome b5 and NADH. Humans cannot introduce double bonds beyond C9 → cannot synthesise omega-3 and omega-6 fatty acids → these are ESSENTIAL (linoleic acid omega-6, alpha-linolenic acid omega-3).\n\n📹 [VIDEO LINK]"},{"id":274,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"VLCFA Beta-oxidation Peroxisome","q":"Beta-oxidation of very-long-chain fatty acids (greater than 22 carbons) occurs in which cellular organelle?","o":["Cytosol","Golgi apparatus","Mitochondria","Peroxisome","Smooth endoplasmic reticulum"],"a":3,"p":"Very-long-chain fatty acids (VLCFA, >C22) are beta-oxidised EXCLUSIVELY in peroxisomes. Medium and long-chain fatty acids (C4-C22) are beta-oxidised in MITOCHONDRIA via the carnitine shuttle. Peroxisomal beta-oxidation generates H₂O₂ (detoxified by catalase) and shortened fatty acids transferred to mitochondria. VLCFA accumulation → X-linked ALD, Zellweger syndrome.\n\n📹 [VIDEO LINK]"},{"id":275,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"H2O2 Peroxisome","q":"Production and degradation of hydrogen peroxide occurs in which organelle?","o":["Golgi apparatus","Lysosome","Mitochondria","Nucleus","Peroxisome"],"a":4,"p":"Peroxisomes both PRODUCE and DEGRADE H₂O₂: Oxidases produce H₂O₂ as byproduct. Catalase immediately degrades H₂O₂ → H₂O + O₂. This self-contained system protects the rest of the cell from oxidative damage. Peroxisomal H₂O₂ also used to oxidise toxic molecules (ethanol → acetaldehyde).\n\n📹 [VIDEO LINK]"},{"id":276,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Sulfation Golgi","q":"Sulfation reactions during glycoprotein processing occur in which organelle?","o":["Endoplasmic reticulum","Golgi apparatus","Lysosome","Mitochondria","Peroxisome"],"a":1,"p":"Sulfation of glycoproteins and proteoglycans occurs in the GOLGI APPARATUS using PAPS (3'-phosphoadenosine-5'-phosphosulfate) as the sulfate donor. Sulfotransferases add sulfate groups to tyrosine residues in proteins and to sugars in glycosaminoglycans (chondroitin sulfate, heparan sulfate, dermatan sulfate, keratan sulfate).\n\n📹 [VIDEO LINK]"},{"id":277,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Plasmalogen Peroxisome","q":"Plasmalogen synthesis begins in which organelle?","o":["Cytosol","Golgi apparatus","Lysosome","Mitochondria","Peroxisome"],"a":4,"p":"Plasmalogens are ether-linked phospholipids abundant in myelin, cardiac muscle, and brain. Synthesis BEGINS in peroxisomes (initial ether bond formation using dihydroxyacetone phosphate acyltransferase and alkyl-DHAP synthase — both peroxisomal enzymes) then continues in the ER. Peroxisomal disorders (Zellweger) cause severe neurological defects partly because myelin plasmalogens cannot be synthesised.\n\n📹 [VIDEO LINK]"},{"id":278,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Glycosylation Golgi","q":"A construction worker injured in an accident is prescribed a gel to enhance collagen synthesis in his wounds. Which organelle glycosylates newly formed collagen in his wound fibroblasts?","o":["Cytosol","Endoplasmic reticulum","Golgi apparatus","Mitochondria","Nucleus"],"a":2,"p":"Glycosylation: N-linked glycosylation BEGINS in the ER (dolichol-phosphate transfers pre-assembled oligosaccharide to asparagine residues). Further processing, trimming, and completion occurs in the GOLGI. O-linked glycosylation also occurs in the Golgi. Final glycoprotein = fully processed in Golgi → sorted and packaged into secretory vesicles.\n\n📹 [VIDEO LINK]"},{"id":279,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Diphtheria EF-2","q":"A 5-year-old child presents with sore throat and a thick grayish membrane over the tonsils. Diphtheria is suspected. The diphtheria toxin inhibits protein synthesis by ADP-ribosylating which component of the translational machinery?","o":["30S ribosomal subunit","50S ribosomal subunit","Elongation factor 2","Initiation factor 2","Ribosomal RNA"],"a":2,"p":"Diphtheria toxin ADP-ribosylates Elongation Factor 2 (EF-2) in EUKARYOTES → EF-2 cannot perform translocation → protein synthesis halted → cell death. EF-2 contains a unique modified histidine called diphthamide (only in EF-2 of eukaryotes and archaea) that is the ADP-ribosylation target. Pseudomonas exotoxin A uses the same mechanism.\n\n📹 [VIDEO LINK]"},{"id":280,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Diphtheria Translocation","q":"Diphtheria toxin arrests protein synthesis by ADP-ribosylating EF-2. EF-2 is specifically required for which single step in the elongation cycle of translation?","o":["Aminoacyl-tRNA binding to A site","Peptide bond formation","Ribosome release at stop codon","Translocation of ribosome along mRNA","tRNA charging by aminoacyl-tRNA synthetase"],"a":3,"p":"EF-2 catalyses TRANSLOCATION — movement of the ribosome along mRNA by exactly one codon AFTER peptide bond formation. The growing peptide moves from A site to P site simultaneously. Diphtheria toxin → ADP-ribosylates diphthamide residue of EF-2 → EF-2 cannot catalyse translocation → ribosome stalls → protein synthesis arrested specifically at the translocation step.\n\n📹 [VIDEO LINK]"},{"id":281,"s":"Cell Membrane","t":"Membrane Transport — Endocytosis","u":"Botulism Membrane Fusion","q":"A young woman develops botulism after eating tinned biryani. Which of the following cellular processes is affected by botulinum toxin?","o":["Endosomal fusion","Lysosomal fusion","Membrane fusion","Nuclear fusion","Phagosome fusion"],"a":2,"p":"Botulinum toxin cleaves SNARE proteins (SNAP-25, synaptobrevin, syntaxin) → SNARE complex cannot assemble → synaptic vesicle MEMBRANE FUSION with the presynaptic plasma membrane is blocked → acetylcholine cannot be released. Membrane fusion is the fundamental cellular process blocked. Endosomal, lysosomal, and phagosomal fusion are different SNARE-mediated events not primarily affected.\n\n📹 [VIDEO LINK]"},{"id":282,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Drug Metabolism Smooth ER","q":"A young boy presents with sore throat and fever. He is prescribed antipyretics and antibiotics. These drugs are metabolised in which organelle?","o":["Golgi apparatus","Lysosome","Mitochondria","Nucleus","Smooth endoplasmic reticulum"],"a":4,"p":"Drug metabolism occurs primarily in the SMOOTH ENDOPLASMIC RETICULUM of hepatocytes via Cytochrome P450 (CYP) enzymes — mixed-function oxidases using NADPH and O₂. Phase I reactions (oxidation, reduction, hydrolysis): mainly CYP enzymes in SER. Phase II reactions (conjugation): cytosol and SER.\n\n📹 [VIDEO LINK]"},{"id":283,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Gemfibrozil PPARalpha","q":"A 55-year-old man with hypertriglyceridaemia is prescribed gemfibrozil. At the molecular level, gemfibrozil enters the cytoplasm and binds which receptor to control lipid metabolism?","o":["Glucocorticoid receptor","Liver X receptor","Oestrogen receptor","Peroxisome proliferator-activated receptor","Thyroid hormone receptor"],"a":3,"p":"Gemfibrozil (fibrate class) binds PPARα (Peroxisome Proliferator-Activated Receptor alpha) — a Type 2 nuclear receptor. PPARα + RXR heterodimer → binds PPRE in DNA → induces gene expression for lipoprotein lipase (↑ triglyceride clearance), apolipoprotein AI and AII (↑ HDL), peroxisome proliferation (↑ fatty acid oxidation).\n\n📹 [VIDEO LINK]"},{"id":284,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Gemfibrozil Peroxisome Proliferation","q":"A man with hypertriglyceridaemia is treated with gemfibrozil. Gene expression for proliferation of which organelle is induced by this drug?","o":["Golgi","Lysosome","Mitochondria","Nucleus","Peroxisome"],"a":4,"p":"Gemfibrozil → PPARα activation → induces genes for PEROXISOME proliferation → more peroxisomes → more beta-oxidation of fatty acids → reduces plasma triglycerides. Also induces lipoprotein lipase → cleaves triglycerides from VLDL → further triglyceride reduction.\n\n📹 [VIDEO LINK]"},{"id":285,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Gemfibrozil Lipoprotein Lipase","q":"A man with hypertriglyceridaemia is prescribed gemfibrozil. Which enzyme's gene expression is induced by gemfibrozil to reduce circulating triglyceride levels?","o":["Gastric lipase","Hepatic lipase","Hormone-sensitive lipase","Lipoprotein lipase","Pancreatic lipase"],"a":3,"p":"PPARα activation by gemfibrozil → induces lipoprotein lipase (LPL) gene expression → LPL hydrolyses triglycerides in VLDL and chylomicrons → fatty acids released → taken up by peripheral tissues → plasma triglycerides fall dramatically. LPL is anchored to endothelial cells by heparan sulfate proteoglycans.\n\n📹 [VIDEO LINK]"},{"id":286,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Proteasome Inhibitors Cancer","q":"A 60-year-old man with multiple myeloma is started on bortezomib. This drug binds the catalytic site of the 26S proteasome — a large cytosolic protein complex — blocking its function. Which class of proteins accumulates in myeloma cells as a direct result of this blockade, triggering cell death?","o":["Catalytic proteins","Proapoptotic proteins","Signalling proteins","Structural proteins","Transport proteins"],"a":1,"p":"Bortezomib blocks proteasome → proteins normally tagged for rapid degradation accumulate. Key: IκB accumulates → NF-κB blocked → cancer cell cannot activate survival response → apoptosis. p53 accumulates → triggers apoptosis. Most effective in multiple myeloma and mantle cell lymphoma. The proteasome (26S = 20S catalytic core + 19S regulatory caps) is a cytosolic protein complex — NOT a membrane-bound organelle.\n\n📹 [VIDEO LINK]"},{"id":287,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Proteasome Short Half-life Proteins","q":"Which of the following proteins are primarily targeted for degradation by the proteasome system?","o":["Extracellular matrix proteins","Long half-life structural proteins","Membrane-bound receptors","Mitochondrial membrane proteins","Short half-life regulatory proteins"],"a":4,"p":"The ubiquitin-proteasome system (UPS) primarily degrades: short-lived regulatory proteins (transcription factors, cyclins, p53, IκB), misfolded proteins, and ubiquitin-tagged proteins. Long-lived structural proteins (collagen, cytoskeletal proteins) are degraded by different mechanisms (lysosomes, calpains). PEST sequences signal rapid ubiquitination and proteasomal degradation.\n\n📹 [VIDEO LINK]"},{"id":288,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Protein Trafficking SRP","q":"Which of the following facilitates ribosome attachment to the rough endoplasmic reticulum?","o":["Clathrin","Dolichol","Mannose-6-phosphate receptor","Signal recognition particle","SNARE complex"],"a":3,"p":"Signal Recognition Particle (SRP) recognises the N-terminal signal sequence on newly synthesised proteins → SRP binds signal sequence → translation pauses → SRP-ribosome complex docks with SRP receptor on ER membrane → ribosome anchors to ER → translation resumes into ER lumen (co-translational translocation). SRP is a ribonucleoprotein complex (6 proteins + 7SL RNA).\n\n📹 [VIDEO LINK]"},{"id":289,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Protein Trafficking Bound Ribosomes","q":"Bound ribosomes on the rough ER synthesise proteins destined for which of the following locations?","o":["Cytosol","Mitochondria","Nucleus","Peroxisomes","Plasma membrane"],"a":4,"p":"Bound ribosomes synthesise proteins destined for: plasma membrane, lysosomes, extracellular space (secretory proteins), Golgi apparatus, ER itself. FREE ribosomes synthesise proteins destined for: cytosol, nucleus, mitochondria, peroxisomes. The key distinction: proteins entering the secretory/endomembrane system → bound ribosomes.\n\n📹 [VIDEO LINK]"},{"id":290,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Protein Trafficking Free Ribosomes","q":"Free ribosomes synthesise proteins destined for which of the following locations?","o":["Extracellular matrix","Lysosome","Mitochondria","Plasma membrane","Secretory vesicles"],"a":2,"p":"Mitochondrial proteins (except the 13 encoded by mtDNA) are synthesised on FREE CYTOSOLIC RIBOSOMES → imported post-translationally via mitochondrial import sequences recognised by TOM/TIM translocase complexes. Peroxisomal proteins also synthesised on free ribosomes → imported via PTS1 (SKL at C-terminus) or PTS2 (N-terminal).\n\n📹 [VIDEO LINK]"},{"id":291,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Protein Trafficking Default Pathway","q":"When no specific targeting signal is incorporated, the default pathway for proteins synthesised on bound ribosomes is secretion into which destination?","o":["Cytosol","Extracellular space","Lysosome","Mitochondria","Nucleus"],"a":1,"p":"The DEFAULT secretory pathway: ER → Golgi → secretory vesicles → EXTRACELLULAR SPACE. Without specific signals: KDEL sequence → kept in ER. M6P tag → diverted to lysosomes. Transmembrane domain → retained in membrane. Without ANY signal → protein is secreted by default.\n\n📹 [VIDEO LINK]"},{"id":292,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"Protein Trafficking N-Terminal Signal","q":"A newly synthesised protein contains an N-terminal hydrophobic signal sequence. This signal sequence causes which of the following events?","o":["Import into mitochondria","Import into peroxisomes","Nuclear localisation","Ribosome attachment to ER","Ubiquitin tagging"],"a":3,"p":"N-terminal hydrophobic signal sequence → recognised by SRP → SRP pauses translation → docks with SRP receptor on ER membrane → ribosome attaches to ER → translation resumes co-translationally into ER lumen → signal peptide cleaved by signal peptidase. Compare: N-terminal positively-charged presequence → mitochondrial import. C-terminal SKL → peroxisomal import.\n\n📹 [VIDEO LINK]"},{"id":293,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"N-End Rule Stable Residues","q":"Proteins containing N-terminal sequences with Met, Ser, Ala, Thr, Val, or Gly (small stable residues) have which of the following half-lives?","o":["Greater than 20 days","Greater than 20 hours","Greater than 20 minutes","Greater than 20 seconds","Less than 20 seconds"],"a":1,"p":"The N-end rule: stabilising residues (Met, Ser, Ala, Thr, Val, Gly) → half-life >20 hours → long-lived proteins. Destabilising residues (Phe, Leu, Asp, Lys, Arg) → half-life ~2-3 minutes → rapidly degraded. Basic and large hydrophobic N-terminal residues recognised by N-recognin E3 ubiquitin ligases → ubiquitination → proteasomal degradation.\n\n📹 [VIDEO LINK]"},{"id":294,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"N-End Rule Destabilising Residues","q":"Proteins containing N-terminal sequences with Phe, Leu, Asp, Lys, or Arg (destabilising residues) have which of the following half-lives?","o":["Greater than 20 hours","Greater than 20 minutes","Greater than 20 seconds","Less than 3 minutes","Less than 20 seconds"],"a":3,"p":"Destabilising N-terminal residues → recognised by N-recognin E3 ligases → ubiquitinated → targeted to 26S proteasome → degraded within minutes. Important for: cell cycle regulators (cyclins destroyed after their target phase), transcription factors (rapid turnover allows rapid gene expression changes), and misfolded proteins (quality control).\n\n📹 [VIDEO LINK]"},{"id":295,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"PEST Sequences","q":"Proteins containing PEST sequences (rich in Pro, Glu, Ser, Thr) are:","o":["Directed to lysosomes","Exported extracellularly","Rapidly degraded","Slowly degraded","Stored in peroxisomes"],"a":2,"p":"PEST sequences signal rapid proteasomal degradation. Ser and Thr within PEST sequences are phosphorylated → signal for ubiquitination → proteasomal degradation. Proteins with PEST sequences: IκB, cyclin D, c-Fos, c-Jun, c-Myc. Their rapid turnover allows quick response to changing cellular signals.\n\n📹 [VIDEO LINK]"},{"id":296,"s":"Cell Organelles","t":"Cell Organelles — Clinical Correlations","u":"KFERQ Chaperone-Mediated Autophagy","q":"Proteins containing the sequence Lys-Phe-Glu-Arg-Gln (KFERQ motif) are degraded by which organelle?","o":["Endosome","Lysosome","Peroxisome","Proteasome","Ribosome"],"a":1,"p":"KFERQ motif → recognised by Hsc70 chaperone → binds LAMP-2A receptor on LYSOSOMAL membrane → protein translocated into lysosomal lumen → degraded by acid hydrolases. This is Chaperone-Mediated Autophagy (CMA). Distinct from macroautophagy (bulk degradation) and the ubiquitin-proteasome system. CMA activated by prolonged starvation and oxidative stress.\n\n📹 [VIDEO LINK]"},{"id":297,"s":"Cell Organelles","t":"Cell Organelles — Mitochondria","u":"Parkinson Alpha-Synuclein","q":"A 62-year-old man with Parkinson's disease has tremor, rigidity, and bradykinesia. Misfolded alpha-synuclein aggregates form pores in the inner mitochondrial membrane of his substantia nigra neurons, releasing which molecule that triggers cell death?","o":["Amyloid","Cytochrome c","Cytochrome P450","NADH","Ubiquitin"],"a":1,"p":"Misfolded alpha-synuclein aggregates (annular protofibrils) insert into the cardiolipin-rich inner mitochondrial membrane → form pores → cytochrome c released into cytosol → apoptosome formation → caspase activation → dopaminergic neuron apoptosis → substantia nigra cell loss → dopamine depletion → Parkinson's motor features.\n\n📹 [VIDEO LINK]"},{"id":298,"s":"Cell Organelles","t":"Cell Organelles — Mitochondria","u":"Parkinson Parkin Mitophagy","q":"A 70-year-old man with Parkinson's disease has dysregulated mitophagy in his substantia nigra neurons. Mutation in which of the following proteins causes this dysregulated mitophagy?","o":["Alpha-synuclein","Cytochrome c","Parkin","Synaptobrevin","Tau"],"a":2,"p":"PARKIN is an E3 ubiquitin ligase involved in mitophagy. Normal: PINK1 accumulates on damaged mitochondria → recruits Parkin → Parkin ubiquitinates outer mitochondrial membrane proteins → autophagosome recognition → mitophagy. In Parkinson's: alpha-synuclein oligomers → oxidative stress → S-nitrosylation of Parkin → Parkin inactivated → mitophagy fails → damaged mitochondria accumulate → neurodegeneration.\n\n📹 [VIDEO LINK]"},{"id":299,"s":"Cell Organelles","t":"Cell Organelles — Mitochondria","u":"Kearns-Sayre Oxidative Phosphorylation","q":"A 19-year-old woman presents with progressive bilateral ptosis and diplopia over 18 months. Unlike myasthenia gravis, her symptoms are NOT fatigable and do NOT improve with anticholinesterase drugs. ECG shows a right bundle branch block and cerebellar ataxia is present. Kearns-Sayre Syndrome is diagnosed. Which cellular process is impaired in her ocular muscles?","o":["Beta oxidation","Glycolysis","Krebs cycle","Oxidative phosphorylation","Protein synthesis"],"a":3,"p":"Kearns-Sayre Syndrome = large mitochondrial DNA deletion → impaired oxidative phosphorylation → insufficient ATP. Key distinguishing features from Myasthenia Gravis: (1) Non-fatigable. (2) Does NOT respond to anticholinesterase drugs. (3) Cardiac conduction defects. (4) Cerebellar ataxia. (5) Pigmentary retinopathy. (6) Elevated CSF protein. (7) Onset before age 20.\n\n📹 [VIDEO LINK]"},{"id":300,"s":"Extracellular Matrix","t":"Collagen Structure","u":"Basal Lamina","q":"A thin specialised layer approximately 100 nanometres thick lies directly beneath epithelial cells. It is composed of type IV collagen, laminin, nidogen, and heparan sulfate proteoglycans. Which structure is this?","o":["Basal lamina","Extracellular matrix","Lamina propria","Mucosa","Submucosa"],"a":0,"p":"Basal lamina (basement membrane) = specialised ECM layer directly beneath epithelial and endothelial cells. Composition: Type IV collagen (non-fibrillar network) + laminin (adhesive glycoprotein) + nidogen/entactin + heparan sulfate proteoglycans (perlecan). Functions: structural support, cell migration scaffold, tumour invasion barrier. Disrupted in Goodpasture syndrome and cancer invasion.\n\n📹 [VIDEO LINK]"},{"id":301,"s":"Extracellular Matrix","t":"Collagen Structure","u":"Lamina Propria","q":"A first-year medical student is studying histology of the small intestine. She notes a layer of loose connective tissue containing blood vessels, lymphatics, plasma cells, and mast cells that lies between the basal lamina above and the muscularis mucosae below. Which layer is she describing?","o":["Adventitia","Lamina propria","Mucosa","Serosa","Submucosa"],"a":1,"p":"Lamina propria = loose connective tissue directly beneath the basal lamina containing blood vessels, lymphatics (lacteals in intestine), nerve fibres, fibroblasts, macrophages, mast cells, plasma cells (IgA-secreting). Together with the epithelium and basal lamina = MUCOSA. Muscularis mucosae marks the boundary between mucosa and submucosa.\n\n📹 [VIDEO LINK]"},{"id":302,"s":"Extracellular Matrix","t":"Collagen Structure","u":"Mucosa Composition","q":"A 40-year-old woman with Crohn's disease has a colonoscopy biopsy showing inflammatory infiltrate, crypt destruction, and goblet cell loss. The pathologist reports disease limited to the mucosa. Which of the following correctly describes the composition of the mucosa?","o":["Basal lamina and epithelium only","Basal lamina and lamina propria only","Epithelium and lamina propria only","Epithelium, basal lamina, and lamina propria","Submucosa and epithelium"],"a":3,"p":"Mucosa = three layers: (1) Epithelium. (2) Basal lamina. (3) Lamina propria. Muscularis mucosae marks the boundary between mucosa and submucosa. In Crohn's disease: transmural inflammation (all layers). In ulcerative colitis: mucosal inflammation only.\n\n[Diagram: Cross-section intestinal wall: Lumen → Epithelium → Basal lamina (thin line) → Lamina propria → Muscularis mucosae. Bracket MUCOSA encompasses these three layers. Below: Submucosa → Muscularis externa → Serosa. Crohn's = red across all layers. UC = red limited to mucosa only.]\n\n📹 [VIDEO LINK]"},{"id":303,"s":"Extracellular Matrix","t":"Glycosaminoglycans","u":"GAG Repeating Disaccharide","q":"Which of the following ECM components is composed of repeating disaccharide units and forms long unbranched heteropolysaccharide chains?","o":["Collagen","Elastin","Fibronectin","Glycosaminoglycans","Proteoglycans"],"a":3,"p":"Glycosaminoglycans (GAGs) = long unbranched heteropolysaccharide chains of repeating disaccharide units (hexuronic acid + hexosamine). Heavily sulfated → highly negatively charged → attract water → resist compression. Types: hyaluronic acid (no sulfate), chondroitin sulfate (cartilage), dermatan sulfate (skin), heparan sulfate (cell surfaces), keratan sulfate (cornea, cartilage). Proteoglycans = GAG chains + protein core.\n\n📹 [VIDEO LINK]"},{"id":304,"s":"Extracellular Matrix","t":"ECM Proteins","u":"ECM Fibrous Proteins","q":"Which of the following are the fibrous protein components of the extracellular matrix?","o":["Cadherins and selectins","Collagen and elastin","Fibronectin and laminin","Integrins and ligands","Proteoglycans and glycosaminoglycans"],"a":1,"p":"ECM protein classification: Fibrous proteins = COLLAGEN (tensile strength) + ELASTIN (elasticity/recoil). Adhesive glycoproteins = FIBRONECTIN (bridges collagen to integrins in connective tissue) + LAMININ (bridges basal lamina to integrins in epithelial tissue). Cadherins and selectins = cell adhesion molecules on cell surfaces, not ECM.\n\n📹 [VIDEO LINK]"},{"id":305,"s":"Extracellular Matrix","t":"ECM Proteins","u":"ECM Adhesive Glycoproteins","q":"Which of the following are the adhesive glycoprotein components of the extracellular matrix secreted by cells into the extracellular space?","o":["Cadherins and selectins","Collagen and elastin","Fibronectin and laminin","Integrins and proteoglycans","Selectins and immunoglobulins"],"a":2,"p":"Fibronectin and laminin = ADHESIVE GLYCOPROTEINS of the ECM. Fibronectin: principal adhesive protein in connective tissue → RGD sequence binds integrin receptors → bridges ECM collagen to cell surface. Laminin: principal adhesive protein in epithelial tissue → binds collagen IV and integrins → critical for basal lamina assembly.\n\n📹 [VIDEO LINK]"},{"id":306,"s":"Extracellular Matrix","t":"ECM Proteins","u":"Fibronectin Connective Tissue","q":"Which of the following is the principal adhesive protein in connective tissue that bridges the ECM to cell surface integrins?","o":["Collagen","Elastin","Fibronectin","Laminin","Vitronectin"],"a":2,"p":"Fibronectin = principal adhesive glycoprotein in CONNECTIVE TISSUE ECM. Contains: collagen-binding domain, fibrin-binding domain, cell-binding domain (RGD sequence binds α5β1 integrin), heparan sulfate binding domain. Cancer cells downregulate fibronectin → lose anchorage → increased motility → metastasis.\n\n📹 [VIDEO LINK]"},{"id":307,"s":"Extracellular Matrix","t":"ECM Proteins","u":"Laminin Epithelial Tissue","q":"Which of the following is the principal adhesive protein in epithelial tissue and the basal lamina?","o":["Collagen","Elastin","Fibronectin","Laminin","Vitronectin"],"a":3,"p":"Laminin = principal adhesive glycoprotein in EPITHELIAL TISSUE and BASAL LAMINA. Large cruciform-shaped glycoprotein (α, β, γ chains). Binds: collagen IV, integrins on epithelial cells, dystroglycan, nidogen. Mutations in laminin-2 (merosin) → congenital muscular dystrophy type 1A (MDC1A). Dystrophin ultimately connects to laminin-2 at the muscle basal lamina.\n\n📹 [VIDEO LINK]"},{"id":308,"s":"Extracellular Matrix","t":"ECM Proteins","u":"Fibronectin Laminin Bridge Integrins","q":"Proteoglycans and collagen are linked to each other and to the cell surface through their interactions with which of the following?","o":["Cadherins and selectins","Collagen and elastin","Fibronectin and laminin","Integrins and ligands","Proteoglycans and glycosaminoglycans"],"a":2,"p":"Fibronectin and laminin serve as molecular bridges: contain binding domains for BOTH ECM components (collagen, proteoglycans) AND cell surface receptors (integrins via RGD sequences). This bridging function links the extracellular matrix network to the intracellular cytoskeleton. Disruption → loss of anchorage-dependent growth → anoikis resistance in cancer.\n\n📹 [VIDEO LINK]"},{"id":309,"s":"Extracellular Matrix","t":"Collagen Disorders","u":"Rickets Hydroxyapatite","q":"An 18-month-old child presents with bowing of legs, frontal bossing, and beading at the costochondral junctions. His mother exclusively breastfed him and avoids taking him outdoors. Which component of bone extracellular matrix is primarily deficient?","o":["Collagen fibrils","Elastin fibres","Fibronectin networks","Glycosaminoglycans","Hydroxyapatite crystals"],"a":4,"p":"Rickets (Vitamin D deficiency): bone ECM has organic (type I collagen = osteoid) and inorganic (hydroxyapatite crystals = Ca₁₀(PO₄)₆(OH)₂) components. Vitamin D deficiency → ↓ calcium and phosphate absorption → ↓ mineralisation → insufficient hydroxyapatite crystal deposition → soft deformable bones → bowing, frontal bossing, rachitic rosary. Collagen matrix (osteoid) is present but unmineralised.\n\n📹 [VIDEO LINK]"},{"id":310,"s":"Extracellular Matrix","t":"Glycosaminoglycans","u":"Glucosamine OA ECM","q":"A 50-year-old man with osteoarthritis takes glucosamine sulfate over the counter. Which component of the extracellular matrix is supplemented by this therapy?","o":["Collagen network","Elastin fibres","Fibronectin bridges","Glycosaminoglycans","Hydroxyapatite crystals"],"a":3,"p":"Glucosamine sulfate is a glycosaminoglycan precursor → supplements chondroitin sulfate and hyaluronic acid in articular cartilage ECM → restores water-binding capacity and resilience. Cartilage ECM: type II collagen (tensile strength) + aggrecan proteoglycan (water retention) + hyaluronic acid (backbone). In OA, GAG content falls → cartilage loses water → brittles → cracks.\n\n📹 [VIDEO LINK]"},{"id":311,"s":"Extracellular Matrix","t":"Collagen Disorders","u":"OI ECM Affected","q":"A little girl with Osteogenesis Imperfecta is described as fragile as glass. Which component of the extracellular matrix is adversely affected in her?","o":["Cytosol matrix","Extracellular matrix","Lysosomal matrix","Mitochondrial matrix","Nuclear matrix"],"a":1,"p":"Osteogenesis Imperfecta = defective type I collagen → abnormal extracellular matrix of bone, skin, sclera, tendons, ligaments → fragile bones, blue sclerae, hearing loss, brittle teeth. Without proper type I collagen → osteoid structurally defective → hydroxyapatite cannot mineralise normally → brittle bones.\n\n📹 [VIDEO LINK]"},{"id":312,"s":"Extracellular Matrix","t":"Collagen Structure","u":"Scurvy ECM Affected","q":"An old man on a tea-and-toast diet presents with pallor and bleeding gums. Vitamin C deficiency is suspected. Composition of which of the following components is primarily impaired in him?","o":["Cytosol matrix","Extracellular matrix","Lysosomal matrix","Mitochondrial matrix","Nuclear matrix"],"a":1,"p":"Vitamin C deficiency → impaired prolyl hydroxylase and lysyl hydroxylase → proline and lysine not hydroxylated → unstable collagen triple helix → defective extracellular matrix in blood vessels, skin, gingiva → fragile capillaries → bleeding gums, petechiae = scurvy.\n\n📹 [VIDEO LINK]"},{"id":313,"s":"Cell Adhesion","t":"Cell Junctions","u":"Tight Junctions Physical Barrier","q":"Physical barriers between individual cells that prevent paracellular movement of molecules are formed by which of the following?","o":["Adherens junctions","Desmosomes","Gap junctions","Hemidesmosomes","Tight junctions"],"a":4,"p":"Tight junctions (zonula occludens) = physical barriers sealing the paracellular space. Components: claudins, occludins, ZO proteins. Prevent ions and molecules from crossing between cells — force transcellular transport. Also establish apical/basolateral membrane polarity. Disrupted in coeliac disease and IBD → leaky gut.\n\n📹 [VIDEO LINK]"},{"id":314,"s":"Cell Adhesion","t":"Cell Junctions","u":"Desmosomes Intermediate Filaments Between Cells","q":"A dermatologist biopsies skin from a patient with pemphigus vulgaris and notes intraepidermal blistering with acantholysis. The junctions disrupted in pemphigus vulgaris link intermediate filaments between adjacent keratinocytes. These junctions are:","o":["Desmosomes","Gap junctions","Hemidesmosomes","Tight junctions","Zonulae adherentes"],"a":0,"p":"Desmosomes link keratin intermediate filaments BETWEEN adjacent cells. Extracellular: desmoglein + desmocollin. Intracellular plaque: desmoplakin + plakophilin → keratin IFs. Pemphigus vulgaris = IgG anti-desmoglein 1 and 3 → acantholysis → intraepidermal blister.\n\n[Diagram: Desmosome structure — extracellular desmoglein/desmocollin → intracellular plaque (desmoplakin + plakophilin) → keratin IFs. IgG antibody blocking desmoglein → gap forming → acantholysis. Tombstone pattern shown.]\n\n📹 [VIDEO LINK]"},{"id":315,"s":"Cell Adhesion","t":"Cell Junctions","u":"Hemidesmosomes Intermediate Filaments to Basal Lamina","q":"A 35-year-old woman with bullous pemphigoid presents with large tense subepidermal blisters. Her IgG antibodies target the attachment structures that anchor the BASE of keratinocytes to the basement membrane. These structures are:","o":["Desmosomes","Gap junctions","Hemidesmosomes","Tight junctions","Zonulae adherentes"],"a":2,"p":"Hemidesmosomes anchor the BASE of epithelial cells to the basal lamina. Components: alpha-6-beta-4 integrin + BP180 (collagen XVII) + BP230 + plectin → linked to keratin IFs. Bullous pemphigoid = IgG anti-BP180/BP230 → subepidermal blister. Compare PV: intraepidermal blister (basal cells still attached to BM via intact hemidesmosomes — tombstone pattern).\n\n[Diagram: Hemidesmosome — alpha-6-beta-4 integrin → intracellular BP230 + plectin → keratin IFs. Extracellular BP180 → laminin-332 → collagen IV in basal lamina. IgG targeting BP180/BP230. Comparison: PV (intraepidermal) vs bullous pemphigoid (subepidermal).]\n\n📹 [VIDEO LINK]"},{"id":316,"s":"Cell Adhesion","t":"Cell Junctions","u":"Zonulae Adherentes Actin Bundles","q":"A histologist describes the junction belt that runs around the apical circumference of intestinal epithelial cells, joining the actin bundles of adjacent cells and maintaining epithelial integrity. This junction is:","o":["Desmosome","Gap junction","Hemidesmosome","Tight junction","Zonula adherens"],"a":4,"p":"Zonula adherens (adherens junction) = belt junction around the apical circumference of epithelial cells. Links ACTIN BUNDLES between cells via E-cadherin (extracellular) → beta-catenin → alpha-catenin → actin filaments (intracellular). Located just BELOW the tight junction. Compare: Desmosomes link INTERMEDIATE FILAMENTS between cells. Hemidesmosomes link INTERMEDIATE FILAMENTS to BASAL LAMINA.\n\n[Diagram: Apical-to-basal junction order: Tight junction → Zonula adherens (actin bundle, E-cadherin/catenin) → Desmosome (keratin IFs, desmoglein) → Hemidesmosome (basal, to BM). Each labelled with key proteins and cytoskeletal connection.]\n\n📹 [VIDEO LINK]"},{"id":317,"s":"Cell Adhesion","t":"Cell Adhesion Molecules","u":"Cadherins Long-lasting Adhesion","q":"A cancer biologist notes that downregulation of a specific cell adhesion molecule allows tumour cells to detach from each other and invade surrounding tissue. This molecule provides long-lasting calcium-dependent adhesion between epithelial cells. Which molecule is this?","o":["Cadherin","Fibronectin","Integrin","Laminin","Selectin"],"a":0,"p":"Cadherins provide LONG-LASTING Ca²⁺-DEPENDENT homophilic cell-cell adhesion. E-cadherin loss = hallmark of epithelial-mesenchymal transition (EMT) in cancer → cells lose adhesion → gain motility → invade and metastasize. E-cadherin is a TUMOUR SUPPRESSOR. Selectins = weak transient adhesion (rolling). Integrins = cell-matrix adhesion.\n\n[Diagram: Two panels — Normal epithelium: E-cadherin linking adjacent cells → tight tissue. Cancer: E-cadherin lost → cells separate → individual cell invades basement membrane → metastasis. Beta-catenin nuclear translocation → Wnt target gene activation shown.]\n\n📹 [VIDEO LINK]"},{"id":318,"s":"Cell Membrane","t":"Membrane Transport","u":"Chromatography","q":"Which technique separates particles of a solute based on their properties such as size, charge, affinity, and solubility?","o":["Centrifugation","Chromatography","Dialysis","Electrophoresis","Radioimmunoassay"],"a":1,"p":"Chromatography separates molecules based on differential distribution between stationary and mobile phases. Types: Ion-exchange (charge), Size-exclusion/gel filtration (size), Affinity chromatography (specific binding), Reverse-phase HPLC (hydrophobicity). Used to purify proteins, nucleic acids, drugs, lipids.\n\n📹 [VIDEO LINK]"},{"id":319,"s":"Cell Membrane","t":"Membrane Transport","u":"Electrophoresis Charged Particles","q":"Which technique separates charged particles by migration in an electric field?","o":["Centrifugation","Chromatography","Dialysis","Electrophoresis","Radioimmunoassay"],"a":3,"p":"Electrophoresis: charged particles placed in electric field → migrate toward opposite electrode at speeds determined by charge, size, and shape. Used for: protein separation (PAGE, SDS-PAGE), DNA/RNA separation (agarose gel). Cannot separate lipids (neutral, uncharged → do not migrate in electric field).\n\n📹 [VIDEO LINK]"},{"id":320,"s":"Cell Membrane","t":"Membrane Transport","u":"Electrophoresis Speed Determinant","q":"The speed of migration of molecules in electrophoresis primarily depends on which of the following?","o":["Charge-to-mass ratio","Colour","Density","Mass","Solubility"],"a":0,"p":"Electrophoretic mobility depends on CHARGE-TO-MASS RATIO: higher charge + smaller size = faster migration. In native PAGE: charge, size, and shape all contribute. In SDS-PAGE: SDS coats all proteins uniformly with negative charge → separation by SIZE ONLY. Isoelectric focusing separates by isoelectric point.\n\n📹 [VIDEO LINK]"},{"id":321,"s":"Cell Membrane","t":"Membrane Transport","u":"SDS-PAGE","q":"If proteins are separated according to their electrophoretic mobility reflecting their molecular weight, which type of electrophoresis is being used?","o":["Affinity electrophoresis","Capillary electrophoresis","Free-flow electrophoresis","Isoelectric focusing","SDS-PAGE"],"a":4,"p":"SDS-PAGE: SDS (anionic detergent) denatures proteins → coats uniformly with negative charge → all proteins migrate by SIZE ONLY (smaller migrate faster through polyacrylamide mesh). Molecular weight determined by comparison with protein ladder. Western blotting follows SDS-PAGE.\n\n📹 [VIDEO LINK]"},{"id":322,"s":"Cell Membrane","t":"Membrane Transport","u":"Electrophoresis Cannot Separate Lipids","q":"Electrophoresis cannot be used to separate which of the following biomolecules?","o":["Amino acids","Lipids","Nucleic acids","Polysaccharides","Proteins"],"a":1,"p":"Lipids are predominantly NEUTRAL and NONPOLAR → carry no electric charge → do not migrate in an electric field → CANNOT be separated by electrophoresis. Lipid separation uses chromatography (TLC, HPLC) or mass spectrometry. All other options carry charges: proteins (amino acid side chains), nucleic acids (phosphate groups), amino acids (amino and carboxyl groups).\n\n📹 [VIDEO LINK]"},{"id":323,"s":"Cell Membrane","t":"Membrane Transport","u":"Radioimmunoassay","q":"A laboratory technician is quantifying insulin levels in a patient's blood sample. She uses a technique in which a fixed quantity of radioactively labelled insulin competes with the patient's unlabelled insulin for a limited number of antibody binding sites. The radioactivity of the antibody-bound fraction is then measured. Which technique is being used?","o":["ELISA","Electrophoresis","Flow cytometry","Radioimmunoassay","Western blot"],"a":3,"p":"RIA (Radioimmunoassay): radioactively labelled antigen (¹²⁵I) competes with unlabelled patient antigen for limited antibody binding sites. More patient antigen present → less labelled antigen binds → lower radioactive count measured. Highly sensitive (detects picogram quantities). Used for hormones (insulin, TSH, LH, FSH), drugs, tumour markers.\n\n📹 [VIDEO LINK]"},{"id":324,"s":"Cell Membrane","t":"Membrane Transport","u":"ELISA","q":"Which of the following techniques measures an enzymatic reaction associated with immune complexes to detect and quantify specific antigens?","o":["Electrophoresis","ELISA","Flow cytometry","Radioimmunoassay","Western blot"],"a":1,"p":"ELISA (Enzyme-Linked Immunosorbent Assay): antibody linked to enzyme (HRP or alkaline phosphatase) → binds antigen → enzyme converts substrate → coloured product → measured by spectrophotometry. Types: direct, indirect, sandwich, competitive. Used for: HIV diagnosis, hepatitis B surface antigen, drug levels, autoantibodies (anti-desmoglein in pemphigus diagnosis).\n\n📹 [VIDEO LINK]"},{"id":325,"s":"Cell Membrane","t":"Membrane Transport","u":"Mass Spectrometry","q":"Which technique determines molecular weight by first ionising particles and then subjecting them to a magnetic field?","o":["Chromatography","Electrophoresis","Mass spectrometry","NMR spectrometry","Radioimmunoassay"],"a":2,"p":"Mass spectrometry: (1) Ionisation (ESI or MALDI) → creates charged ions. (2) Acceleration by electric field. (3) Deflection by magnetic field (lighter ions deflected more). (4) Detection → mass-to-charge ratio (m/z) measured → molecular weight determined. MALDI-TOF used for protein fingerprinting and microbial identification.\n\n📹 [VIDEO LINK]"},{"id":326,"s":"Cell Membrane","t":"Membrane Transport","u":"NMR Spectrometry","q":"Which technique determines the three-dimensional structure of molecules by exploiting the magnetic properties of certain atomic nuclei?","o":["Chromatography","Electrophoresis","Mass spectrometry","NMR spectrometry","X-ray crystallography"],"a":3,"p":"NMR (Nuclear Magnetic Resonance) spectrometry: atomic nuclei with odd mass numbers (¹H, ¹³C, ¹⁵N, ³¹P) have spin → in magnetic field → absorb radiofrequency radiation at specific frequencies → resonance signals → 3D structure determined. Works in solution. MRI uses ¹H NMR of water. Advantage over X-ray crystallography: no crystal needed.\n\n📹 [VIDEO LINK]"},{"id":327,"s":"Cytoskeleton","t":"Hereditary Spherocytosis","u":"RBC Glycolysis Only","q":"A red blood cell is circulating in peripheral blood. Which of the following processes is helping it to meet its energy needs?","o":["Beta oxidation","Gluconeogenesis","Glycolysis","Krebs cycle","Oxidative phosphorylation"],"a":2,"p":"Mature RBCs have NO MITOCHONDRIA → cannot perform beta-oxidation, Krebs cycle, or oxidative phosphorylation → rely EXCLUSIVELY on GLYCOLYSIS (anaerobic) for ATP production → 2 ATP per glucose. Also use the pentose phosphate pathway (PPP) for NADPH → regenerates glutathione → protects against oxidative damage. RBCs use Rapoport-Luebering shunt → produces 2,3-BPG → reduces haemoglobin oxygen affinity → facilitates O₂ delivery to tissues.\n\n📹 [VIDEO LINK]"},{"id":328,"s":"Cytoskeleton","t":"Hereditary Spherocytosis","u":"Sickle Cell HbS Polymerisation","q":"A 10-year-old boy from West Africa presents with painful crisis, anaemia, and splenomegaly. Blood smear shows sickle-shaped red blood cells. The sickling of RBCs is caused by polymerisation of which abnormal protein under low oxygen conditions?","o":["Haemoglobin A","Haemoglobin F","Haemoglobin S","Methaemoglobin","Myoglobin"],"a":2,"p":"Sickle cell disease: Glu6Val substitution in beta-globin → HbS. In low O₂: HbS deoxygenated → hydrophobic valine creates sticky patch → HbS molecules polymerise → long fibres → distort RBC into sickle shape → vascular occlusion → pain crises, organ damage, haemolysis. HbF INHIBITS HbS polymerisation → hydroxyurea induces HbF → reduces crises. Heterozygotes (sickle trait) = protection against malaria.\n\n📹 [VIDEO LINK]"},{"id":329,"s":"Extracellular Matrix","t":"Collagen Structure","u":"Copper Enzymes Lysyl Oxidase","q":"A patient with Menkes disease has impaired copper transport leading to copper deficiency in multiple tissues. Which of the following enzymes requires copper as a cofactor for collagen crosslinking in the extracellular matrix?","o":["Lysyl hydroxylase","Lysyl oxidase","Prolyl hydroxylase","Prolyl isomerase","Transglutaminase"],"a":1,"p":"Lysyl oxidase = COPPER-containing enzyme that oxidises lysine and hydroxylysine → allysine → spontaneous crosslinks between tropocollagen molecules → insoluble mature collagen fibrils. Also crosslinks elastin. Menkes disease (X-linked, ATP7A mutation) → copper deficiency → defective lysyl oxidase → weak connective tissue → kinky hair, neurodegeneration, vascular aneurysms. Also copper-requiring: dopamine β-hydroxylase, ceruloplasmin, superoxide dismutase, tyrosinase, cytochrome c oxidase.\n\n📹 [VIDEO LINK]"},{"id":330,"s":"Cell Organelles","t":"Cell Organelles — Mitochondria","u":"Carnitine Shuttle CAT-I","q":"A patient with carnitine deficiency cannot transport long-chain fatty acids into mitochondria for beta-oxidation. Which enzyme catalyses the transfer of acyl groups from acyl-CoA to carnitine on the outer mitochondrial membrane?","o":["Acyl-CoA dehydrogenase","Carnitine acyltransferase I","Carnitine acyltransferase II","Malonyl-CoA carboxylase","Thiolase"],"a":1,"p":"Carnitine shuttle: Long-chain fatty acyl-CoA → CARNITINE ACYLTRANSFERASE I (CAT-I, outer mitochondrial membrane) → acylcarnitine. Transported across inner membrane by carnitine-acylcarnitine translocase. Inside matrix: CARNITINE ACYLTRANSFERASE II (CAT-II) → reforms acyl-CoA → beta-oxidation begins. CAT-I is inhibited by MALONYL-CoA — prevents simultaneous fatty acid synthesis and oxidation. Medium/short-chain fatty acids cross independently without carnitine.\n\n📹 [VIDEO LINK]"},{"id":331,"s":"Cell Organelles","t":"Cell Organelles — Lysosomal Storage","u":"Lysosomal Storage Disease Comparison","q":"A 6-month-old infant presents with hepatosplenomegaly, progressive neurodegeneration, and a cherry-red spot on fundoscopy. No features of bony involvement are present. Which enzyme is most likely deficient?","o":["Alpha-galactosidase A","Glucocerebrosidase","Hexosaminidase A","Sphingomyelinase","Sulphatase"],"a":3,"p":"Cherry-red spot + hepatosplenomegaly + neurodegeneration WITHOUT bone involvement = Niemann-Pick disease Type A (sphingomyelinase deficiency). Compare: Gaucher = hepatosplenomegaly + bone disease + NO cherry-red spot. Tay-Sachs = neurodegeneration + cherry-red spot + NO hepatosplenomegaly. Fabry = X-linked, pain + skin + kidney, no cherry-red spot. The combination of hepatosplenomegaly AND cherry-red spot narrows to Niemann-Pick.\n\n📹 [VIDEO LINK]"},{"id":332,"s":"Protein Structure — Clinical Correlations","t":"Alzheimer's Disease","u":"APP Normal Cleavage","q":"A 87-year-old man with Alzheimer's disease had normal amyloid precursor protein (APP) processing 3 years ago. Which enzyme was active in him at that time, cleaving APP in a non-amyloidogenic manner?","o":["Alpha secretase","Beta secretase","Delta secretase","Gamma secretase","Sigma secretase"],"a":0,"p":"ALPHA secretase was normally active — cleaves APP within the amyloid sequence → non-amyloidogenic pathway → produces sAPPα (neuroprotective) + C83 fragment → gamma secretase cleaves C83 → non-toxic p3 peptide. When BETA secretase becomes predominant → cleaves APP differently → C99 fragment → gamma secretase cleaves C99 → Aβ42 peptide → beta pleated sheet → insoluble aggregates → extracellular plaques in hippocampus and cortex → Alzheimer's disease.\n\n📹 [VIDEO LINK]"},{"id":333,"s":"Protein Structure — Clinical Correlations","t":"Alzheimer's Disease","u":"Aβ42 Beta Sheet Conformation","q":"A 87-year-old man with Alzheimer's disease has amyloid plaques in his hippocampus. These plaques form when amyloid beta 42 peptide adopts which abnormal secondary structure causing it to aggregate?","o":["Alpha helix","Beta barrel","Beta pleated sheet","Beta turn","Random coil"],"a":2,"p":"Amyloid beta 42 (Aβ42) normally soluble → undergoes conformational change → adopts BETA PLEATED SHEET conformation → insoluble → self-aggregates → extracellular amyloid plaques (neuritic plaques) in hippocampus and cortex. Beta pleated sheet conformation is resistant to proteolytic cleavage → accumulates progressively. This same beta sheet aggregation operates in prion disease, Parkinson's (alpha-synuclein), and type 2 diabetes (IAPP).\n\n[Diagram: Two panels — Left (normal): APP in neuronal membrane → alpha secretase cleaves → soluble non-toxic fragments. Right (Alzheimer's): APP → beta secretase → C99 → gamma secretase → Aβ42 → beta pleated sheet conformation → aggregation → extracellular plaque in hippocampus labelled]\n\n📹 [VIDEO LINK]"},{"id":334,"s":"Protein Structure — Clinical Correlations","t":"Alzheimer's Disease","u":"Tau Normal Function Impaired","q":"A 87-year-old man with Alzheimer's disease has neurofibrillary tangles in his neurons due to hyperphosphorylated tau protein. Which of the following functions is impaired in this man due to malfunctioning tau?","o":["Actin polymerisation","Amyloid clearance","Gene transcription","Microtubule assembly","Synaptic vesicle fusion"],"a":3,"p":"Tau protein normally stabilises MICROTUBULES in neurons → promotes tubulin polymerisation → maintains axonal transport. In Alzheimer's: abnormal kinases hyperphosphorylate tau → tau dissociates from microtubules → microtubules collapse → tau aggregates → paired helical filaments → neurofibrillary tangles → axonal transport blocked → apoptosis → neuron loss. Tangles are INTRACELLULAR (plaques are extracellular — key distinction).\n\n📹 [VIDEO LINK]"},{"id":335,"s":"Protein Structure — Clinical Correlations","t":"Alzheimer's Disease","u":"Neurofibrillary Tangles Intracellular","q":"A 87-year-old man with Alzheimer's disease has neurofibrillary tangles in his hippocampal neurons. The location of these tangles compared to amyloid plaques is:","o":["Extracellular","Intracellular","Intranuclear","Intrasynaptic","Intravascular"],"a":1,"p":"Key distinction in Alzheimer's pathology: AMYLOID PLAQUES = EXTRACELLULAR (Aβ42 aggregates outside neurons in hippocampus and cortex). NEUROFIBRILLARY TANGLES = INTRACELLULAR (hyperphosphorylated tau aggregates inside neurons as paired helical filaments). Both are hallmarks of Alzheimer's. Amyloid hypothesis: plaques form first → trigger tau phosphorylation → tangles form secondarily → neuron death.\n\n[Diagram: Neuron diagram — extracellular space showing amyloid plaque (Aβ42 aggregates, labelled EXTRACELLULAR). Inside neuron: paired helical filaments of phosphorylated tau forming tangle (labelled INTRACELLULAR). Microtubule collapse shown inside neuron. Arrow: axonal transport blocked by tangles → neuron death]\n\n📹 [VIDEO LINK]"},{"id":336,"s":"Protein Structure — Clinical Correlations","t":"Alzheimer's Disease","u":"Amyloid Plaque Core Protein","q":"A 60-year-old man with Alzheimer's disease confirmed at autopsy has characteristic lesions in his brain. Which protein forms the core of the amyloid plaques found in his brain?","o":["Alpha-synuclein","Amyloid beta","Huntingtin","Prion protein","Tau protein"],"a":1,"p":"Amyloid plaques in Alzheimer's disease have AMYLOID BETA (Aβ42) as their core protein — derived from faulty beta/gamma secretase cleavage of APP. Tau = neurofibrillary tangles (intracellular, different structure). Alpha-synuclein = Lewy bodies in Parkinson's. Huntingtin = Huntington's disease. Prion protein = prion diseases (CJD, Kuru). Each neurodegenerative disease has its own characteristic aggregating protein.\n\n📹 [VIDEO LINK]"},{"id":337,"s":"Protein Structure — Clinical Correlations","t":"Alzheimer's Disease","u":"Abnormal Secretase in Alzheimer's","q":"A 87-year-old man with Alzheimer's disease has amyloid plaques forming due to faulty APP cleavage. Which enzyme is responsible for the abnormal cleavage producing toxic fragments over the past 3 years?","o":["Alpha secretase","Beta secretase","Delta secretase","Gamma secretase","Sigma secretase"],"a":1,"p":"BETA secretase (BACE1) cleaves APP differently from alpha secretase → produces C99 fragment → gamma secretase cleaves C99 → Aβ42 (42 amino acid peptide) → beta pleated sheet → insoluble aggregates → neuritic plaques in extracellular space of hippocampus and cortex → Alzheimer's disease. When ALPHA secretase was active (normal) → non-amyloidogenic pathway → no plaques.\n\n📹 [VIDEO LINK]"},{"id":338,"s":"Protein Structure — Clinical Correlations","t":"Prion Disease","u":"Kuru Causative Agent","q":"An 18-year-old man from New Guinea presents with unsteady gait, dysarthria, and rapid memory loss after participating in ritualistic cannibalism. CSF analysis confirms prion disease. Which of the following is the causative agent?","o":["Amyloid beta","PrPc","PrPi","PrPs","PrPsc"],"a":4,"p":"PrPsc (sc = scrapie) = the INFECTIOUS misfolded form of prion protein. PrPc (c = cellular) = normal non-infectious form present on surface of neurons and glial cells — involved in signal transduction and neuroprotection. PrPsc contains predominantly beta sheets (PrPc is alpha helix rich). PrPsc acts as a TEMPLATE — converts normal PrPc into PrPsc → exponential accumulation → aggregates → spongiform changes → neurodegeneration. This disease = Kuru — linked to ritualistic cannibalism.\n\n📹 [VIDEO LINK]"},{"id":339,"s":"Protein Structure — Clinical Correlations","t":"Prion Disease","u":"Alpha Helix to Beta Sheet Conversion","q":"A 19-year-old woman from the USA develops unsteady gait, dysarthria, and memory loss after receiving cadaveric growth hormone injections. CSF confirms prion disease. The key biochemical change converting normal prion protein to its infectious form is replacement of which secondary structure by beta sheets?","o":["Alpha barrels","Alpha helix","Beta bends","Beta turns","Random coils"],"a":1,"p":"PrPc (normal) = predominantly ALPHA HELIX rich (42% alpha helix, 3% beta sheet). PrPsc (infectious) = predominantly BETA SHEET rich (43% beta sheet, 30% alpha helix). Key conformational change: ALPHA HELICES REPLACED BY BETA SHEETS → PrPsc becomes resistant to protease digestion → accumulates → acts as template for converting more PrPc → exponential propagation → aggregates → spongiform encephalopathy. No change in primary structure (identical amino acid sequence).\n\n[Diagram: Two protein structure diagrams side by side — Left (PrPc): predominantly alpha helix coils, labelled 42% alpha helix, 3% beta sheet, NORMAL, NON-INFECTIOUS. Right (PrPsc): predominantly beta sheet arrows, labelled 43% beta sheet, INFECTIOUS, PROTEASE RESISTANT. Arrow between them: conformational change, alpha helix → beta sheet]\n\n📹 [VIDEO LINK]"},{"id":340,"s":"Protein Structure — Clinical Correlations","t":"Prion Disease","u":"Prion Template Mechanism","q":"A 45-year-old man with sporadic CJD dies within 18 months. Prion protein causes neurodegeneration by acting as which of the following?","o":["Antigen","Antibody","Chaperonin","Enzyme","Template"],"a":4,"p":"PrPsc causes neurodegeneration by acting as a TEMPLATE — it binds to normal PrPc molecules → induces conformational change → converts PrPc to PrPsc → newly formed PrPsc converts more PrPc → exponential self-propagating cascade. This is NOT enzymatic (no catalytic activity), NOT immunological (no antigen-antibody reaction). PrPsc is therefore an infectious protein with no nucleic acid — contradicts the central dogma. Nobel Prize to Stanley Prusiner 1997.\n\n📹 [VIDEO LINK]"},{"id":341,"s":"Protein Structure — Clinical Correlations","t":"Prion Disease","u":"Sporadic CJD Causative Agent","q":"A 60-year-old man presents with rapidly progressive dementia, periodic sharp wave complexes on EEG, and increased signal in caudate and putamen on MRI. He dies within 6 months. Brain autopsy shows spongiform changes. Which of the following is the causative agent?","o":["Amyloid beta protein","Huntingtin protein","Prion protein","Tau protein","TDP-43 protein"],"a":2,"p":"Sporadic CJD: rapidly progressive dementia + periodic sharp waves on EEG + DWI MRI changes in caudate/putamen/cortex + death within months. Caused by PRION PROTEIN (PrPsc). Spongiform changes = hallmark histology. Note: VARIANT CJD (vCJD) = human form of Mad Cow Disease (BSE) — acquired by consuming BSE-contaminated beef. It is NOT hereditary. Hereditary prion diseases (familial CJD, Fatal Familial Insomnia, GSS) are caused by PRNP gene mutations.\n\n📹 [VIDEO LINK]"},{"id":342,"s":"Protein Structure — Clinical Correlations","t":"Prion Disease","u":"BSE vCJD Causative Agent","q":"A farmer develops rapidly progressive neurological decline after handling cattle with bovine spongiform encephalopathy. Which protein is the causative agent of both BSE and variant CJD?","o":["Amyloid beta protein","Alpha-synuclein protein","Huntingtin protein","Prion protein","Tau protein"],"a":3,"p":"BSE (Mad cow disease), Scrapie (sheep), CJD and variant CJD (humans), Kuru (humans), Fatal Familial Insomnia (humans) — ALL caused by PRION PROTEIN (PrPsc). The infectious agent contains no nucleic acid — protein only. Prion diseases = transmissible spongiform encephalopathies (TSEs). Variant CJD = human form of BSE — acquired by consuming BSE-contaminated beef. Younger patients than sporadic CJD. Psychiatric symptoms precede neurological features.\n\n📹 [VIDEO LINK]"},{"id":343,"s":"Protein Structure — Clinical Correlations","t":"Prion Disease","u":"Kuru Linked to Cannibalism","q":"Which of the following prion diseases is linked to ritualistic cannibalism?","o":["Fatal familial insomnia","Gerstmann-Sträussler-Scheinker syndrome","Kuru","Scrapie","Variant CJD"],"a":2,"p":"Kuru = prion disease found among the Fore people of Papua New Guinea — transmitted by ritualistic cannibalism (eating brain of deceased relatives during mourning ceremonies). Women and children most affected. Cerebellar signs predominate (kuru = trembling in Fore language). Elimination of cannibalism led to disappearance of Kuru. First human prion disease identified — led Carleton Gajdusek to Nobel Prize 1976.\n\n📹 [VIDEO LINK]"},{"id":344,"s":"Protein Structure — Clinical Correlations","t":"Prion Disease","u":"PrPc vs PrPsc Difference","q":"Normal prion protein (PrPc) and infectious prion protein (PrPsc) have identical amino acid sequences but differ in which of the following?","o":["Glycosylation pattern","Lipid anchor type","Primary structure","Three-dimensional conformation","Transcriptional modification"],"a":3,"p":"PrPc and PrPsc have IDENTICAL PRIMARY STRUCTURES (same amino acid sequence, same gene PRNP) — they differ ONLY in THREE-DIMENSIONAL CONFORMATION. PrPc = alpha helix rich, protease sensitive, membrane-anchored via GPI anchor. PrPsc = beta sheet rich, PROTEASE RESISTANT, aggregates. Same sequence, completely different properties — this is why prions were so revolutionary and initially so controversial.\n\n📹 [VIDEO LINK]"},{"id":345,"s":"Protein Structure — Clinical Correlations","t":"Prion Disease","u":"PrPc Normal Composition","q":"Normal cellular prion protein (PrPc) is present on the surface of neurons where it is involved in signal transduction and neuroprotection. PrPc is composed of which secondary structure that is lost when it converts to the infectious form?","o":["Alpha helix","Beta barrel","Beta pleated sheet","Beta turn","Random coil"],"a":0,"p":"PrPc (normal cellular prion protein) = ALPHA HELIX RICH (approximately 42% alpha helix, only 3% beta sheet). Attached to outer surface of neurons and glial cells via GPI anchor. Normal functions: signal transduction and neuroprotection. When PrPsc contacts PrPc → conformational change → alpha helices replaced by beta sheets → PrPc becomes PrPsc → infectious form → protease resistant → accumulates → neurodegeneration.\n\n📹 [VIDEO LINK]"},{"id":346,"s":"Protein Structure — Clinical Correlations","t":"Prion Disease","u":"PrPsc Protease Resistance","q":"A 55-year-old woman develops rapidly progressive dementia, ataxia, and myoclonus. Spongiform encephalopathy is confirmed. The infectious prion protein (PrPsc) resists degradation because it is resistant to which of the following?","o":["Denaturation","Glycosylation","Phosphorylation","Proteolytic cleavage","Ubiquitination"],"a":3,"p":"PrPsc is RESISTANT TO PROTEOLYTIC CLEAVAGE — beta sheet conformation hides protease cleavage sites → resists digestion by proteinase K → accumulates → aggregates → spongiform changes. PrPsc also resists: heat (survives normal cooking temperatures), formalin fixation, UV radiation, standard sterilisation — explains why prion diseases are so difficult to prevent.\n\n📹 [VIDEO LINK]"},{"id":347,"s":"Extracellular Matrix","t":"Fibrous Proteins — Clinical","u":"EDS Collagen Type V","q":"A 27-year-old woman presents with hyperextensible joints, fragile skin that stretches easily and heals poorly, and recurrent joint dislocations. A diagnosis of Ehlers-Danlos syndrome is made. Which type of collagen is most likely defective in her?","o":["Collagen type I","Collagen type II","Collagen type III","Collagen type IV","Collagen type V"],"a":4,"p":"Ehlers-Danlos syndrome with SKIN and JOINT hypermobility = COLLAGEN TYPE V defect. Type V collagen regulates collagen fibril diameter — its deficiency causes abnormally large collagen fibrils → poor skin tensile strength → hyperextensibility → poor wound healing → joint hypermobility. Compare: EDS vascular type (aortic aneurysm rupture) = COLLAGEN TYPE III. OI = type I. Alport syndrome = type IV. Cartilage = type II.\n\n📹 [VIDEO LINK]"},{"id":348,"s":"Extracellular Matrix","t":"Fibrous Proteins — Clinical","u":"EDS Vascular Collagen Type III","q":"A 35-year-old man with a known collagenopathy presents with rupture of an aortic aneurysm. Which collagen type is defective in this vascular form of Ehlers-Danlos syndrome?","o":["Collagen type I","Collagen type II","Collagen type III","Collagen type IV","Collagen type V"],"a":2,"p":"Vascular EDS (EDS Type IV) = COLLAGEN TYPE III defect. Collagen type III is the major collagen in blood vessels, hollow organs (intestines, uterus), and skin. Its deficiency → fragile vessel walls → spontaneous aortic/arterial aneurysm and rupture → life-threatening. Most dangerous form of EDS. Collagen type III forms reticulin fibres in liver, spleen, lymph nodes.\n\n📹 [VIDEO LINK]"},{"id":349,"s":"Extracellular Matrix","t":"Fibrous Proteins — Clinical","u":"EDS Lysyl Hydroxylase Vitamin C","q":"A 27-year-old woman with Ehlers-Danlos syndrome has defective collagen crosslinking because lysyl hydroxylase is deficient. Lysyl hydroxylase requires which vitamin as a cofactor?","o":["Vitamin A","Vitamin B12","Vitamin C","Vitamin D","Vitamin E"],"a":2,"p":"LYSYL HYDROXYLASE requires VITAMIN C (ascorbic acid) as a cofactor — it maintains iron as Fe²⁺ → enzyme remains active → hydroxylates lysine residues → hydroxylysine formed → provides crosslinking sites. Without Vitamin C → lysyl hydroxylase fails → poor collagen crosslinking → weak connective tissue = scurvy/EDS features. Distinguish: LYSYL HYDROXYLASE (Vitamin C + Iron dependent) vs LYSYL OXIDASE (copper dependent — defective in Menkes disease).\n\n📹 [VIDEO LINK]"},{"id":350,"s":"Extracellular Matrix","t":"Fibrous Proteins — Clinical","u":"EDS Lysyl Hydroxylase Iron vs Copper","q":"A 25-year-old woman with Ehlers-Danlos syndrome has defective lysyl hydroxylase causing impaired collagen crosslinking. Lysyl hydroxylase requires iron in its reduced form (Fe²⁺) as a cofactor. Which vitamin maintains iron in this reduced state to keep lysyl hydroxylase active?","o":["Vitamin A","Vitamin B12","Vitamin C","Vitamin D","Vitamin E"],"a":2,"p":"VITAMIN C (ascorbate) maintains iron as Fe²⁺ (reduced form) → keeps lysyl hydroxylase and prolyl hydroxylase active → hydroxylation of lysine and proline → stable collagen triple helix. Without Vitamin C → Fe²⁺ oxidised to Fe³⁺ → enzymes inactive → underhydroxylated collagen → scurvy/EDS features. Key distinction: LYSYL HYDROXYLASE requires IRON + VITAMIN C. LYSYL OXIDASE requires COPPER (defective in Menkes disease).\n\n📹 [VIDEO LINK]"},{"id":351,"s":"Extracellular Matrix","t":"Collagen Disorders","u":"OI Type II Lethal","q":"A neonate is born with multiple fractures, severe bone deformity, and dies within hours of birth. Mutation in a collagen synthesis gene is identified. Which type of Osteogenesis Imperfecta is most consistent with this presentation?","o":["Osteogenesis imperfecta type I","Osteogenesis imperfecta type II","Osteogenesis imperfecta type III","Osteogenesis imperfecta type IV","Osteogenesis imperfecta type V"],"a":1,"p":"OI TYPE II = LETHAL PERINATAL form. Multiple fractures at birth, severe bone deformity, death within days to weeks. Caused by dominant negative mutation in COL1A1 or COL1A2 → ONE abnormal alpha chain poisons the ENTIRE collagen triple helix → structurally abnormal collagen → catastrophic bone fragility. Compare OI TYPE I = REDUCED AMOUNT of normal collagen (haploinsufficiency) → blue sclerae, moderate fractures, normal life expectancy. Type I = quantity defect. Type II = quality defect.\n\n[Diagram: Two panels — Left (OI Type I): normal collagen triple helix but fewer fibrils, labelled reduced amount of normal collagen, mild phenotype, blue sclerae. Right (OI Type II): one abnormal alpha chain in triple helix shown in different colour, labelled dominant negative effect, entire helix abnormal, lethal perinatal]\n\n📹 [VIDEO LINK]"},{"id":352,"s":"Extracellular Matrix","t":"Collagen Disorders","u":"OI Type I vs Type II","q":"A 10-year-old girl has blue sclerae, moderate bone fragility with recurrent fractures after minor trauma, and hearing impairment. She has normal stature and intelligence. Which type of Osteogenesis Imperfecta is most consistent with this presentation?","o":["OI type I","OI type II","OI type III","OI type IV","OI type V"],"a":0,"p":"OI TYPE I = MILDEST form. REDUCED AMOUNT of structurally NORMAL type I collagen (haploinsufficiency). Features: blue sclerae, moderate fractures (decrease after puberty), hearing impairment (otosclerosis), normal stature, normal life expectancy. Compare OI TYPE II = STRUCTURALLY ABNORMAL collagen (dominant negative Gly substitution) → lethal perinatal. Key distinction: Type I = quantity defect (less of normal collagen). Type II = quality defect (abnormal collagen structure).\n\n📹 [VIDEO LINK]"},{"id":353,"s":"Extracellular Matrix","t":"AAT Deficiency","u":"AAT Methionine Oxidation Smoking","q":"A 30-year-old woman with alpha-1 antitrypsin deficiency develops emphysema after years of smoking. Cigarette smoke inactivates alpha-1 antitrypsin by oxidising a specific amino acid residue essential for its activity. Which amino acid is oxidised?","o":["Arginine","Cysteine","Lysine","Methionine","Serine"],"a":3,"p":"METHIONINE 358 in alpha-1 antitrypsin (AAT) is essential for binding to neutrophil elastase. Cigarette smoke oxidises methionine 358 → methionine sulphoxide → AAT cannot bind elastase → elastase acts unopposed → destroys elastin of alveolar walls → emphysema. This is why smokers with even NORMAL AAT levels develop emphysema — their AAT is inactivated. Connects methionine side chain chemistry directly to lung disease.\n\n📹 [VIDEO LINK]"},{"id":354,"s":"Extracellular Matrix","t":"AAT Deficiency","u":"AAT Z Variant Lysine Substitution","q":"A young man with alpha-1 antitrypsin deficiency (Z variant) has PiZZ genotype. The Z variant results from substitution of which amino acid for glutamate at position 342?","o":["Alanine","Glycine","Lysine","Proline","Valine"],"a":2,"p":"AAT Z variant: single nucleotide mutation (GAG → AAG) → GLUTAMATE replaced by LYSINE at position 342 → AAT protein misfolds → polymerises and aggregates in the ENDOPLASMIC RETICULUM of hepatocytes → liver accumulates AAT polymers → cirrhosis. Simultaneously reduced AAT in blood → lungs unprotected → neutrophil elastase unchecked → emphysema. Two consequences of one mutation: liver disease (ER accumulation) and lung disease (reduced circulating AAT).\n\n📹 [VIDEO LINK]"},{"id":355,"s":"Extracellular Matrix","t":"AAT Deficiency","u":"AAT Z Variant Liver Complication","q":"A young man with Z-variant alpha-1 antitrypsin deficiency has misfolded AAT accumulating in the endoplasmic reticulum of hepatocytes. This accumulation may lead to which complication in the liver?","o":["Carcinoma","Cirrhosis","Hepatitis","Steatosis","Thrombosis"],"a":1,"p":"AAT Z variant → misfolded AAT polymers accumulate in ER of hepatocytes → hepatocyte injury → progressive liver damage → CIRRHOSIS. The lung complication (emphysema) occurs due to reduced circulating AAT. Two organs affected: LIVER (ER accumulation → cirrhosis) and LUNGS (reduced AAT → elastase unopposed → emphysema). Treatment: intravenous AAT augmentation therapy. Liver transplant corrects the genetic defect.\n\n📹 [VIDEO LINK]"},{"id":356,"s":"Extracellular Matrix","t":"AAT Deficiency","u":"AAT PiZZ Genotype","q":"A 28-year-old non-smoker with alpha-1 antitrypsin deficiency develops panlobular emphysema predominantly affecting the lung bases. Which genotype is most likely responsible?","o":["PiMM","PiMS","PiMZ","PiSS","PiZZ"],"a":4,"p":"PiZZ = most SEVERE AAT deficiency (<15% normal AAT levels). Panlobular emphysema affecting lung BASES (compare with smoking-related centrilobular emphysema affecting UPPER lobes). Even non-smokers with PiZZ develop emphysema due to unchecked neutrophil elastase. PiMM = normal. PiMZ = heterozygote (usually asymptomatic). Panlobular + bases + non-smoker + young onset = classic PiZZ presentation.\n\n📹 [VIDEO LINK]"},{"id":357,"s":"Extracellular Matrix","t":"AAT Deficiency","u":"AAT PiMM Normal","q":"A 28-year-old non-smoker with alpha-1 antitrypsin deficiency (PiZZ genotype) develops emphysema. Which genotype represents a normal individual with full alpha-1 antitrypsin function?","o":["PiMM","PiMS","PiMZ","PiSS","PiSZ"],"a":0,"p":"AAT genotype nomenclature: PiMM = NORMAL (two normal M alleles, full AAT activity ~100%). PiZZ = most SEVERE deficiency (<15%). PiMZ = carrier (intermediate levels, usually asymptomatic). PiSZ = moderate deficiency (~35%). PiSS = mild deficiency (~60%). Pi = protease inhibitor. Z allele = Glu342Lys mutation. S allele = Glu264Val mutation. PiZZ is the most clinically significant genotype.\n\n📹 [VIDEO LINK]"},{"id":358,"s":"Plasma Proteins","t":"Immunoglobulins","u":"IgA Mucosal Protection","q":"A 4-year-old boy is exposed to infective droplets from a coughing friend but does not develop a throat infection. Which immunoglobulin in his mucosal secretions prevents attachment of pathogens to his respiratory epithelium?","o":["IgA","IgD","IgE","IgG","IgM"],"a":0,"p":"Secretory IgA (sIgA) is the dominant immunoglobulin in mucosal secretions (saliva, tears, breast milk, respiratory and GI secretions). Exists as a DIMER joined by J chain + secretory component (protects from proteolysis). Prevents pathogen attachment to mucosa (immune exclusion). Does NOT fix complement. First line of mucosal defence. IgA deficiency = most common primary immunodeficiency → recurrent sinopulmonary infections.\n\n[Diagram: sIgA dimer structure — two IgA monomers joined by J chain at Fc region, secretory component wrapping around the dimer, labelled: Fab arms (antigen binding), Fc region, J chain, secretory component]\n\n📹 [VIDEO LINK]"},{"id":359,"s":"Plasma Proteins","t":"Immunoglobulins","u":"IgM Complement Activation","q":"A medical student acquires throat infection after inhaling infective droplets. A serum immunoglobulin activates the complement system by the classical pathway and is the first to be produced in a primary immune response. Which of the following best describes this activated immunoglobulin?","o":["Dimeric structure with secretory component","Monomeric structure with four constant domains","Monomeric structure with three constant domains","Pentameric structure with J chain","Tetrameric structure with hinge region"],"a":3,"p":"IgM = PENTAMERIC structure (5 monomers joined by J chain) → 10 antigen binding sites → most efficient complement activator via classical pathway → C3b deposited on bacteria → phagocytosis enhanced. IgM does NOT directly opsonise — kills primarily through complement activation. First immunoglobulin of primary immune response. Compare: IgA = dimer with secretory component. IgG = monomer with 3 constant domains and hinge region.\n\n📹 [VIDEO LINK]"},{"id":360,"s":"Plasma Proteins","t":"Immunoglobulins","u":"IgE Worm Infestation","q":"A 3-year-old boy develops intestinal worm infestation. Which immunoglobulin is elevated in his blood and mediates defence against helminths?","o":["IgA","IgD","IgE","IgG","IgM"],"a":2,"p":"IgE mediates IMMEDIATE HYPERSENSITIVITY (Type I) and ANTI-HELMINTHIC DEFENCE. IgE binds Fc receptors on mast cells and basophils → allergen crosslinks IgE → degranulation → histamine, leukotrienes → allergic reaction. Against helminths: IgE on eosinophils → eosinophil degranulation → major basic protein → toxic to worm cuticle. IgE does NOT fix complement. Lowest serum concentration of all immunoglobulins. Elevated in: atopy, parasitic infections.\n\n📹 [VIDEO LINK]"},{"id":361,"s":"Plasma Proteins","t":"Immunoglobulins","u":"Heavy Chain Class Determination","q":"Which component of an immunoglobulin molecule determines its class and mediates its effector functions such as complement fixation and placental transfer?","o":["Fab region","Heavy chain","Hinge region","Kappa chain","Lambda chain"],"a":1,"p":"The HEAVY CHAIN determines immunoglobulin CLASS (isotype) and EFFECTOR FUNCTION. Heavy chain types: mu (IgM), gamma (IgG), alpha (IgA), epsilon (IgE), delta (IgD). Each class has different constant region domains → different effector functions: IgG = complement fixation + placental transfer + opsonisation. IgM = complement fixation (most efficient). IgA = mucosal protection. IgE = mast cell/eosinophil binding. Light chains do NOT determine class.\n\n[Diagram: Basic immunoglobulin Y-shape — two heavy chains (grey, forming stem and upper arms), two light chains (black, upper arms only). Fc region (stem) labelled: determines class and effector function. Fab region (arms) labelled: determines antigen binding. Hinge region between CH1 and CH2 labelled]\n\n📹 [VIDEO LINK]"},{"id":362,"s":"Plasma Proteins","t":"Immunoglobulins","u":"IgG Heavy Chain Domains","q":"The complete heavy chain of IgG (gamma chain) consists of which combination of domains?","o":["CH1, CH2, CH3","CH1, CH2, CH3, CH4","VH, CH1, CH2, CH3","VH, CH1, CH2, CH3, CH4","VH, CH1, CH3"],"a":2,"p":"Complete IgG heavy chain (gamma chain) = VH + CH1 + CH2 + CH3. VH = variable domain (antigen binding). CH1, CH2, CH3 = three constant domains. Complement fixation: CH2 domain binds C1q. Placental transfer (FcRn receptor): CH2-CH3 interface. Protein A binding (Staphylococcus): CH2-CH3. Compare IgM and IgE: mu and epsilon chains have VH + CH1 + CH2 + CH3 + CH4 (four constant domains, no hinge region).\n\n📹 [VIDEO LINK]"},{"id":363,"s":"Plasma Proteins","t":"Immunoglobulins","u":"IgM Heavy Chain Domains","q":"The complete heavy chain of IgM (mu chain) consists of which combination of domains?","o":["CH1, CH2, CH3","CH1, CH2, CH3, CH4","VH, CH1, CH2, CH3","VH, CH1, CH2, CH3, CH4","VH, CH1, CH3, CH4"],"a":3,"p":"IgM mu chain = VH + CH1 + CH2 + CH3 + CH4 (FOUR constant domains). IgE epsilon chain also has 4 constant domains. Both IgM and IgE lack a hinge region — the CH2 domain functionally compensates. IgG, IgA, IgD = 3 constant domains with a hinge region between CH1 and CH2. The extra constant domain in IgM and IgE replaces the absent hinge region.\n\n📹 [VIDEO LINK]"},{"id":364,"s":"Plasma Proteins","t":"Immunoglobulins","u":"Hinge Region Location","q":"The hinge region of an immunoglobulin molecule confers flexibility and allows both Fab arms to move independently. Between which constant domains is the hinge region located?","o":["Between CH1 and CH2","Between CH2 and CH3","Between CH3 and CH4","Between VH and CH1","Between VL and CL"],"a":0,"p":"Hinge region = flexible segment BETWEEN CH1 and CH2 of IgG (and IgA, IgD). Rich in proline and cysteine residues → proline provides flexibility, cysteine forms inter-heavy chain disulfide bonds. Hinge region is ABSENT in IgM and IgE (replaced by an extra CH2 domain). Papain cleaves ABOVE the hinge → two Fab fragments + one Fc fragment. Pepsin cleaves BELOW the hinge → one F(ab')2 fragment.\n\n[Diagram: IgG structure showing hinge region between CH1 and CH2 on both heavy chains, labelled: Fab arm, Hinge region (proline rich, flexible, disulfide bonds shown), Fc region (CH2 + CH3). Papain cleavage site (above hinge) and Pepsin cleavage site (below hinge) both labelled]\n\n📹 [VIDEO LINK]"},{"id":365,"s":"Plasma Proteins","t":"Plasma Protein Disorders","u":"Multiple Myeloma Bence Jones","q":"A 60-year-old man presents with bone pain, anaemia, and renal failure. Serum protein electrophoresis shows a sharp monoclonal spike in the gamma region equal to the albumin peak. Which protein is most likely present in his urine?","o":["Albumin","Bence Jones protein","Ceruloplasmin","Fibrinogen","Transferrin"],"a":1,"p":"Multiple myeloma = malignant proliferation of plasma cells → monoclonal immunoglobulin (M protein) → monoclonal spike on serum electrophoresis in gamma region. Bence Jones proteins = FREE LIGHT CHAINS (kappa or lambda) secreted into urine → detected by urine electrophoresis → cause renal tubular damage (cast nephropathy). Classic triad: bone pain (lytic lesions) + anaemia (marrow infiltration) + renal failure. Rouleaux formation on blood smear.\n\n📹 [VIDEO LINK]"},{"id":366,"s":"Plasma Proteins","t":"Albumin","u":"Albumin Fatty Acid Transport","q":"A man trapped in earthquake rubble has been without food for 5 days. Gluconeogenesis was exhausted after 72 hours and ketogenesis has now taken over as the primary energy source. Hormone-sensitive lipase is releasing free fatty acids from adipose tissue for beta-oxidation. Which plasma protein is transporting these free fatty acids to their destinations for beta-oxidation at this point in time?","o":["Albumin","Ceruloplasmin","Haptoglobin","Transferrin","Transthyretin"],"a":0,"p":"ALBUMIN transports up to 10 FREE FATTY ACIDS per molecule to peripheral tissues for beta-oxidation. After 72 hours of starvation gluconeogenesis becomes insufficient → ketogenesis takes over → adipose tissue releases free fatty acids via hormone-sensitive lipase → fatty acids bind albumin → transported to liver (ketone body synthesis) and muscle (direct beta-oxidation). Albumin also transports: unconjugated bilirubin, drugs (warfarin, aspirin), calcium, bile acids, thyroid hormones.\n\n📹 [VIDEO LINK]"},{"id":367,"s":"Plasma Proteins","t":"Albumin","u":"Albumin Oncotic Pressure Kwashiorkor","q":"An infant with kwashiorkor presents with generalised oedema and a protuberant abdomen. Which plasma protein is deficient in him causing these symptoms?","o":["Alpha-1 antitrypsin","Albumin","Ceruloplasmin","Fibrinogen","Immunoglobulin G"],"a":1,"p":"Kwashiorkor = protein malnutrition → ↓ dietary protein → ↓ liver albumin synthesis → hypoalbuminaemia → ↓ plasma oncotic pressure. Albumin normally holds water in capillaries by oncotic pressure. When albumin falls → water moves from intravascular → interstitial space → generalised oedema and ascites (protuberant abdomen). Fatty liver also occurs due to ↓ apolipoprotein synthesis → ↓ VLDL export. Distinguishing feature from Menkes disease: Menkes = copper deficiency → kinky hair + neurodegeneration + connective tissue defects. No oedema in Menkes.\n\n📹 [VIDEO LINK]"},{"id":368,"s":"Plasma Proteins","t":"Acute Phase Proteins","u":"CRP Positive Acute Phase","q":"A 7-year-old boy presents with sore throat, fever, and cervical lymphadenopathy due to bacterial tonsillitis. Which of the following plasma proteins is elevated as part of his acute phase response?","o":["Albumin","C-reactive protein","Haemoglobin","Transferrin","Transthyretin"],"a":1,"p":"POSITIVE acute phase reactants RISE during infection/inflammation: C-reactive protein (CRP) — rises first and fastest (100-1000 fold), made by liver, binds phosphocholine on bacteria → activates complement. Others: fibrinogen, haptoglobin, ceruloplasmin, alpha-1 antitrypsin, alpha-2 macroglobulin, alpha-1 acid glycoprotein, ferritin, serum amyloid A. NEGATIVE acute phase reactants FALL: albumin, transferrin, transthyretin.\n\n📹 [VIDEO LINK]"},{"id":369,"s":"Plasma Proteins","t":"Acute Phase Proteins","u":"Albumin Negative Acute Phase","q":"A 45-year-old woman with severe pneumonia has markedly elevated CRP and fibrinogen. Her serum albumin is unexpectedly low despite normal nutritional status before admission. Which best explains her hypoalbuminaemia?","o":["Increased renal excretion","Increased hepatic degradation","Negative acute phase response","Reduced dietary protein","Reduced intestinal absorption"],"a":2,"p":"Albumin is a NEGATIVE ACUTE PHASE REACTANT — its synthesis by the liver DECREASES during acute infection and inflammation. The liver reprioritises synthetic capacity toward positive acute phase reactants (CRP, fibrinogen, haptoglobin, ceruloplasmin). This is NOT due to malnutrition — it is a deliberate hepatic response. Negative acute phase reactants: albumin, transferrin, transthyretin (prealbumin), retinol-binding protein.\n\n📹 [VIDEO LINK]"},{"id":370,"s":"Plasma Proteins","t":"Acute Phase Proteins","u":"Ferritin Acute Phase Reactant","q":"A 45-year-old woman with rheumatoid arthritis has anaemia. Her serum ferritin is elevated at 850 ng/mL (normal 12-150) yet her serum iron is low. Which of the following best explains the elevated ferritin?","o":["Haemolytic anaemia","Increased dietary iron absorption","Iron deficiency anaemia","Positive acute phase response","Renal iron loss"],"a":3,"p":"FERRITIN is the primary intracellular iron STORAGE protein and is also a POSITIVE ACUTE PHASE REACTANT → rises during inflammation regardless of iron stores. In anaemia of chronic disease (rheumatoid arthritis, IBD, malignancy): ferritin elevated (inflammation) + serum iron low + TIBC low. This pattern distinguishes anaemia of chronic disease from iron deficiency anaemia (low ferritin + low iron + HIGH TIBC). Ferritin = reliable iron overload marker but unreliable in inflammation.\n\n📹 [VIDEO LINK]"},{"id":371,"s":"Plasma Proteins","t":"Transport Proteins","u":"Transferrin Iron Transport","q":"A 25-year-old woman presents with fatigue, pallor, and brittle nails. Blood tests show microcytic hypochromic anaemia with serum iron of 5 µmol/L (normal 10-30). She is diagnosed with iron deficiency. Levels of which plasma protein are likely to be increased in her blood?","o":["Albumin","Ceruloplasmin","Ferritin","Haemoglobin","Transferrin"],"a":4,"p":"In iron deficiency the liver upregulates TRANSFERRIN SYNTHESIS to maximise iron scavenging from whatever iron is available → elevated TIBC (total iron binding capacity). Iron deficiency pattern: ↓ serum iron + ↑ TIBC + ↓ ferritin + ↓ transferrin saturation. Compare anaemia of chronic disease: ↓ serum iron + ↓ TIBC + ↑ ferritin. Transferrin is a NEGATIVE ACUTE PHASE REACTANT — falls during inflammation.\n\n📹 [VIDEO LINK]"},{"id":372,"s":"Plasma Proteins","t":"Transport Proteins","u":"Haptoglobin Haemolysis Marker","q":"A 25-year-old man with G6PD deficiency develops acute intravascular haemolysis after eating fava beans. Free haemoglobin is released into his plasma. Which plasma protein protects his kidneys from haemoglobin-mediated damage by rapidly binding free haemoglobin?","o":["Albumin","Ceruloplasmin","Haptoglobin","Transferrin","Transthyretin"],"a":2,"p":"HAPTOGLOBIN binds FREE HAEMOGLOBIN in plasma → haptoglobin-haemoglobin complex → cleared by Kupffer cells in liver (CD163 receptor) → PREVENTS HAEMOGLOBIN-INDUCED RENAL TUBULAR DAMAGE (free haemoglobin is nephrotoxic → precipitates in tubules → acute tubular necrosis). When haemolysis is massive → haptoglobin completely consumed → free haemoglobin reaches kidneys → red/brown urine (haemoglobinuria) → renal damage. Low haptoglobin = reliable marker of intravascular haemolysis.\n\n📹 [VIDEO LINK]"},{"id":373,"s":"Plasma Proteins","t":"Transport Proteins","u":"Ceruloplasmin Copper Wilson Disease","q":"A 19-year-old woman presents with jaundice, tremor, and psychiatric symptoms. Slit-lamp examination reveals golden-brown rings at the corneal periphery. Serum copper is elevated. Which plasma protein is likely to be deficient in her?","o":["Albumin","Ceruloplasmin","Haptoglobin","Transferrin","Transthyretin"],"a":1,"p":"Kayser-Fleischer rings + liver disease + neuropsychiatric symptoms + elevated free copper + LOW ceruloplasmin = WILSON DISEASE (ATP7B mutation → copper cannot be incorporated into ceruloplasmin → ceruloplasmin falls → free copper deposits in liver, brain, cornea, kidney). CERULOPLASMIN normally carries 90% plasma copper. Diagnostic triad: low ceruloplasmin + elevated 24-hour urine copper + Kayser-Fleischer rings. Treatment: penicillamine.\n\n[Diagram: Frontal view of eye — iris visible centrally, golden-brown ring encircling the outer edge of the cornea at the limbus, labelled: Kayser-Fleischer ring = copper deposits in Descemet's membrane at corneal periphery. Caption: pathognomonic of Wilson disease when seen with neurological or hepatic features]\n\n📹 [VIDEO LINK]"},{"id":374,"s":"Plasma Proteins","t":"Immunoglobulins","u":"Monoclonal Antibody Therapeutic Use","q":"A 45-year-old woman with rheumatoid arthritis is started on a biologic therapy that is a monoclonal antibody targeting tumour necrosis factor alpha. Monoclonal antibodies used therapeutically are derived from a single B cell clone and therefore bind:","o":["All antigens nonspecifically","Multiple epitopes on multiple antigens","Multiple epitopes on one antigen","One epitope on one specific antigen","One epitope on several antigens"],"a":3,"p":"MONOCLONAL ANTIBODIES bind ONE SPECIFIC EPITOPE on ONE SPECIFIC ANTIGEN — produced from a single B cell clone (hybridoma technique). Examples: infliximab/adalimumab (anti-TNF-α → RA, IBD), rituximab (anti-CD20 → lymphoma, autoimmune), trastuzumab (anti-HER2 → breast cancer), bevacizumab (anti-VEGF → colorectal cancer). Suffix -mab = monoclonal antibody. Compare: POLYCLONAL antibodies = multiple B cell clones → multiple epitopes → mixture of antibodies (normal serum immunoglobulins, antivenom).\n\n📹 [VIDEO LINK]"},{"id":375,"s":"Plasma Proteins","t":"Immunoglobulins","u":"Lectin Pathway Complement","q":"A 2-year-old child has recurrent bacterial and fungal infections despite normal immunoglobulin levels and intact classical complement pathway. Investigations reveal he cannot opsonise pathogens with mannose-rich capsules without first generating antibodies. Which complement activation pathway is deficient in this child?","o":["Alternative pathway","Classical pathway","Lectin pathway","Lytic pathway","Terminal pathway"],"a":2,"p":"LECTIN PATHWAY: Mannose-Binding Lectin (MBL) binds mannose residues on pathogen surfaces → MBL-associated serine proteases (MASP-1, MASP-2) activated → cleave C4 and C2 → C3 convertase → C3b deposition → opsonisation. ANTIBODY-INDEPENDENT. MBL deficiency → cannot opsonise encapsulated organisms without antibody → recurrent infections. Normal immunoglobulins = classical pathway intact. Three pathways: CLASSICAL (Ag-Ab → C1q), LECTIN (MBL → MASP), ALTERNATIVE (spontaneous C3 hydrolysis). All converge at C3 convertase.\n\n[Diagram: Three complement pathways converging at C3 convertase — Left: Classical (Ag-Ab → C1q → C4, C2 → C3 convertase). Middle: Lectin (MBL binds mannose → MASP-1/2 → C4, C2 → C3 convertase) with RED X through MBL. Right: Alternative (spontaneous C3 hydrolysis → Factor B, D → C3 convertase). All three → C3b (opsonisation) + MAC (C5b-9) → lysis]\n\n📹 [VIDEO LINK]"},{"id":376,"s":"Plasma Proteins","t":"Plasma Protein Disorders","u":"Polyclonal vs Monoclonal Antibodies","q":"A 65-year-old man has bone pain, fatigue, and recurrent infections. Serum protein electrophoresis shows a sharp narrow spike in the gamma region. A 25-year-old healthy volunteer donating blood has a broad diffuse band in the gamma region on his electrophoresis. The difference between these two electrophoresis patterns reflects the difference between which types of antibody responses?","o":["Active and passive immunity","Cellular and humoral immunity","Innate and adaptive immunity","Monoclonal and polyclonal antibodies","Primary and secondary immune responses"],"a":3,"p":"MONOCLONAL (myeloma patient): single malignant plasma cell clone → all antibodies identical → same charge → same electrophoretic mobility → sharp narrow M-spike. POLYCLONAL (healthy donor): thousands of different B cell clones → different antibodies → different charges → range of mobilities → broad diffuse gamma band. Polyclonal hypergammaglobulinaemia (broad increase) = chronic infection, autoimmune disease, liver disease. Monoclonal spike = multiple myeloma, Waldenström macroglobulinaemia, MGUS.\n\n[Diagram: Two electrophoresis strip patterns side by side — Top (myeloma): albumin peak, then broad alpha/beta bands, then sharp tall narrow spike in gamma region labelled M-SPIKE = MONOCLONAL. Bottom (healthy): albumin peak, then broad diffuse gamma band labelled POLYCLONAL. Arrow M-spike: one clone, identical antibodies. Arrow broad band: thousands of clones, diverse antibodies]\n\n📹 [VIDEO LINK]"},{"id":377,"s":"Plasma Proteins","t":"Immunoglobulins","u":"Autoimmune vs Immunodeficiency","q":"A 25-year-old woman has recurrent joint pain, butterfly rash over her cheeks, and proteinuria. ANA is strongly positive. Her condition results from the immune system attacking which of the following?","o":["Environmental pathogens","External allergens","Foreign transplanted tissue","Self antigens","Tumour neoantigens"],"a":3,"p":"AUTOIMMUNE DISEASE = immune system attacks SELF ANTIGENS (loss of tolerance). SLE: anti-dsDNA + anti-Smith antibodies → immune complexes → deposit in kidneys (lupus nephritis), skin (butterfly rash), joints. Compare: IMMUNODEFICIENCY = immune system fails to respond to external pathogens → recurrent infections. Primary immunodeficiency = genetic (X-linked agammaglobulinaemia, SCID, DiGeorge). Secondary = acquired (HIV, chemotherapy). Autoimmunity (overactive) vs Immunodeficiency (underactive) = opposite ends of immune dysregulation.\n\n📹 [VIDEO LINK]"},{"id":378,"s":"Plasma Proteins","t":"Transport Proteins","u":"Hepcidin Iron Regulation","q":"A 65-year-old woman with chronic inflammation has anaemia of chronic disease. Elevated interleukin-6 from inflammation stimulates the liver to produce a peptide hormone that causes iron sequestration by reducing intestinal iron absorption and trapping iron inside macrophages. Which hormone is responsible?","o":["Albumin","Erythropoietin","Ferritin","Hepcidin","Transferrin"],"a":3,"p":"HEPCIDIN is the master regulator of iron homeostasis. Mechanism: hepcidin binds FERROPORTIN (the only known iron export protein) on enterocytes, macrophages, and hepatocytes → ferroportin internalised and degraded → iron trapped inside cells → cannot enter plasma. In inflammation: IL-6 → ↑ hepcidin → ↓ iron absorption + ↓ iron release from macrophages → low serum iron → anaemia of chronic disease. In iron deficiency and hypoxia: ↓ hepcidin → ↑ ferroportin → ↑ iron absorption.\n\n[Diagram: Hepcidin mechanism — Liver produces hepcidin → travels in blood → binds ferroportin on (1) intestinal enterocyte, (2) macrophage, (3) hepatocyte → ferroportin degraded → iron trapped inside cell. Normal state (low hepcidin): ferroportin present → iron exported → transferred to transferrin. Inflammation state (high hepcidin): ferroportin absent → iron sequestered → low serum iron → anaemia of chronic disease]\n\n📹 [VIDEO LINK]"},{"id":379,"s":"Protein Structure — Clinical Correlations","t":"Protein Misfolding Disorders","u":"CFTR Phe508 Deletion","q":"An 8-year-old girl with cystic fibrosis has thick mucosal secretions clogging her respiratory passages and pancreatic ducts. The CFTR protein is misfolded due to deletion of which amino acid at position 508?","o":["Alanine","Glycine","Leucine","Phenylalanine","Valine"],"a":3,"p":"CFTR most common mutation: ΔF508 = deletion of PHENYLALANINE at position 508 → CFTR protein misfolded → retained in ER → degraded by proteasome → no CFTR at cell surface → chloride channel absent → thick mucus in lungs and pancreas → recurrent pulmonary infections, pancreatic insufficiency, infertility (males). ΔF508 = 70% of CF alleles in northern Europeans. Diagnosis: sweat chloride test >60 mmol/L.\n\n📹 [VIDEO LINK]"},{"id":380,"s":"Protein Structure — Clinical Correlations","t":"Protein Misfolding Disorders","u":"PKU Proteasomal Degradation","q":"A 4-month-old boy is diagnosed with Phenylketonuria. The misfolded enzyme in this condition is degraded by which cellular system?","o":["Autophagy","Lysosomal degradation","Proteasomal degradation","Ribosomal quality control","Secretory pathway degradation"],"a":2,"p":"PKU: mutation in phenylalanine hydroxylase gene → misfolded phenylalanine hydroxylase → recognised by Hsp70 chaperone → if cannot be refolded → ubiquitinated → targeted to 26S PROTEASOME → degraded. This is the fate of most misfolded proteins: ER quality control → if unable to fold → ERAD (ER-associated degradation) → retrotranslocation to cytosol → ubiquitin-proteasome degradation. Consequence: no functional phenylalanine hydroxylase → phenylalanine accumulates → phenylketones → intellectual disability.\n\n📹 [VIDEO LINK]"},{"id":381,"s":"Protein Structure — Clinical Correlations","t":"Protein Misfolding Disorders","u":"LDL Receptor Misfolding","q":"A 40-year-old man with familial hypercholesterolaemia has premature atherosclerosis due to misfolded LDL receptors. The fate of these misfolded LDL receptors is:","o":["Autophagy","Export to plasma","Lysosomal degradation","Proteasomal degradation","Secretion into ECM"],"a":3,"p":"Misfolded LDL receptors in familial hypercholesterolaemia → retained in ER by quality control machinery (calnexin) → cannot be exported to cell surface → ubiquitinated → ERAD → PROTEASOMAL DEGRADATION. Consequence: no LDL receptors on hepatocyte surface → LDL cannot be cleared from blood → hypercholesterolaemia → atherosclerosis → premature MI. Treatment: statins (upregulate LDL receptor gene expression).\n\n📹 [VIDEO LINK]"},{"id":382,"s":"Protein Structure and Chemistry","t":"Protein Classification","u":"Structural Proteins","q":"Laminin, collagen, and elastin provide mechanical support to cells and tissues without enzymatic or signalling activity. These proteins are classified as:","o":["Derived proteins","Genomic caretaker proteins","Signalling proteins","Structural proteins","Transport proteins"],"a":3,"p":"Structural proteins provide mechanical support. Examples: LAMININ and COLLAGEN (basal lamina and ECM), elastin (elastic recoil), keratin (skin, hair, nails), actin and tubulin (cytoskeleton), spectrin (RBC membrane scaffold), dystrophin (muscle membrane scaffold). Compare: Signalling = GPCRs, kinases, nuclear receptors. Transport = albumin, transferrin, haemoglobin. Derived proteins = peptones, peptides (hydrolysis products). Conjugated = glycoproteins, lipoproteins, metalloproteins.\n\n📹 [VIDEO LINK]"},{"id":383,"s":"Protein Structure and Chemistry","t":"Protein Classification","u":"Signalling Proteins","q":"Which of the following is an example of a signalling protein?","o":["Collagen","G protein-coupled receptor","Laminin","Porin","Transferrin"],"a":1,"p":"Signalling proteins transmit information between or within cells. Examples: G protein-coupled receptors (GPCRs), receptor tyrosine kinases, G proteins (Ras, Gαs, Gαi), nuclear receptors (steroid hormone receptors), adenylyl cyclase, phospholipase C. Compare: structural = collagen, laminin. Transport = transferrin, porin. The GPCR superfamily is the largest family of cell surface receptors — target of ~34% of all approved drugs.\n\n📹 [VIDEO LINK]"},{"id":384,"s":"Protein Structure and Chemistry","t":"Protein Classification","u":"Transport Proteins","q":"Which of the following is an example of a transport protein?","o":["Catenin","Collagen","G protein-coupled receptor","Laminin","Porin"],"a":4,"p":"Transport proteins move ions, molecules, or electrons across membranes or through body fluids. Examples: PORINS (channel proteins in outer mitochondrial membrane, bacterial outer membrane), albumin (plasma transport), haemoglobin (oxygen), transferrin (iron), ceruloplasmin (copper), GLUT/SGLT (glucose transporters), aquaporins (water). Catenins = intracellular scaffolding/signalling proteins. Collagen and laminin = structural proteins.\n\n📹 [VIDEO LINK]"},{"id":385,"s":"Protein Structure and Chemistry","t":"Protein Classification","u":"Genomic Caretaker Proteins","q":"Which of the following best describes DNA polymerases?","o":["Derived proteins","Genomic caretaker proteins","Motor proteins","Signalling proteins","Structural proteins"],"a":1,"p":"Genomic caretaker proteins maintain genome integrity. DNA polymerases replicate DNA with high fidelity and participate in DNA repair. Other genomic caretaker proteins: mismatch repair proteins (MLH1, MSH2 — defective in Lynch syndrome → colorectal cancer), nucleotide excision repair proteins (defective in Xeroderma Pigmentosum), BRCA1/BRCA2 (repair scaffold — defective in hereditary breast/ovarian cancer), topoisomerases, helicases. These are true caretaker proteins — not transcription factors or tumour suppressors.\n\n📹 [VIDEO LINK]"},{"id":386,"s":"Protein Structure and Chemistry","t":"Axial Ratio","u":"Globular Protein Axial Ratio","q":"An enzyme assay measures a plasma protein with an axial ratio of 3 that functions as a catalyst in solution. Based on its axial ratio, this protein belongs to which class?","o":["Amyloid protein","Fibrous protein","Globular protein","Membrane protein","Structural protein"],"a":2,"p":"Axial ratio = length:width. GLOBULAR proteins have axial ratio <10 (typically 1-4) → compact, roughly spherical → water-soluble → buried hydrophobic core → exposed hydrophilic surface. Examples: all enzymes, albumin, immunoglobulins, haemoglobin, signalling proteins. FIBROUS proteins: axial ratio >10 → elongated → mostly insoluble → collagen, elastin, keratin, myosin. Enzymes are always globular — their active sites require precise 3D folding only achievable in globular conformation.\n\n📹 [VIDEO LINK]"},{"id":387,"s":"Protein Structure and Chemistry","t":"Axial Ratio","u":"Fibrous Protein Axial Ratio","q":"A protein has an axial ratio of 13. Based on this measurement, this protein is most likely classified as:","o":["Enzyme","Fibrous protein","Globular protein","Membrane protein","Transport protein"],"a":1,"p":"Axial ratio >10 = FIBROUS protein. Examples: collagen (triple helix, axial ratio ~200), elastin (cross-linked network), keratin (alpha helix coiled coil), myosin (rod domain), fibrinogen (elongated structure). Fibrous proteins are predominantly structural — provide tensile strength (collagen), elasticity (elastin), waterproofing (keratin). Their elongated shape means hydrophobic residues are distributed along the molecule → mostly insoluble in water → form fibrils or sheets.\n\n📹 [VIDEO LINK]"},{"id":388,"s":"Protein Structure and Chemistry","t":"Protein Structure","u":"Secondary Structure Alzheimer Historical","q":"In 1906 Dr. Alzheimer described a woman who died of unusual mental illness. Autopsy showed abnormal protein clumps and tangled fibres in her brain. Which level of protein structure was primarily affected in her brain proteins?","o":["Primary structure","Quaternary structure","Secondary structure","Tertiary structure","Transmembrane structure"],"a":2,"p":"The abnormal clumps (amyloid plaques) and tangled fibres (neurofibrillary tangles) in Alzheimer's disease result from SECONDARY STRUCTURE changes — normal proteins adopt abnormal beta pleated sheet conformation → insoluble → aggregate. Secondary structure = local regular folding patterns stabilised by HYDROGEN BONDS between backbone NH and C=O groups. Types: alpha helix (H-bonds parallel to helix axis), beta pleated sheet (H-bonds perpendicular between strands), beta bends, super-secondary structure (motifs).\n\n[Diagram: Four panels — (1) Alpha helix: spiral structure, H-bonds parallel to axis, 3.6 amino acids per turn labelled. (2) Beta pleated sheet: parallel and antiparallel arrows, H-bonds perpendicular between strands. (3) Beta bend: sharp turn connecting beta strands. (4) Disease panel: normal alpha helix → conformational change → abnormal beta sheet → aggregation → plaque/tangle]\n\n📹 [VIDEO LINK]"},{"id":389,"s":"Protein Structure and Chemistry","t":"Protein Folding and Misfolding","u":"Chaperone Proteins Hsp70 Hsp90","q":"A newly synthesised protein is partially unfolded and at risk of aggregation. Which class of proteins binds this unfolded polypeptide to prevent premature aggregation and assist correct folding?","o":["Chaperone proteins","Integrin proteins","Proteasome proteins","Ribosomal proteins","Ubiquitin proteins"],"a":0,"p":"CHAPERONE PROTEINS (Hsp70, Hsp90, Hsp60/GroEL) bind exposed hydrophobic regions of unfolded/partially folded proteins → prevent premature aggregation → assist correct folding in an ATP-dependent manner. Hsp70 = recognises unfolded proteins in cytosol and ER (BiP/GRP78 in ER lumen). Hsp90 = stabilises signalling proteins (steroid hormone receptors, kinases). Hsp60 = barrel-shaped, provides protected chamber for folding. Failure → misfolded proteins → proteasomal degradation OR aggregation → protein misfolding diseases (Alzheimer's, Parkinson's, CJD, CF).\n\n[Diagram: Chaperone cycle — unfolded protein with exposed hydrophobic patches → Hsp70 binds → ATP hydrolysis → protein partially folded → transferred to Hsp60 barrel → protein enters protected chamber → correct folding → native protein released. Alternative pathway: if cannot fold → ubiquitination → proteasomal degradation]\n\n📹 [VIDEO LINK]"},{"id":390,"s":"Protein Structure and Chemistry","t":"Amino Acid Classification","u":"Hydroxyl Side Chains Phosphorylation","q":"A 35-year-old man with type 2 diabetes has elevated blood glucose. Insulin binds its receptor and activates a cascade that phosphorylates specific amino acid residues on downstream proteins to promote glucose uptake. Which amino acid on the insulin receptor is phosphorylated when insulin binds?","o":["Arginine","Aspartate","Glutamate","Serine","Tyrosine"],"a":4,"p":"Insulin receptor = receptor TYROSINE KINASE. Insulin binding → receptor autophosphorylates TYROSINE residues on its own intracellular domain → IRS-1 phosphorylated → PI3K → Akt → GLUT4 translocation. Tyrosine kinases phosphorylate tyrosine (hydroxyl group on aromatic ring). Serine/threonine kinases (PKA, PKC, CDKs) phosphorylate serine and threonine (hydroxyl group on aliphatic chain). Three phosphorylatable residues: SERINE, THREONINE, TYROSINE — all have hydroxyl (–OH) side chains.\n\n📹 [VIDEO LINK]"},{"id":391,"s":"Protein Structure and Chemistry","t":"Amino Acid Classification","u":"Sulphur Side Chains Met Cys","q":"Cigarette smoke damages alpha-1 antitrypsin by oxidising a sulphur-containing amino acid at position 358, rendering the protein unable to inhibit elastase. Which amino acid contains the sulphur-containing side chain oxidised by cigarette smoke?","o":["Alanine","Cysteine","Glycine","Methionine","Proline"],"a":3,"p":"METHIONINE contains a thioether side chain (–CH₂–CH₂–S–CH₃) → oxidised by reactive oxygen species in cigarette smoke → methionine sulphoxide → AAT cannot bind elastase → emphysema. CYSTEINE also contains sulphur (–CH₂–SH, thiol group) → forms DISULFIDE BONDS (–S–S–) in immunoglobulins, insulin, keratin, albumin → stabilises tertiary structure. Two sulphur-containing amino acids: METHIONINE (thioether, start codon AUG, methyl donor via SAM) and CYSTEINE (thiol, disulfide bonds, active site of cysteine proteases).\n\n📹 [VIDEO LINK]"},{"id":392,"s":"Protein Structure and Chemistry","t":"Amino Acid Classification","u":"Proline Helix Breaker","q":"A 30-year-old man with Osteogenesis Imperfecta has a glycine substitution in his collagen. Substitution of glycine by which of the following amino acids causes the most severe OI due to its helix-breaking properties?","o":["Alanine","Leucine","Proline","Serine","Valine"],"a":2,"p":"PROLINE substituting for glycine in the Gly-X-Y repeat causes the most severe collagen disruption because: (1) proline is larger than glycine → steric clash in central axis. (2) Proline has a rigid pyrrolidine ring → helix breaker → disrupts the regular Gly-X-Y periodicity that the triple helix depends on. Proline cannot participate in normal hydrogen bonding → cannot be accommodated at the central axis position. Proline is an IMINO ACID. Essential in collagen: Gly-X-Pro repeat where hydroxyproline at Y position stabilises triple helix.\n\n📹 [VIDEO LINK]"},{"id":393,"s":"Protein Structure and Chemistry","t":"Amino Acid Classification","u":"Glycine Achiral Collagen","q":"A 30-year-old man with Osteogenesis Imperfecta has multiple bone fractures. His collagen has glycine substitutions in the triple helix. Glycine must occupy every third position because it is the only amino acid without a side chain. This makes glycine the only amino acid that is also:","o":["Acidic","Achiral","Aromatic","Basic","Ketogenic"],"a":1,"p":"GLYCINE is the only ACHIRAL amino acid — its alpha carbon has two identical hydrogen atoms → no asymmetric carbon → no L or D isomer. All other amino acids in proteins are L-isomers. Glycine is also: smallest amino acid (H side chain), fits into sterically restricted spaces (collagen central axis, active sites, beta turns), inhibitory neurotransmitter in spinal cord (glycine receptor — target of strychnine poisoning), precursor of haem synthesis (glycine + succinyl-CoA → ALA synthase → first step of porphyrin synthesis).\n\n[Diagram: Collagen triple helix cross-section — three chains coiled around each other. Central axis labelled: only Gly fits here (side chain = H only). Outer positions X and Y showing proline and hydroxyproline with larger side chains. Gly-Pro-Hyp repeat labelled on one chain. Arrow: Gly→Ala mutation → steric clash → helix disrupted → OI]\n\n📹 [VIDEO LINK]"},{"id":394,"s":"Protein Structure and Chemistry","t":"Amino Acid Classification","u":"Positively Charged Arg Lys His","q":"Cholera toxin ADP-ribosylates G protein alpha-s at arginine 201 causing permanent activation of adenylyl cyclase. Pertussis toxin ADP-ribosylates G protein alpha-i at cysteine 351. Histidine is the only amino acid that changes its ionisation state around physiological pH. Which of the following best describes histidine?","o":["Acidic amino acid","Aromatic amino acid","Basic amino acid","Neutral amino acid","Sulphur-containing amino acid"],"a":2,"p":"Histidine is classified as a BASIC amino acid (along with lysine and arginine) due to its imidazole side chain (pKa ~6.0). What makes histidine uniquely important: it is the ONLY amino acid that changes its ionisation state around physiological pH (7.4) → the primary physiological buffer and most common amino acid in enzyme active sites. Histidine 146 of haemoglobin beta chain mediates the Bohr effect — protonation of histidine by CO₂-driven pH drop promotes oxygen release in tissues.\n\n📹 [VIDEO LINK]"},{"id":395,"s":"Protein Structure and Chemistry","t":"Amino Acid Classification","u":"Negatively Charged Asp Glu","q":"The Z variant of alpha-1 antitrypsin results from substitution of a negatively charged amino acid at position 342 by lysine. Which negatively charged amino acid is replaced?","o":["Aspartate","Cysteine","Glutamate","Histidine","Serine"],"a":2,"p":"AAT Z variant: GLUTAMATE (Glu342) replaced by lysine → negative charge replaced by positive charge → local electrostatic disruption → AAT misfolds. GLUTAMATE and ASPARTATE are the two negatively charged amino acids at physiological pH (pKa ~4.0 and ~3.9 respectively → fully deprotonated → carry –1 charge at pH 7.4). Glutamate is also substituted in SICKLE CELL DISEASE (Glu6Val in beta-globin). Aspartate = active site of beta-secretase (BACE1) in Alzheimer's disease.\n\n📹 [VIDEO LINK]"},{"id":396,"s":"Protein Structure and Chemistry","t":"Amino Acid Classification","u":"Aromatic Amino Acids UV Absorption","q":"A laboratory measures protein concentration by UV spectrophotometry at 280 nm. A sample with high protein content shows strong absorbance. Which amino acid contributes most to UV absorbance at 280 nm due to its indole ring system?","o":["Histidine","Leucine","Phenylalanine","Tryptophan","Tyrosine"],"a":3,"p":"TRYPTOPHAN (indole ring) has the strongest UV absorbance at 280 nm (molar extinction coefficient ~5500 M⁻¹cm⁻¹). TYROSINE also absorbs at 280 nm (~1490 M⁻¹cm⁻¹). PHENYLALANINE absorbs weakly at 257 nm. Protein concentration at A280 primarily reflects tryptophan content. Tryptophan = precursor of serotonin and niacin. Tyrosine = precursor of dopamine, noradrenaline, adrenaline, melanin, thyroid hormones. Phenylalanine accumulates in PKU.\n\n📹 [VIDEO LINK]"},{"id":397,"s":"Protein Structure and Chemistry","t":"Amino Acid Classification","u":"Isoelectric Point Haemoglobin Electrophoresis","q":"A 35-year-old woman with sickle cell disease undergoes haemoglobin electrophoresis. Haemoglobin S migrates differently from haemoglobin A because valine replaces glutamate at position 6, removing a negative charge. At which condition would haemoglobin S and haemoglobin A have the same net charge, making electrophoretic separation impossible?","o":["At their combined isoelectric point","At their individual isoelectric points","At pH 14","At pH 2","At the isoelectric point of haemoglobin A"],"a":1,"p":"ISOELECTRIC POINT (pI) = pH at which a protein has ZERO NET CHARGE → no migration in electric field → separation impossible at this pH. HbA and HbS have DIFFERENT pI values because HbS has Glu→Val substitution (removes negative charge) → HbS has slightly higher pI than HbA. At alkaline pH (8.6 — standard haemoglobin electrophoresis): HbA has greater negative charge → migrates faster toward anode. HbS migrates slower → separated from HbA → diagnosis confirmed.\n\n📹 [VIDEO LINK]"},{"id":398,"s":"Protein Structure and Chemistry","t":"Amino Acid Classification","u":"Essential Amino Acids","q":"A 3-year-old child from a food-insecure household presents with stunted growth and hypoalbuminaemia. Deficiency of which of the following categories of amino acids cannot be corrected by endogenous synthesis and must come from diet?","o":["Acidic amino acids","Basic amino acids","Essential amino acids","Glucogenic amino acids","Nonpolar amino acids"],"a":2,"p":"ESSENTIAL AMINO ACIDS cannot be synthesised by humans → must come from diet. Mnemonic PVT TIM HaLL: Phenylalanine, Valine, Tryptophan, Threonine, Isoleucine, Methionine, Histidine (conditionally essential), Arginine (conditionally essential in growth/stress), Leucine, Lysine. Tryptophan deficiency → pellagra (niacin deficiency). Phenylalanine accumulates in PKU. Lysine and leucine = only purely KETOGENIC essential amino acids.\n\n📹 [VIDEO LINK]"},{"id":399,"s":"Protein Structure and Chemistry","t":"Amino Acid Classification","u":"Purely Ketogenic Amino Acids Leu Lys","q":"Muhammad Ali Sadpara was trapped at the Bottleneck on K2 at 8,400 metres for 5 days without food in February 2021. Catabolism of which of the following amino acids may have contributed towards ketone body formation in him?","o":["Alanine and glutamine","Aspartate and glutamate","Isoleucine and phenylalanine","Leucine and lysine","Threonine and valine"],"a":3,"p":"LEUCINE and LYSINE are the ONLY purely KETOGENIC amino acids — their carbon skeletons are converted exclusively to acetyl-CoA or acetoacetyl-CoA → ketone bodies only. They CANNOT contribute to gluconeogenesis. All other amino acids are either purely glucogenic (alanine, glutamine, aspartate) or both glucogenic and ketogenic (Phe, Tyr, Trp, Ile, Thr). Memory aid: LL = Leucine and Lysine = purely ketogenic.\n\n📹 [VIDEO LINK]"},{"id":400,"s":"Extracellular Matrix","t":"Collagen Structure","u":"Proline Lysine Hydroxylation Scurvy","q":"A 25-year-old woman presents with bleeding gums, perifollicular haemorrhages, and a non-healing wound after minor surgery. Her surgeon notes that wound edges cannot hold sutures because collagen is structurally defective. Hydroxylation of which two amino acids in the collagen Gly-X-Y repeat is impaired in her condition?","o":["Alanine and glycine","Glycine and lysine","Lysine and arginine","Proline and lysine","Serine and threonine"],"a":3,"p":"Scurvy (Vitamin C deficiency): prolyl hydroxylase and lysyl hydroxylase both inactive → PROLINE not hydroxylated → no hydroxyproline → collagen triple helix unstable → unravels → weak collagen → bleeding and poor wound healing. LYSINE not hydroxylated → no hydroxylysine → no crosslinking sites → collagen fibrils mechanically weak → sutures pull through tissue. Both hydroxylations require Vitamin C as cofactor for Fe²⁺ maintenance.\n\n📹 [VIDEO LINK]"},{"id":401,"s":"Protein Structure and Chemistry","t":"Amino Acid Classification","u":"Tyrosine Dopamine Precursor Parkinson","q":"A 55-year-old man with Parkinson's disease has progressive tremor, rigidity, and bradykinesia. Impaired metabolism of which of the following amino acids is contributing to decreased dopamine in his brain?","o":["Glycine","Histidine","Tryptophan","Tyrosine","Valine"],"a":3,"p":"TYROSINE → DOPA (tyrosine hydroxylase, rate-limiting, requires BH4) → Dopamine (DOPA decarboxylase) → Noradrenaline (dopamine beta-hydroxylase, copper) → Adrenaline (PNMT, SAM). In Parkinson's disease: dopaminergic neurons of substantia nigra degenerate → tyrosine metabolism to dopamine impaired → dopamine deficiency → motor features. Levodopa (L-DOPA) bypasses rate-limiting tyrosine hydroxylase and crosses the blood-brain barrier.\n\n[Diagram: Tyrosine pathway — Tyrosine → (tyrosine hydroxylase, BH4, rate-limiting) → L-DOPA → (DOPA decarboxylase) → Dopamine (labelled: DEFICIENT in Parkinson's) → (dopamine β-hydroxylase, copper) → Noradrenaline → (PNMT, SAM) → Adrenaline. Side branch: Tyrosine → (tyrosinase, copper) → Melanin. Levodopa entry point shown bypassing tyrosine hydroxylase. Separate: Tryptophan → Serotonin → Melatonin. Histidine → Histamine]\n\n📹 [VIDEO LINK]"},{"id":402,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Vitamin D Synthesis UV Light","q":"A 25-year-old woman from northern latitudes with insufficient ultraviolet light exposure presents with backache and vitamin D levels of 10 ng/mL. Failure of formation of which of the following compounds is the primary precipitating factor for her backache?","o":["1,25 dihydroxycholecalciferol","7-dehydrocholesterol","Calcidiol","Calcitriol","Cholecalciferol"],"a":4,"p":"UV light converts 7-dehydrocholesterol in skin → CHOLECALCIFEROL (Vitamin D3). Without UV light → cholecalciferol cannot be formed → downstream compounds (calcidiol in liver, calcitriol in kidney) also fail → vitamin D deficiency → impaired calcium absorption → backache and bone pain. Vitamin D synthesis pathway: 7-dehydrocholesterol (skin, UV light) → Cholecalciferol → 25-hydroxylase (liver) → Calcidiol (25-OH D3) → 1-alpha-hydroxylase (kidney) → Calcitriol (1,25-OH D3) — the active form.\n\n[Diagram: Vitamin D synthesis pathway — Skin: 7-dehydrocholesterol → (UV light) → Cholecalciferol. Blood transport to Liver: Cholecalciferol → (25-hydroxylase) → Calcidiol (25-OH D3). Blood transport to Kidney: Calcidiol → (1-alpha-hydroxylase, activated by PTH) → Calcitriol (1,25-OH D3) = ACTIVE FORM. Red X at UV light step in this patient. Each step labelled with organ and enzyme.]\n\n📹 [VIDEO LINK]"},{"id":403,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Vitamin D Deficiency Wide Fontanelle","q":"An 18-month-old boy is brought for routine check-up. He has a wide anterior fontanelle and is exclusively breastfed with minimal sunlight exposure. Supplementation with which of the following vitamins is most urgently required?","o":["Vitamin A","Vitamin B12","Vitamin D","Vitamin E","Vitamin K"],"a":2,"p":"Wide anterior fontanelle + exclusively breastfed + minimal sunlight = VITAMIN D DEFICIENCY. Breast milk contains very little vitamin D. Sunlight is the primary source. Vitamin D deficiency in infants → impaired calcium absorption → hypocalcaemia → rickets → wide fontanelle, delayed tooth eruption, bowing of legs, frontal bossing, rachitic rosary. All exclusively breastfed infants should receive vitamin D supplementation (400 IU/day).\n\n📹 [VIDEO LINK]"},{"id":404,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Rickets Bone ECM Hydroxyapatite","q":"A 1-year-old boy presents with bowing of legs, frontal bossing, and bead-like prominences at the costochondral junctions. His mother reports exclusive breastfeeding for 8 months and avoids taking him outdoors. Which component of his bone extracellular matrix is primarily deficient?","o":["Collagen fibrils","Elastin fibres","Fibronectin networks","Glycosaminoglycans","Hydroxyapatite crystals"],"a":4,"p":"Rickets = Vitamin D deficiency → impaired calcium and phosphate absorption → decreased mineralisation of bone ECM → insufficient HYDROXYAPATITE CRYSTAL deposition (Ca₁₀(PO₄)₆(OH)₂) → soft bones → bowing, frontal bossing, rachitic rosary. Bone ECM: MINERAL = hydroxyapatite crystals. ORGANIC = collagen type I (osteoid), fibronectin, laminin, proteoglycans. The organic matrix (osteoid) is present but unmineralised in rickets.\n\n📹 [VIDEO LINK]"},{"id":405,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"PTH Phosphaturia","q":"A 1-year-old boy with rickets has elevated parathyroid hormone. Elevated PTH in this child acts on the kidney to decrease reabsorption of which of the following?","o":["Bicarbonate","Calcium","Glucose","Phosphate","Sodium"],"a":3,"p":"PTH actions on kidney: (1) INCREASES calcium reabsorption (distal tubule). (2) DECREASES phosphate reabsorption → phosphaturia → lowers serum phosphate → prevents calcium-phosphate precipitation → allows serum calcium to rise. (3) Activates 1-alpha-hydroxylase → calcitriol → increases intestinal calcium absorption. In rickets: vitamin D deficiency → hypocalcaemia → ↑ PTH → phosphaturia + calcium reabsorption + calcitriol production → compensatory attempt to restore calcium.\n\n📹 [VIDEO LINK]"},{"id":406,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"PTH Calcium Reabsorption","q":"A 1-year-old boy with rickets has elevated parathyroid hormone. Elevated PTH in this child acts on the kidney to increase reabsorption of which of the following minerals?","o":["Calcium","Chloride","Magnesium","Phosphate","Sodium"],"a":0,"p":"PTH increases CALCIUM reabsorption in the distal convoluted tubule by activating calcium channels (TRPV5) and calcium-binding proteins (calbindin). Simultaneously PTH causes phosphate excretion (phosphaturia) in the proximal tubule. Net result: calcium retained, phosphate excreted. Without PTH → hypocalcaemia → tetany.\n\n📹 [VIDEO LINK]"},{"id":407,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"PTH 1-Alpha Hydroxylase Activation","q":"A malnourished 2-year-old with rickets has low serum calcium, low serum phosphate, and elevated PTH. Elevated PTH activates which enzyme in the kidney to increase calcitriol production?","o":["1-alpha hydroxylase","24-hydroxylase","25-hydroxylase","Alpha hydroxylase","Dehydrogenase"],"a":0,"p":"PTH activates 1-ALPHA HYDROXYLASE in the proximal tubule of the kidney → converts calcidiol (25-OH D3) → calcitriol (1,25-OH D3) → active vitamin D → increased intestinal calcium and phosphate absorption. 25-hydroxylase is in the LIVER (not regulated by PTH). 24-hydroxylase is activated by HIGH calcitriol → converts calcitriol to inactive 24,25-dihydroxy D3 (degradation pathway). PTH and low phosphate STIMULATE 1-alpha hydroxylase. High calcitriol and high calcium INHIBIT it.\n\n📹 [VIDEO LINK]"},{"id":408,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"PTH Bone Demineralisation","q":"A malnourished 2-year-old with rickets has elevated PTH. Which of the following best describes the main effect of elevated PTH on bone in this child?","o":["Bone calcification","Bone demineralisation","Bone metastasis","Bone mineralisation","Bone ossification"],"a":1,"p":"Elevated PTH → stimulates OSTEOCLAST activity + inhibits osteoblast activity → BONE DEMINERALISATION → releases calcium and phosphate from bone → attempts to restore serum calcium. In rickets this is a secondary compensatory response to hypocalcaemia. Chronic PTH excess → severe bone demineralisation → pathological fractures. Compare: normal pulsatile PTH = anabolic effect on bone. Continuous elevated PTH = catabolic effect → bone resorption.\n\n📹 [VIDEO LINK]"},{"id":409,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Calcitriol Bone Mineralisation Healthy Infant","q":"A healthy infant receives adequate vitamin D from sunlight and diet. Vitamin D (calcitriol) in this healthy infant promotes which of the following?","o":["Bone demineralisation","Bone metastasis","Bone mineralisation","Bone resorption","Osteoclast activation"],"a":2,"p":"In a HEALTHY infant with adequate vitamin D: calcitriol → increased intestinal calcium and phosphate absorption → adequate serum levels → BONE MINERALISATION → hydroxyapatite crystal deposition into osteoid → strong bones. Contrast with RICKETS: insufficient calcitriol → inadequate calcium/phosphate → bone demineralisation. Calcitriol binds VDR (nuclear receptor Type 2) → gene transcription → calbindin synthesis → calcium absorption.\n\n📹 [VIDEO LINK]"},{"id":410,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Renal Osteodystrophy 1-Alpha Hydroxylase","q":"An anaemic 14-year-old with chronic kidney disease presents with bowing of legs and recalcitrant hypocalcaemia. Serum vitamin D3 is 10 ng/mL. Which enzyme is malfunctioning in this patient due to renal osteodystrophy?","o":["1-alpha hydroxylase","24-hydroxylase","25-hydroxylase","1,25-hydroxylase","Alpha hydroxylase"],"a":0,"p":"Renal osteodystrophy: chronic kidney disease → loss of renal tubular cells → 1-ALPHA HYDROXYLASE deficient → cannot convert calcidiol → calcitriol → vitamin D deficiency despite adequate sunlight and liver function → hypocalcaemia → secondary hyperparathyroidism → bone disease. Treatment: activated vitamin D (calcitriol or alfacalcidol) given directly, bypassing the deficient renal 1-alpha hydroxylase.\n\n📹 [VIDEO LINK]"},{"id":411,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Hypocalcaemia Trousseau Sign","q":"A first-year medical student develops carpopedal spasm (flexion of wrist, thumb, and metacarpophalangeal joints) when a sphygmomanometer cuff is inflated on her arm during skills lab practice. Decreased ionised levels of which of the following is the likely biochemical basis?","o":["Serum calcium","Serum magnesium","Serum phosphate","Serum potassium","Serum sodium"],"a":0,"p":"Trousseau sign = carpopedal spasm induced by ischaemia (inflated BP cuff) = classic sign of HYPOCALCAEMIA. Decreased ionised calcium → decreased threshold potential → increased neuronal excitability → spontaneous action potentials → tetany → carpopedal spasm, laryngospasm, Chvostek sign (facial twitch on tapping). Causes: hypoparathyroidism, vitamin D deficiency, hypomagnesaemia (blocks PTH secretion and action), acute pancreatitis.\n\n📹 [VIDEO LINK]"},{"id":412,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Magnesium Refractory Hypocalcaemia","q":"A 44-year-old woman presents with muscle cramps, perioral tingling, and carpopedal spasm. Her serum calcium is 6.8 mg/dL (normal 8.5–10.5 mg/dL) and PTH is 18 pg/mL (normal 10–60 pg/mL) and these symptoms are refractory to calcium and vitamin D supplementation. Which of the following minerals is likely to be deficient in her?","o":["Calcium","Magnesium","Manganese","Phosphorus","Zinc"],"a":1,"p":"MAGNESIUM deficiency causes REFRACTORY HYPOCALCAEMIA: (1) Mg²⁺ required for PTH SECRETION — hypomagnesaemia impairs PTH release from parathyroid glands (inappropriately low PTH for degree of hypocalcaemia). (2) Mg²⁺ required for PTH ACTION on target tissues (PTH receptor-G protein signalling requires Mg²⁺). Therefore calcium and vitamin D supplementation FAILS until magnesium is replenished. Classic scenario: alcoholic patient or malabsorption. Treatment: intravenous magnesium sulphate.\n\n📹 [VIDEO LINK]"},{"id":413,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Hypercalcaemia Bone Metastasis Low PTH","q":"A 70-year-old woman with breast cancer presents with abdominal pain and backache. Serum calcium = 3.96 mmol/L (normal 2.15–2.55), serum phosphate = 1.12 mmol/L (normal), serum PTH = 10 ng/L (low). Which of the following best describes her electrolyte disturbance?","o":["Hypocalcaemia","Hypercalcaemia","Hypochloraemia","Hypophosphataemia","Hyperphosphataemia"],"a":1,"p":"Elevated calcium + LOW PTH = PTH-independent hypercalcaemia. Bone metastasis from breast cancer → osteolytic lesions → calcium released from bone → HYPERCALCAEMIA. PTH is suppressed by the high calcium (negative feedback). Other causes with LOW PTH: hypervitaminosis D, sarcoidosis (ectopic 1-alpha hydroxylase), PTHrP-secreting tumours. Hypercalcaemia symptoms: bones (pain), stones (renal), groans (abdominal), psychic moans (confusion).\n\n📹 [VIDEO LINK]"},{"id":414,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Primary Hyperparathyroidism","q":"A 58-year-old postmenopausal woman presents with bone pain and constipation. Serum calcium = 2.96 mmol/L (normal 2.15–2.55), serum phosphate = 0.62 mmol/L (normal 0.80–1.35), serum PTH = 157 ng/L (normal 20–65). Which condition does this pattern indicate?","o":["Hyperthyroidism","Hypoparathyroidism","Primary hyperparathyroidism","Secondary hyperparathyroidism","Tertiary hyperparathyroidism"],"a":2,"p":"Primary hyperparathyroidism: HIGH calcium + LOW phosphate + HIGH PTH = autonomous PTH secretion (parathyroid adenoma 80-85%, hyperplasia 15%). Classic presentation: postmenopausal woman with hypercalcaemia and phosphaturia. Most common cause of hypercalcaemia in OUTPATIENTS. Secondary hyperparathyroidism = LOW calcium (renal failure/vitamin D deficiency) → ↑ PTH appropriately. Tertiary = longstanding secondary → parathyroid gland becomes autonomous.\n\n📹 [VIDEO LINK]"},{"id":415,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Alcohol Hypocalcaemia Magnesium","q":"A 60-year-old woman with alcohol use disorder presents with muscle cramps and perioral tingling. Serum calcium is 6.4 mg/dL (normal 8.4–10.2) and PTH is 21 pg/mL (normal 10–60). Symptoms are refractory to calcium supplementation. Which investigation is the best next step?","o":["Serum albumin","Serum calcitriol","Serum creatinine","Serum magnesium","Serum phosphate"],"a":3,"p":"Alcohol use disorder → HYPOMAGNESAEMIA (poor dietary intake + renal magnesium wasting by alcohol). Magnesium deficiency → impaired PTH secretion → PTH inappropriately low/normal despite hypocalcaemia → REFRACTORY hypocalcaemia. The inappropriately low PTH for the degree of hypocalcaemia indicates magnesium deficiency is impairing the parathyroid response. Best next step = SERUM MAGNESIUM. Treatment: magnesium replacement → PTH secretion restored → calcium normalises.\n\n📹 [VIDEO LINK]"},{"id":416,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Hypervitaminosis D Hypercalcaemia","q":"A 62-year-old health-conscious woman routinely takes high-dose vitamin D supplements for general wellbeing. She is incidentally found to have serum calcium of 2.90 mmol/L (normal 2.15–2.55) on routine blood tests. She reports no symptoms. Repeat testing confirms persistent hypercalcaemia. Which of the following is the most likely cause?","o":["Hypoparathyroidism","Hypervitaminosis D","Iron deficiency","Renal tubular acidosis","Vitamin D deficiency"],"a":1,"p":"HYPERVITAMINOSIS D from excessive supplementation causes hypercalcaemia — excess calcitriol → excessive intestinal calcium absorption + increased bone resorption → elevated serum calcium. Often asymptomatic when mild but can cause nausea, vomiting, weakness, polyuria, nephrocalcinosis. The history of high-dose vitamin D supplementation is the key diagnostic clue. Compare: RICKETS/OSTEOMALACIA = LOW vitamin D → LOW calcium. OSTEOPOROSIS = NORMAL calcium (structural bone loss). HYPOPARATHYROIDISM = LOW calcium.\n\n📹 [VIDEO LINK]"},{"id":417,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Osteoporosis T-Score","q":"An 80-year-old man falls on an outstretched hand and fractures his wrist. DEXA scan shows T-score of -2.5 SD at neck of femur and -3.0 SD at vertebral column. Serum calcium and phosphate are normal. Which condition is the most likely precipitating factor?","o":["Osteoarthritis","Osteomalacia","Osteopenia","Osteopetrosis","Osteoporosis"],"a":4,"p":"T-score ≤ -2.5 = OSTEOPOROSIS (WHO definition). T-score -1.0 to -2.5 = OSTEOPENIA. T-score ≥ -1.0 = NORMAL. This patient has T-score -2.5 and -3.0 = OSTEOPOROSIS. Normal calcium and phosphate = structural bone loss without mineralisation defect (unlike osteomalacia where calcium/phosphate are low). Osteoporosis = reduced QUANTITY of normally mineralised bone. Osteomalacia = reduced QUALITY of mineralisation (unmineralised osteoid).\n\n📹 [VIDEO LINK]"},{"id":418,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Postmenopausal Osteoporosis Osteoclast","q":"A 60-year-old oestrogen-deficient postmenopausal woman fractures her hip after minor trauma. The underlying precipitating factor is increased activity of which of the following cells?","o":["Chondroblast","Chondrocyte","Osteoblast","Osteoclast","Osteocyte"],"a":3,"p":"Oestrogen normally INHIBITS osteoclast activity by suppressing RANKL expression and promoting osteoprotegerin (OPG). Oestrogen deficiency (postmenopause) → ↑ RANKL → ↑ osteoclast activation → ↑ bone resorption → osteoporosis → fractures. Treatment: bisphosphonates (inhibit osteoclasts), SERMs (raloxifene), denosumab (anti-RANKL antibody), teriparatide (intermittent PTH). Osteoblasts BUILD bone (reduced in osteoporosis). Osteoclasts RESORB bone (increased in osteoporosis).\n\n📹 [VIDEO LINK]"},{"id":419,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Osteoarthritis Heberden Bouchard Nodes","q":"A 60-year-old male presents with knee pain and stiffness worse at end of day. Bony swellings are observed at the distal interphalangeal joints and proximal interphalangeal joints of his hands. Which condition does this presentation indicate?","o":["Osteoarthritis","Osteomalacia","Osteopenia","Osteopetrosis","Osteoporosis"],"a":0,"p":"Heberden nodes (bony swellings at DISTAL interphalangeal joints — DIP) + Bouchard nodes (bony swellings at PROXIMAL interphalangeal joints — PIP) + pain worse at END OF DAY (mechanical) = OSTEOARTHRITIS. OA = degenerative joint disease — cartilage loss → subchondral bone exposure → osteophyte formation. Compare RA: morning stiffness >1 hour, MCP and PIP joints (NOT DIP), symmetrical, systemic features, no Heberden nodes.\n\n[Diagram: Outline of right hand — DIP joints highlighted in red with swellings labelled HEBERDEN NODES (OA). PIP joints highlighted in blue with swellings labelled BOUCHARD NODES (OA). MCP joints highlighted in green labelled: RHEUMATOID ARTHRITIS (MCP + PIP, spares DIP). Two columns below: OA features vs RA features for comparison.]\n\n📹 [VIDEO LINK]"},{"id":420,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Renal Disease Low Ca High Phosphate","q":"A 45-year-old man with a 5-year history of chronic kidney disease presents with bone pain and proximal muscle weakness. Which of the following lab patterns is most consistent with his condition?","o":["High calcium, high phosphate, high ALP","High calcium, low phosphate, normal ALP","Low calcium, high phosphate, normal ALP","Low calcium, low phosphate, high ALP","Normal calcium, normal phosphate, high ALP"],"a":2,"p":"Chronic kidney disease → renal osteodystrophy: ↓ 1-alpha hydroxylase → ↓ calcitriol → ↓ calcium absorption → LOW calcium. ↓ GFR → phosphate retention → HIGH phosphate. ALP NORMAL initially. Compare: RICKETS/OSTEOMALACIA = ↓ Ca + ↓ phosphate + ↑ ALP. PAGET'S DISEASE = normal Ca + normal phosphate + MARKEDLY ↑ ALP. PRIMARY HYPERPARATHYROIDISM = ↑ Ca + ↓ phosphate + ↑ ALP. The CKD pattern: low Ca + HIGH phosphate (phosphate retention unique to renal disease) + normal ALP initially.\n\n📹 [VIDEO LINK]"},{"id":421,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Calcitonin Inhibits Osteoclasts","q":"A 45-year-old woman with hypercalcaemia from hyperparathyroidism is given a hormone to lower her serum calcium. This hormone is produced by parafollicular C cells of the thyroid and acts by inhibiting osteoclasts. Which hormone is this?","o":["Aldosterone","Calcitonin","Calcitriol","Cortisol","Parathyroid hormone"],"a":1,"p":"CALCITONIN produced by parafollicular C cells of the thyroid → inhibits OSTEOCLAST activity → ↓ bone resorption → ↓ serum calcium. Physiological antagonist of PTH on bone. Also promotes renal calcium and phosphate excretion. Clinical uses: Paget's disease, hypercalcaemia of malignancy, osteoporosis (second-line). Medullary thyroid carcinoma = C cell tumour → very elevated calcitonin = tumour marker.\n\n📹 [VIDEO LINK]"},{"id":422,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Osteomalacia vs Osteoporosis","q":"A 40-year-old woman presents with bone pain and muscle weakness. Serum calcium is low, serum phosphate is low, and alkaline phosphatase is elevated. Her bones show unmineralised osteoid on biopsy. This condition in adults is called:","o":["Osteoarthritis","Osteomalacia","Osteopenia","Osteopetrosis","Osteoporosis"],"a":1,"p":"OSTEOMALACIA (adults) = RICKETS (children) = deficient mineralisation of newly formed bone matrix (osteoid). Lab: ↓ Ca + ↓ phosphate + ↑ ALP (↑ osteoblast activity). Causes: vitamin D deficiency, malabsorption, renal disease, anticonvulsants. OSTEOPOROSIS = REDUCED QUANTITY of normally mineralised bone → normal Ca, normal phosphate, normal/mildly elevated ALP. OSTEOPETROSIS = INCREASED bone density (failed osteoclast activity). Key: mineralisation defect = osteomalacia. Quantity defect = osteoporosis.\n\n📹 [VIDEO LINK]"},{"id":423,"s":"Vitamins and Minerals","t":"Calcium and Vitamin D","u":"Vitamin D Liver First Hydroxylation","q":"A 55-year-old man with chronic liver disease secondary to hepatitis C presents with bone pain and proximal muscle weakness. He has adequate sun exposure. Despite being prescribed standard vitamin D supplements for 6 months his symptoms have not improved and his vitamin D levels remain low. Which of the following biochemical steps is most likely impaired in this patient?","o":["1-alpha hydroxylation","1,25 hydroxylation","Calcidiol formation","Calcitriol formation","Cholecalciferol formation"],"a":2,"p":"Standard vitamin D supplements provide CHOLECALCIFEROL. Cholecalciferol must be converted to CALCIDIOL by 25-HYDROXYLASE in the LIVER — the first hydroxylation step. Chronic liver disease → hepatocyte loss → 25-hydroxylase deficient → CALCIDIOL FORMATION impaired → no substrate for kidney → calcitriol also falls → vitamin D deficiency despite adequate sun exposure AND supplementation. Treatment: give CALCIDIOL directly (bypasses liver). Compare: renal disease → 1-alpha hydroxylation fails → calcitriol formation impaired → give CALCITRIOL directly.\n\n[Diagram: Vitamin D synthesis pathway — Skin: 7-dehydrocholesterol → (UV light) → Cholecalciferol (working normally, green tick). LIVER: Cholecalciferol → (25-hydroxylase) → Calcidiol (25-OH D3) — RED X: 25-hydroxylase deficient in chronic liver disease → CALCIDIOL FORMATION BLOCKED. KIDNEY: Calcidiol → (1-alpha-hydroxylase) → Calcitriol (1,25-OH D3 = ACTIVE FORM) — dotted arrow, no substrate. Standard supplement arrow enters at cholecalciferol → blocked at liver. Calcidiol supplement arrow bypasses liver. Compare box: Renal disease → 1-alpha hydroxylation blocked → give calcitriol directly.]\n\n📹 [VIDEO LINK]"},{"id":424,"s":"Vitamins and Minerals","t":"Phosphorus and Magnesium","u":"Refeeding Syndrome Gaza Child","q":"A 6-year-old severely malnourished child from Gaza is admitted and started on a high-calorie feeding regimen. On day 3 he develops muscle weakness, confusion, and cardiac arrhythmia. Which of the following electrolyte changes is most likely responsible?","o":["Hypercalcaemia","Hyperkalemia","Hypermagnesemia","Hypernatraemia","Hypophosphataemia"],"a":4,"p":"REFEEDING SYNDROME: severely malnourished patient → high-calorie feeding → insulin released → glucose enters cells → phosphate used to phosphorylate carbohydrate intermediates (glucose-6-phosphate, ATP synthesis) → HYPOPHOSPHATAEMIA → impaired ATP production → muscle weakness, confusion, cardiac arrhythmia, respiratory failure. Also: hypokalaemia, hypomagnesaemia, thiamine deficiency. Prevention: start feeding slowly, monitor and supplement phosphate. Dietary supplement: milk (rich in phosphate).\n\n📹 [VIDEO LINK]"},{"id":425,"s":"Vitamins and Minerals","t":"Phosphorus and Magnesium","u":"Refeeding Syndrome Carbohydrate Phosphorylation","q":"A 72-year-old man with oesophageal cancer has negligible oral intake for 8 days. On day 5 of nutritional support he develops psychomotor agitation, respiratory distress, and cardiogenic shock. Phosphorylation of which of the following compounds during refeeding is responsible for his deteriorating condition?","o":["Amino acid carbon skeletons","Carbohydrate intermediates","Fatty acid esters","Ketone bodies","Nucleic acid residues"],"a":1,"p":"Refeeding syndrome: carbohydrate reintroduction → insulin spike → glucose enters cells → phosphorylation of CARBOHYDRATE INTERMEDIATES (glucose → glucose-6-phosphate in glycolysis, ADP → ATP) rapidly consumes intracellular phosphate → plasma phosphate falls → hypophosphataemia → ATP depletion → cardiac muscle fails (cardiogenic shock) → respiratory muscle fails (respiratory distress) → neuronal dysfunction (psychomotor agitation). Milk given as phosphate supplement.\n\n📹 [VIDEO LINK]"},{"id":426,"s":"Vitamins and Minerals","t":"Phosphorus and Magnesium","u":"Refeeding Syndrome Marasmus Thar","q":"A 10-month-old boy with Marasmus from Thar region is started on refeeding and presents with hypophosphataemia. The biochemical reason is that available phosphate has been utilised to phosphorylate which of the following?","o":["Amino acid residues","Amino acid carbon skeletons","Carbohydrate intermediates","Fatty acid esters","Ketone bodies"],"a":2,"p":"Refeeding hypophosphataemia in marasmus: prolonged starvation → phosphate depleted from cells. Refeeding with carbohydrates → insulin spike → glucose uptake → phosphate used to phosphorylate CARBOHYDRATE INTERMEDIATES in glycolysis → rapid intracellular shift → serum phosphate falls. Phosphate supplement (milk) must be given before and during refeeding. Start with 10-20% of caloric requirement on day 1 and increase gradually.\n\n📹 [VIDEO LINK]"},{"id":427,"s":"Vitamins and Minerals","t":"Phosphorus and Magnesium","u":"Aluminium Antacids Phosphorus Deficiency","q":"A 65-year-old woman routinely consumes aluminium-containing antacids for heartburn. She is likely to develop deficiency of which of the following minerals?","o":["Calcium","Iron","Magnesium","Phosphorus","Sodium"],"a":3,"p":"Aluminium-containing antacids bind PHOSPHATE in the intestinal lumen → form insoluble aluminium phosphate → phosphate cannot be absorbed → HYPOPHOSPHATAEMIA. Used therapeutically in CKD (phosphate binders reduce hyperphosphataemia). However chronic use in normal individuals → phosphate depletion → bone demineralisation → osteomalacia. Aluminium antacids also reduce gastric acid → impair iron absorption (iron requires acidic environment). PRIMARY deficiency from aluminium antacids = phosphorus.\n\n📹 [VIDEO LINK]"},{"id":428,"s":"Vitamins and Minerals","t":"Phosphorus and Magnesium","u":"Magnesium Kinase Cofactor","q":"A chronic alcohol user is admitted with muscle cramps, tetany, and cardiac arrhythmias. Serum magnesium is low. Magnesium deficiency is suspected. Which of the following enzyme functions is most likely to be impaired in him?","o":["Hydrolase activity","Isomerase activity","Kinase activity","Ligase activity","Lyase activity"],"a":2,"p":"Magnesium forms the Mg-ATP complex that serves as the actual substrate for ALL KINASE REACTIONS — kinases transfer phosphate groups from ATP to target molecules and all require Mg²⁺ to chelate the ATP beta and gamma phosphates. Without Mg²⁺: kinases cannot function → impaired signal transduction → PTH signalling fails → refractory hypocalcaemia. Also: DNA and RNA polymerases require Mg²⁺ for phosphodiester bond formation. Magnesium = cofactor for more than 300 enzymatic reactions.\n\n📹 [VIDEO LINK]"},{"id":429,"s":"Vitamins and Minerals","t":"Phosphorus and Magnesium","u":"Magnesium Polymerase Cofactor","q":"Which of the following enzymes requires magnesium as a cofactor for phosphodiester bond formation?","o":["Helicase","Polymerase","Reverse transcriptase","Topoisomerase","Transcriptase"],"a":1,"p":"DNA and RNA POLYMERASES require Mg²⁺ as cofactor for phosphodiester bond formation — Mg²⁺ coordinates the phosphate groups of the incoming nucleotide and the 3'-OH of the growing chain → facilitates nucleophilic attack → phosphodiester bond forms. All polymerases (DNA pol I, II, III, RNA polymerase, reverse transcriptase) require Mg²⁺ or Mn²⁺. Helicase unwinds DNA (ATP-dependent but no phosphodiester bond formation). Topoisomerase relieves torsional stress.\n\n📹 [VIDEO LINK]"},{"id":430,"s":"Vitamins and Minerals","t":"Phosphorus and Magnesium","u":"PTH Calcium and Phosphate Combined","q":"Which of the following best describes PTH action on calcium and phosphate in the kidney?","o":["Calcium and phosphate excretion","Calcium and phosphate reabsorption","Calcium excretion and phosphate reabsorption","Calcium reabsorption and phosphate excretion","Phosphate reabsorption only"],"a":3,"p":"PTH acts on kidney: CALCIUM reabsorption (distal convoluted tubule → activates TRPV5 channels + calbindin → calcium retained) + PHOSPHATE excretion (proximal tubule → internalises NaPi cotransporters → phosphaturia). Net: calcium rises, phosphate falls → prevents calcium-phosphate precipitation. PTH also activates 1-alpha hydroxylase → calcitriol → more intestinal calcium absorption. The COMBINATION of calcium reabsorption + phosphaturia is the KEY distinguishing feature of PTH action.\n\n📹 [VIDEO LINK]"},{"id":431,"s":"Vitamins and Minerals","t":"Iron Metabolism","u":"Iron Deficiency Lab Pattern","q":"A 19-year-old female presents with fatigue, exertional dyspnoea, and craving for ice. She reports heavy menstrual periods. Examination shows pale conjunctiva and spoon-shaped nails. Which of the following lab changes is most consistent with her condition?","o":["Decreased serum TIBC","Decreased serum iron","Increased hepcidin","Increased serum ferritin","Increased transferrin saturation"],"a":1,"p":"Iron deficiency anaemia lab pattern: ↓ serum iron + ↑ TIBC (transferrin elevated to scavenge available iron) + ↓ ferritin (iron stores depleted) + ↓ transferrin saturation. Koilonychia (spoon-shaped nails), pica (craving for ice/clay), glossitis, angular cheilitis = classic iron deficiency features. Hepcidin FALLS in iron deficiency (to allow maximum intestinal iron absorption). Microcytic hypochromic anaemia on blood smear.\n\n📹 [VIDEO LINK]"},{"id":432,"s":"Vitamins and Minerals","t":"Iron Metabolism","u":"Iron Deficiency Lab Profile","q":"A 45-year-old man with peptic ulcer disease and long-term NSAID use presents with Hb 9 g/dL and MCV 70 fL. Which of the following lab profiles is most likely?","o":["Decreased ferritin, decreased TIBC, decreased serum iron","Decreased ferritin, increased TIBC, decreased serum iron","Increased ferritin, decreased TIBC, decreased serum iron","Increased ferritin, increased TIBC, increased serum iron","Normal ferritin, normal TIBC, decreased serum iron"],"a":1,"p":"Iron deficiency anaemia from chronic blood loss: ↓ ferritin (iron stores depleted — most sensitive early marker) + ↑ TIBC (liver upregulates transferrin synthesis) + ↓ serum iron. Compare: anaemia of chronic disease = ↓ serum iron + ↓ TIBC + NORMAL/↑ ferritin (iron sequestered in macrophages by hepcidin). Haemochromatosis = ↑ serum iron + ↓ TIBC + ↑ ferritin + ↑ transferrin saturation. ↓ ferritin + ↑ TIBC = pathognomonic of iron deficiency.\n\n📹 [VIDEO LINK]"},{"id":433,"s":"Vitamins and Minerals","t":"Iron Metabolism","u":"Anaemia of Chronic Disease Iron Sequestration","q":"A 60-year-old male with rheumatoid arthritis presents with anaemia. Lab results show low serum iron, low TIBC, and elevated ferritin. Which of the following is the most likely underlying mechanism?","o":["Decreased hepcidin production","Decreased transferrin synthesis","Increased intestinal iron absorption","Increased iron sequestration in macrophages","Increased transferrin synthesis"],"a":3,"p":"Anaemia of chronic disease (ACD): inflammation → IL-6 → ↑ HEPCIDIN → hepcidin degrades ferroportin on macrophages → macrophages cannot release iron → IRON SEQUESTERED IN MACROPHAGES → low serum iron despite adequate stores. Lab pattern: ↓ serum iron + ↓ TIBC (transferrin is a negative acute phase reactant) + NORMAL/↑ ferritin (iron trapped in macrophages). ACD is the most common anaemia in hospitalised patients. Treatment: treat underlying disease.\n\n📹 [VIDEO LINK]"},{"id":434,"s":"Vitamins and Minerals","t":"Iron Metabolism","u":"Haemochromatosis Hepcidin Deficiency","q":"A 48-year-old man presents with fatigue, joint pain, and skin hyperpigmentation. Investigations reveal elevated serum ferritin and transferrin saturation. Genetic testing confirms a disorder causing excessive intestinal iron absorption. Decreased activity of which protein is most likely responsible for the increased intestinal iron absorption?","o":["Albumin","Ceruloplasmin","Ferritin","Hepcidin","Hephaestin"],"a":3,"p":"Hereditary haemochromatosis (HFE mutation): ↓ HEPCIDIN → ferroportin NOT degraded → iron freely exported from intestinal cells → excessive iron absorption → iron overload → deposited in liver (cirrhosis), heart (cardiomyopathy), pancreas (bronze diabetes), skin (hyperpigmentation), joints, pituitary. Lab: ↑ serum iron + ↑ ferritin + ↑ transferrin saturation (>45%) + ↓ TIBC. Treatment: phlebotomy.\n\n[Diagram: Iron absorption in enterocyte — Lumen: dietary Fe³⁺ → (DCYTB + Vitamin C) → Fe²⁺ → enters enterocyte via DMT1 (apical membrane) → stored as Ferritin OR → exported via Ferroportin (basolateral membrane) → Hephaestin oxidises Fe²⁺ → Fe³⁺ → binds Transferrin in blood → delivered to bone marrow/liver. HEPCIDIN shown: High iron/inflammation → Liver produces Hepcidin → Hepcidin binds Ferroportin → Ferroportin degraded → iron trapped in enterocyte. In haemochromatosis: HFE mutation → LOW Hepcidin → Ferroportin preserved → excess iron absorbed → iron overload organs shown.]\n\n📹 [VIDEO LINK]"},{"id":435,"s":"Vitamins and Minerals","t":"Iron Metabolism","u":"Iron Storage Ferritin Enterocyte","q":"Iron enters intestinal mucosal cells via DMT1 on the apical membrane. Which of the following proteins stores iron temporarily within the intestinal mucosal cell?","o":["Ceruloplasmin","Ferritin","Haemosiderin","Transferrin","Transthyretin"],"a":1,"p":"Iron absorption pathway: Dietary Fe³⁺ → reduced to Fe²⁺ by duodenal cytochrome b (DCYTB, requires Vitamin C) → Fe²⁺ enters enterocyte via DMT1 → stored intracellularly as FERRITIN → when body needs iron → ferroportin exports Fe²⁺ from enterocyte → hephaestin oxidises Fe²⁺ → Fe³⁺ → binds transferrin in blood. If iron is not needed → stored as ferritin in enterocyte → shed when enterocyte is shed into intestinal lumen. Ferritin = temporary storage that regulates how much iron actually reaches circulation.\n\n📹 [VIDEO LINK]"},{"id":436,"s":"Vitamins and Minerals","t":"Iron Metabolism","u":"Iron DMT1 Duodenum Absorption","q":"Iron enters intestinal mucosal cells via DMT1 on the apical membrane. Which of the following proteins exports iron across the basolateral membrane into the bloodstream?","o":["DMT1","Ferritin","Ferroportin","Hepcidin","Transferrin"],"a":2,"p":"Iron absorption route: Lumen → Fe²⁺ enters enterocyte via DMT1 (apical) → stored as FERRITIN inside enterocyte OR → exported from BASOLATERAL membrane via FERROPORTIN → hephaestin oxidises Fe²⁺ → Fe³⁺ → binds TRANSFERRIN in plasma → delivered to tissues. DMT1 = ENTRY (apical). FERROPORTIN = EXIT (basolateral). FERRITIN = intracellular STORAGE. TRANSFERRIN = PLASMA TRANSPORT. HEPCIDIN = REGULATOR (degrades ferroportin → reduces export).\n\n📹 [VIDEO LINK]"},{"id":437,"s":"Vitamins and Minerals","t":"Iron Metabolism","u":"Iron Absorption Increased Iron Deficiency","q":"In which of the following conditions is intestinal iron absorption increased 2 to 10 times compared to normal?","o":["Aplastic anaemia","Iron deficiency anaemia","Megaloblastic anaemia","Sickle cell anaemia","Thalassaemia"],"a":1,"p":"Iron deficiency anaemia → ↓ hepcidin → ferroportin NOT degraded → ↑ DMT1 expression → iron absorption increased 2-10 times. Pregnancy also increases iron absorption. Haemolytic anaemias (sickle cell, thalassaemia) increase absorption due to ineffective erythropoiesis → iron overload risk. Aplastic anaemia: ↓ erythropoiesis → ↑ hepcidin → DECREASED iron absorption. Megaloblastic anaemia: iron stores normal or increased — no stimulus for increased absorption.\n\n📹 [VIDEO LINK]"},{"id":438,"s":"Vitamins and Minerals","t":"Iron Metabolism","u":"Sideroblastic Anaemia Pyridoxine","q":"A hypochromic microcytic anaemia with increased iron stores in the bone marrow is seen in a patient. This anaemia is most likely responsive to which of the following?","o":["Folic acid","Iron","Pyridoxine","Vitamin B12","Vitamin C"],"a":2,"p":"Hypochromic microcytic anaemia + INCREASED bone marrow iron stores = SIDEROBLASTIC ANAEMIA. Iron is present but cannot be incorporated into haem because ALA SYNTHASE is deficient (requires pyridoxal phosphate/Vitamin B6). Iron accumulates in mitochondria → ring sideroblasts on bone marrow smear. PYRIDOXINE (Vitamin B6) responsive. Causes: hereditary (X-linked, ALAS2 mutation), acquired (isoniazid blocks B6, lead poisoning, alcohol). Iron supplementation would worsen iron overload.\n\n📹 [VIDEO LINK]"},{"id":439,"s":"Vitamins and Minerals","t":"Iron Metabolism","u":"Haemochromatosis Transferrin Saturation","q":"A 52-year-old man with haemochromatosis presents with fatigue, joint pain, and skin hyperpigmentation. Lab findings reveal increased serum iron and decreased TIBC. Which of the following lab findings is most consistent with iron overload?","o":["Decreased serum ferritin","Decreased transferrin saturation","Increased TIBC","Increased transferrin saturation","Normal serum iron"],"a":3,"p":"Haemochromatosis lab pattern: ↑ serum iron + ↑ ferritin + INCREASED TRANSFERRIN SATURATION (>45%, often >80%) + ↓ TIBC (transferrin downregulated when iron stores are full). Transferrin saturation = (serum iron / TIBC) × 100. In iron overload: iron occupies all transferrin binding sites → high saturation. Tissue deposition: liver (cirrhosis), pancreas (bronze diabetes), heart (cardiomyopathy), skin (hyperpigmentation), joints, pituitary. Classic triad: cirrhosis + diabetes + skin pigmentation.\n\n📹 [VIDEO LINK]"},{"id":440,"s":"Vitamins and Minerals","t":"Iron Metabolism","u":"Iron Storage Ferritin vs Haemosiderin","q":"A 55-year-old man with hereditary haemochromatosis has serum ferritin of 2500 ng/mL (normal 12-150). His liver biopsy is sent for histological analysis. The pathologist notes the iron is stored in a readily mobilisable form that can be released when needed. Which is this storage form?","o":["Ferritin","Haemoglobin","Haemosiderin","Heme","Transferrin"],"a":0,"p":"FERRITIN = primary intracellular iron storage form — readily mobilisable. Shell of 24 apoferritin subunits stores up to 4500 Fe³⁺ atoms. HAEMOSIDERIN = insoluble, aggregated, partially digested ferritin → forms when iron OVERLOAD exceeds ferritin capacity → seen in haemochromatosis → Prussian blue positive → LESS readily mobilisable than ferritin. In early haemochromatosis = ferritin elevated. In severe overload = haemosiderin deposits. The question specifies READILY MOBILISABLE → ferritin.\n\n📹 [VIDEO LINK]"},{"id":441,"s":"Vitamins and Minerals","t":"Iron Metabolism","u":"Hepcidin Degrades Ferroportin","q":"A 65-year-old woman with anaemia of chronic disease secondary to rheumatoid arthritis has low serum iron despite adequate dietary intake and normal total body iron stores. IL-6 secreted by inflamed synovial tissue is driving macrophages to retain iron intracellularly. Which of the following peptides is mediating this iron sequestration?","o":["Aldosterone","Erythropoietin","Ferritin","Hepcidin","Transferrin"],"a":3,"p":"HEPCIDIN is a PEPTIDE HORMONE produced by the liver. In inflammation: IL-6 → liver → ↑ HEPCIDIN PRODUCTION → hepcidin binds FERROPORTIN on macrophages → ferroportin internalised and degraded → macrophages RETAIN iron → low serum iron despite adequate stores → anaemia of chronic disease. Note: hepcidin is functioning NORMALLY here — the problem is EXCESSIVE production driven by chronic inflammation, not impaired function.\n\n📹 [VIDEO LINK]"},{"id":442,"s":"Vitamins and Minerals","t":"Iron Metabolism","u":"Ferroportin Basolateral Export","q":"Iron enters intestinal mucosal cells via DMT1 on the apical membrane. A 28-year-old woman with iron deficiency anaemia secondary to coeliac disease is unable to absorb iron normally. Her gastroenterologist explains that even when iron enters the intestinal mucosal cell, a specific protein must export it across the basolateral membrane into the bloodstream. Which of the following proteins performs this basolateral export?","o":["DMT1","Ferritin","Ferroportin","Hepcidin","Transferrin"],"a":2,"p":"Iron absorption route: Fe²⁺ enters enterocyte via DMT1 (apical) → stored as ferritin OR → FERROPORTIN exports Fe²⁺ from basolateral membrane → hephaestin oxidises Fe²⁺ → Fe³⁺ → binds transferrin in plasma. DMT1 = ENTRY (apical). FERROPORTIN = EXIT (basolateral). FERRITIN = intracellular storage. TRANSFERRIN = plasma transport. HEPCIDIN = regulator (degrades ferroportin). In coeliac disease: damaged intestinal mucosa → impaired absorption at multiple steps.\n\n📹 [VIDEO LINK]"},{"id":443,"s":"Vitamins and Minerals","t":"Iron Metabolism","u":"Ferroportin DMT1 Distinction","q":"Iron enters intestinal enterocytes via DMT1 on the apical membrane. Iron is then transported across the basolateral membrane into the portal blood. Which of the following proteins performs this basolateral export?","o":["DMT1","Ferritin","Ferroportin","Hepcidin","Transferrin"],"a":2,"p":"DMT1 (Divalent Metal Transporter 1) = APICAL ENTRY of Fe²⁺ into enterocyte. FERROPORTIN = BASOLATERAL EXPORT of Fe²⁺ from enterocyte into portal blood. HEPCIDIN degrades ferroportin → blocks export → iron trapped in enterocyte → reduced serum iron. FERRITIN = intracellular storage (enterocyte sheds with iron inside when ferroportin is blocked). TRANSFERRIN = plasma iron transport protein (picks up Fe³⁺ after hephaestin oxidation). Ferroportin is the ONLY known iron exporter from cells.\n\n📹 [VIDEO LINK]"},{"id":444,"s":"Vitamins and Minerals","t":"Iron Metabolism","u":"Iron Deficiency Increased Absorption Pregnancy","q":"A 35-year-old woman presents to a haematology clinic with microcytic hypochromic anaemia and fatigue. Her haematologist notes her serum iron is very low and explains that her body is now maximising iron extraction from whatever dietary iron is available. In which of the following conditions is intestinal iron absorption increased 2 to 10 times compared to normal?","o":["Aplastic anaemia","Iron deficiency anaemia","Megaloblastic anaemia","Sickle cell anaemia","Thalassaemia"],"a":1,"p":"Iron deficiency anaemia → ↓ hepcidin → ferroportin NOT degraded → ↑ DMT1 expression → iron absorption increased 2-10 times. The body maximises extraction of every available dietary iron atom. Pregnancy also increases iron absorption (placental hormones suppress hepcidin). Haemolytic anaemias also increase absorption due to ineffective erythropoiesis → iron overload risk. Aplastic anaemia: ↓ erythropoiesis → ↑ hepcidin → DECREASED iron absorption.\n\n📹 [VIDEO LINK]"},{"id":445,"s":"Vitamins and Minerals","t":"Copper Metabolism","u":"Menkes Disease Dopamine Beta Hydroxylase","q":"A 4-year-old boy with kinky hair presents with progressive neurodegeneration, low urinary and serum free copper, and an occipital horn on skull X-ray. Activity of which of the following enzymes is most likely impaired?","o":["Arginase","Asparaginase","Dopamine beta-hydroxylase","Glucocerebrosidase","Lysyl oxidase"],"a":2,"p":"Menkes disease (ATP7A mutation) → copper cannot be exported from intestinal cells → LOW plasma copper → deficient copper-dependent enzymes: DOPAMINE BETA-HYDROXYLASE (→ cannot hydroxylate dopamine → noradrenaline → neurodegeneration), lysyl oxidase (→ defective collagen/elastin crosslinking → occipital horn, vascular aneurysms), tyrosinase (→ depigmented kinky hair), cytochrome c oxidase (→ ATP deficiency), superoxide dismutase. Arginase = manganese-containing. Asparaginase = no metal cofactor. Glucocerebrosidase = no metal cofactor.\n\n📹 [VIDEO LINK]"},{"id":446,"s":"Vitamins and Minerals","t":"Copper Metabolism","u":"Wilson's Disease Kayser-Fleischer","q":"A 4-year-old boy with hepatic dysfunction, Kayser-Fleischer rings, and elevated urinary and serum copper is likely to be suffering from which condition?","o":["Cretinism","Graves disease","Menkes syndrome","Parkinson's disease","Wilson's disease"],"a":4,"p":"Wilson's disease (ATP7B mutation) → copper cannot be incorporated into ceruloplasmin NOR excreted into bile → copper accumulates in LIVER (cirrhosis) → leaks into blood → deposited in BRAIN (psychiatric symptoms, tremor, dysarthria), CORNEA (Kayser-Fleischer rings), KIDNEY (Fanconi syndrome), SKIN. Lab: ↑ serum free copper + ↑ 24-hour urine copper + ↓ ceruloplasmin. Compare Menkes: LOW copper. Wilson's: HIGH copper/HIGH urine copper but LOW ceruloplasmin.\n\n📹 [VIDEO LINK]"},{"id":447,"s":"Vitamins and Minerals","t":"Copper Metabolism","u":"Wilson's Disease Bile Excretion Defect","q":"In Wilson's disease, copper accumulates because of which primary defect?","o":["Decreased biliary copper excretion","Decreased renal copper excretion","Decreased urinary copper excretion","Increased hepatic copper synthesis","Increased intestinal copper absorption"],"a":0,"p":"Wilson's disease (ATP7B mutation) → defective ATP7B copper-transporting ATPase in hepatocytes → CANNOT excrete copper into BILE → BILIARY EXCRETION FAILS → copper accumulates in liver → hepatotoxicity → leaks into circulation → deposits in brain, cornea, kidney, skin. Biliary excretion is the ONLY route for copper elimination — when this fails, copper accumulates. Urinary copper is INCREASED (consequence, not cause). Treatment: penicillamine, zinc (blocks intestinal absorption).\n\n📹 [VIDEO LINK]"},{"id":448,"s":"Vitamins and Minerals","t":"Copper Metabolism","u":"Copper Ferroxidase Iron Metabolism","q":"A 23-year-old with Wilson's disease has elevated copper in his blood. Copper-containing ferroxidases such as ceruloplasmin and hephaestin are impaired. This impairs iron metabolism in which of the following ways?","o":["Decreased haem synthesis","Decreased iron absorption","Decreased iron oxidation for transport","Increased ferritin storage","Increased transferrin production"],"a":2,"p":"Copper-containing ferroxidases (ceruloplasmin and hephaestin) oxidise Fe²⁺ (ferrous) → Fe³⁺ (ferric). Fe³⁺ is REQUIRED for: (1) Binding to transferrin for plasma transport. (2) Storage as ferritin. Without copper → ferroxidases non-functional → Fe²⁺ cannot be oxidised → iron cannot be loaded onto transferrin → impaired iron transport and storage. This explains why copper deficiency causes microcytic hypochromic anaemia similar to iron deficiency.\n\n📹 [VIDEO LINK]"},{"id":449,"s":"Vitamins and Minerals","t":"Copper Metabolism","u":"Cytochrome C Oxidase Copper","q":"A copper-containing enzyme is essential for passing electrons to oxygen during ATP generation in the mitochondria. Which of the following is this enzyme?","o":["Cytochrome c oxidase","Glucose oxidase","Monoamine oxidase","NADPH oxidase","Xanthine oxidase"],"a":0,"p":"CYTOCHROME C OXIDASE (Complex IV) = copper-containing enzyme in the inner mitochondrial membrane. Contains CuA and CuB centres → transfers electrons from cytochrome c to O₂ → water formed → proton gradient generated → ATP synthesis. Deficiency → impaired ATP generation. Monoamine oxidase = FAD-containing (not copper). NADPH oxidase = FAD and haem (generates superoxide in neutrophils). Glucose oxidase = FAD-containing (used in glucometers). Xanthine oxidase = molybdenum-containing.\n\n📹 [VIDEO LINK]"},{"id":450,"s":"Vitamins and Minerals","t":"Copper Metabolism","u":"Copper Iron Oxidation Transport","q":"Which of the following minerals is required for the oxidation of ferrous iron to ferric iron for transport and storage?","o":["Calcium","Cobalt","Copper","Potassium","Zinc"],"a":2,"p":"COPPER-containing FERROXIDASES (ceruloplasmin in plasma, hephaestin in intestinal enterocytes) oxidise Fe²⁺ (ferrous) → Fe³⁺ (ferric). Fe³⁺ is required for binding to transferrin for plasma transport and for storage as ferritin. Without copper → ferroxidases inactive → iron cannot be properly transported or stored → functional iron deficiency despite adequate iron intake. This is why copper deficiency causes microcytic hypochromic anaemia.\n\n📹 [VIDEO LINK]"},{"id":451,"s":"Vitamins and Minerals","t":"Copper Metabolism","u":"Menkes Disease Copper Deficiency","q":"A 1-year-old boy with kinky hair and mental retardation is likely to have deficiency of which of the following minerals?","o":["Calcium","Cobalt","Copper","Potassium","Sodium"],"a":2,"p":"Kinky/steely hair + mental retardation + connective tissue abnormalities in an infant = MENKES DISEASE (X-linked, ATP7A mutation). ATP7A normally exports copper from intestinal enterocytes into blood → mutation → copper trapped in enterocytes → LOW plasma copper → deficient copper-dependent enzymes → tyrosinase (depigmented kinky hair), lysyl oxidase (occipital horn, vascular tortuosity), dopamine beta-hydroxylase (neurodegeneration), cytochrome c oxidase (ATP deficiency). Dietary copper sources: meat, shellfish, nuts.\n\n📹 [VIDEO LINK]"},{"id":452,"s":"Vitamins and Minerals","t":"Copper Metabolism","u":"Menkes Disease Tyrosinase Impaired","q":"A 1-year-old boy with kinky hair and mental retardation is likely to have impaired functioning of which of the following enzymes?","o":["Asparaginase","Ferrochelatase","Glutaminase","Maltase","Tyrosinase"],"a":4,"p":"In Menkes disease: low copper → TYROSINASE (copper-containing enzyme) deficient → cannot catalyse tyrosine → DOPA → melanin → depigmented, kinky/steely hair. Tyrosinase is a copper-dependent monooxygenase. Also impaired: dopamine beta-hydroxylase (neurodegeneration), lysyl oxidase (loose joints, vascular aneurysms, occipital horn), superoxide dismutase (oxidative damage). Menkes = X-linked recessive → affects males predominantly.\n\n📹 [VIDEO LINK]"},{"id":453,"s":"Vitamins and Minerals","t":"Copper Metabolism","u":"Ceruloplasmin Ferroxidase Activity","q":"Which of the following best describes the activity of ceruloplasmin?","o":["Ferroxidase","Hydrolase","Ligase","Reductase","Transferase"],"a":0,"p":"Ceruloplasmin = FERROXIDASE — oxidises Fe²⁺ (ferrous) → Fe³⁺ (ferric) for loading onto transferrin for plasma transport and storage as ferritin. Contains 6-7 copper atoms per molecule → carries 90% plasma copper. Positive acute phase reactant. Reduced in Wilson's disease and Menkes disease. The FERROXIDASE activity is its primary and clinically most important biochemical function — linking copper metabolism to iron metabolism.\n\n📹 [VIDEO LINK]"},{"id":454,"s":"Vitamins and Minerals","t":"Copper Metabolism","u":"Haemochromatosis Iron Infiltration","q":"In haemochromatosis, which of the following metals is infiltrating the liver?","o":["Chromium","Copper","Iron","Lead","Manganese"],"a":2,"p":"Haemochromatosis = IRON overload → IRON infiltrates liver parenchyma (hepatocytes) → stains with Prussian blue on liver biopsy → hepatic cirrhosis → hepatocellular carcinoma. Also: pancreas (bronze diabetes), heart (dilated cardiomyopathy), skin (hyperpigmentation), pituitary (hypogonadism), joints (chondrocalcinosis). HFE gene mutation → ↓ hepcidin → unrestrained iron absorption. Treatment: phlebotomy. Wilson's disease = COPPER infiltration.\n\n📹 [VIDEO LINK]"},{"id":455,"s":"Vitamins and Minerals","t":"Copper Metabolism","u":"Copper Deficiency Demyelination","q":"A 45-year-old woman undergoes bariatric surgery and subsequently takes excessive zinc supplements. Two years later she develops patchy demyelination and peripheral neuropathy. Deficiency of which of the following minerals is responsible?","o":["Cobalt","Copper","Iron","Lead","Zinc"],"a":1,"p":"Excessive ZINC supplementation → zinc competes with COPPER for intestinal metallothionein binding → COPPER DEFICIENCY → patchy demyelination and peripheral neuropathy (impaired cytochrome c oxidase → ↓ ATP for myelination + impaired superoxide dismutase → oxidative myelin damage). Why NOT cobalt: cobalt/vitamin B12 deficiency causes SUBACUTE COMBINED DEGENERATION (posterior AND lateral column demyelination — specific symmetrical pattern) not patchy peripheral demyelination. The term patchy + peripheral neuropathy specifically indicates COPPER deficiency.\n\n📹 [VIDEO LINK]"},{"id":456,"s":"Vitamins and Minerals","t":"Copper Metabolism","u":"Cytochrome C Oxidase Copper Required","q":"Which of the following minerals is required for the proper functioning of cytochrome c oxidase in ATP generation?","o":["Calcium","Cobalt","Copper","Potassium","Sodium"],"a":2,"p":"Cytochrome c oxidase (Complex IV) contains COPPER at its catalytic core (CuA and CuB centres) → accepts electrons from cytochrome c → reduces O₂ to H₂O → drives proton pumping → ATP synthesis. Copper deficiency (Menkes disease, severe malnutrition) → cytochrome c oxidase dysfunctional → ATP production impaired → neurodegeneration. Other copper-requiring enzymes: dopamine beta-hydroxylase, ferroxidase/ceruloplasmin, lysyl oxidase, tyrosinase, superoxide dismutase.\n\n📹 [VIDEO LINK]"},{"id":457,"s":"Vitamins and Minerals","t":"Zinc and Lead","u":"Zinc Alcohol Dehydrogenase Cofactor","q":"Alcohol dehydrogenase from the liver contains which of the following as a mineral cofactor?","o":["Cobalt","Copper","Magnesium","Sodium","Zinc"],"a":4,"p":"ZINC is the mineral cofactor of ALCOHOL DEHYDROGENASE (ADH) — the liver enzyme that oxidises ethanol → acetaldehyde. Zinc coordinates substrate binding at the active site. Zinc also the mineral cofactor of: carbonic anhydrase, carboxypeptidase, DNA polymerase, RNA polymerase, superoxide dismutase (structural role), ALA dehydratase (haem synthesis — inhibited by lead). Zinc deficiency → impaired wound healing, growth retardation, hypogonadism, anosmia, dysgeusia, dermatitis, alopecia.\n\n📹 [VIDEO LINK]"},{"id":458,"s":"Vitamins and Minerals","t":"Zinc and Lead","u":"Lead Displaces Zinc ALA Dehydratase Ferrochelatase","q":"A young boy earns tips by posing as a golden statue on the Islamabad Expressway. He sprays his face, arms, and neck with gold-coloured metallic paint every day. Several months later he presents with fatigue, colicky abdominal pain, and developmental delay. His blood smear shows basophilic stippling. Displacement of which of the following minerals involved in haem synthesis is most likely occurring in him?","o":["Calcium","Copper","Magnesium","Manganese","Zinc"],"a":4,"p":"Gold spray paint contains LEAD → absorbed through skin → LEAD DISPLACES ZINC at the active sites of two zinc-containing haem synthesis enzymes: (1) ALA DEHYDRATASE (condenses 2 ALA → porphobilinogen) — inhibited → urine ALA elevated. (2) FERROCHELATASE (inserts Fe²⁺ into protoporphyrin IX → haem) — inhibited → free erythrocyte protoporphyrin elevated → anaemia. Basophilic stippling = ribosomal aggregates in RBCs = hallmark of lead poisoning. Lead lines in gums (Burton lines) + neurological effects + abdominal colic complete the picture.\n\n[Diagram: Illustration of golden boy standing on a road with spray paint can. Panel below shows haem synthesis pathway with two red X marks: (1) ALA dehydratase step — Pb²⁺ displaces Zn²⁺ → ALA accumulates. (2) Ferrochelatase step — Pb²⁺ displaces Zn²⁺ → protoporphyrin IX accumulates → no haem formed. Blood smear inset showing basophilic stippling in RBCs. Note: image of golden boy from Islamabad Expressway to be inserted by editor.]\n\n📹 [VIDEO LINK]"},{"id":459,"s":"Vitamins and Minerals","t":"Zinc and Lead","u":"Lead Displaces Zinc Ferrochelatase","q":"A 7-year-old boy presents with colicky abdominal pain, constipation, and developmental delay. His parents report he plays in a house with old paint. Blood smear shows basophilic stippling and urine aminolaevulinic acid (ALA) is elevated. Displacement of which mineral at the active site of ferrochelatase is impairing the final step of haem synthesis?","o":["Calcium","Copper","Magnesium","Manganese","Zinc"],"a":4,"p":"FERROCHELATASE (haem synthase) = ZINC-containing enzyme → inserts Fe²⁺ into protoporphyrin IX → HAEM. LEAD displaces ZINC → ferrochelatase inhibited → protoporphyrin IX accumulates → elevated free erythrocyte protoporphyrin → anaemia. Lead also inhibits ALA DEHYDRATASE (also zinc-containing) → ALA accumulates → elevated urine ALA (explains this child's elevated urine ALA). Two zinc-requiring haem synthesis enzymes inhibited by lead: ALA dehydratase AND ferrochelatase. Basophilic stippling = hallmark of lead poisoning.\n\n📹 [VIDEO LINK]"},{"id":460,"s":"Vitamins and Minerals","t":"Zinc and Lead","u":"Zinc Excess Copper Deficiency Anaemia","q":"A patient receiving long-term zinc supplementation develops microcytic hypochromic anaemia. Excess zinc impairs absorption of which of the following minerals, leading to this anaemia?","o":["Calcium","Copper","Iron","Magnesium","Selenium"],"a":1,"p":"Excess ZINC competes with COPPER for absorption at intestinal METALLOTHIONEIN. High zinc → ↑ metallothionein → sequesters copper → copper trapped in enterocytes → copper deficiency → ↓ ceruloplasmin → ↓ ferroxidase activity → iron cannot be oxidised to Fe³⁺ → cannot bind transferrin → functional IRON DEFICIENCY → microcytic hypochromic anaemia. Excessive zinc supplementation causes anaemia indistinguishable from iron deficiency — through copper depletion impairing iron metabolism.\n\n📹 [VIDEO LINK]"},{"id":461,"s":"Vitamins and Minerals","t":"Selenium and Antioxidants","u":"Selenium Glutathione Peroxidase","q":"Which of the following elements is an essential component of the enzyme glutathione peroxidase, providing it antioxidant activity?","o":["Calcium","Cobalt","Magnesium","Selenium","Zinc"],"a":3,"p":"SELENIUM is incorporated as SELENOCYSTEINE into GLUTATHIONE PEROXIDASE → reduces hydrogen peroxide (H₂O₂) and lipid peroxides using glutathione (GSH) as reductant → protects cells from oxidative damage. Selenium also required for thioredoxin reductase and iodothyronine deiodinase (converts T4 → T3 in peripheral tissues — connects selenium to thyroid function). Selenium deficiency: Keshan disease (cardiomyopathy, selenium-deficient soil areas of China), impaired thyroid hormone conversion.\n\n📹 [VIDEO LINK]"},{"id":462,"s":"Vitamins and Minerals","t":"Selenium and Antioxidants","u":"Selenium Glutathione Peroxidase Constituent","q":"Selenium is a constituent of which of the following enzymes?","o":["Glutathione peroxidase","Homogentisate oxidase","Myeloperoxidase","Phenylalanine hydroxylase","Tyrosine hydroxylase"],"a":0,"p":"GLUTATHIONE PEROXIDASE contains SELENOCYSTEINE at its active site → catalyses: 2 GSH + H₂O₂ → GSSG + 2 H₂O (detoxifies hydrogen peroxide) and GSH + lipid-OOH → GSSG + lipid-OH (detoxifies lipid peroxides). Myeloperoxidase = contains iron (haem) → generates hypochlorous acid in neutrophils. Phenylalanine hydroxylase = BH4-dependent. Tyrosinase = copper-containing. Homogentisate oxidase = iron-containing (defective in alkaptonuria).\n\n📹 [VIDEO LINK]"},{"id":463,"s":"Vitamins and Minerals","t":"Selenium and Antioxidants","u":"Selenium Antioxidant Role","q":"Which of the following has an antioxidant role by being a component of glutathione peroxidase?","o":["Cobalt","Copper","Iron","Selenium","Zinc"],"a":3,"p":"SELENIUM is incorporated as selenocysteine into glutathione peroxidase → provides antioxidant protection by reducing H₂O₂ and lipid peroxides. COPPER also has antioxidant role (via copper-zinc superoxide dismutase → converts O₂⁻ to H₂O₂). ZINC in superoxide dismutase (structural role, copper is catalytic). IRON in catalase (H₂O₂ → H₂O + O₂) but iron also generates free radicals via Fenton reaction. Selenium specifically = glutathione peroxidase antioxidant.\n\n📹 [VIDEO LINK]"},{"id":464,"s":"Vitamins and Minerals","t":"Iodine and Thyroid","u":"Iodine Deficiency Goitre TSH","q":"A 54-year-old female presents with goitre and elevated TSH. She is likely to be deficient in which of the following?","o":["Copper","Iodine","Iron","Molybdenum","Selenium"],"a":1,"p":"IODINE deficiency → cannot synthesise T3/T4 → low thyroid hormones → elevated TSH (negative feedback lost) → TSH drives thyroid gland growth → GOITRE. Iodine incorporated into thyroglobulin by thyroid peroxidase (TPO) → MIT and DIT → coupled → T3 (MIT+DIT) and T4 (DIT+DIT). Most common preventable cause of intellectual disability worldwide. Endemic goitre in iodine-deficient regions. Prevented by iodised salt. Cretinism = iodine deficiency in utero → irreversible intellectual disability.\n\n📹 [VIDEO LINK]"},{"id":465,"s":"Vitamins and Minerals","t":"Iodine and Thyroid","u":"Iodine Deficiency Hypothyroidism","q":"A child presents with iodine deficiency. Which of the following conditions is most likely to develop?","o":["Hyperparathyroidism","Hyperthyroidism","Hypoaldosteronism","Hypothyroidism","Hypogonadism"],"a":3,"p":"Iodine deficiency → insufficient thyroid hormone synthesis (T3/T4) → HYPOTHYROIDISM → goitre, weight gain, cold intolerance, bradycardia, constipation, dry skin, coarse hair, myxoedema, delayed reflexes, cognitive impairment. Neonatal hypothyroidism (cretinism): irreversible intellectual disability if untreated → detected by newborn screening (TSH measurement). Selenium also required for thyroid function (iodothyronine deiodinase converts T4 → active T3).\n\n📹 [VIDEO LINK]"},{"id":466,"s":"Vitamins and Minerals","t":"Iodine and Thyroid","u":"Hashimoto TPO Iodine","q":"A 35-year-old woman presents with fatigue, weight gain, cold intolerance, and constipation for 6 months. Examination reveals a diffusely enlarged thyroid gland. Anti-TPO antibodies are strongly positive. TSH is elevated at 12 mIU/L (normal 0.4–4.0). Deficiency of which mineral is most likely to exacerbate her underlying condition?","o":["Calcium","Copper","Iodine","Selenium","Zinc"],"a":2,"p":"This patient has HASHIMOTO'S THYROIDITIS (autoimmune hypothyroidism). IODINE deficiency exacerbates hypothyroidism because iodine is essential for T3/T4 synthesis — thyroid peroxidase incorporates iodide into thyroglobulin → MIT + DIT → T3 and T4. Without iodine → cannot synthesise thyroid hormones → worsens existing hypothyroidism. SELENIUM deficiency also impairs thyroid function (iodothyronine deiodinase — converts T4 → active T3), but iodine is the primary mineral for thyroid hormone synthesis.\n\n📹 [VIDEO LINK]"},{"id":467,"s":"Vitamins and Minerals","t":"Trace Elements","u":"Cobalt Vitamin B12","q":"Cobalt forms an integral part of which of the following vitamins?","o":["Vitamin B1","Vitamin B6","Vitamin B12","Vitamin D","Folic acid"],"a":2,"p":"COBALT is the metallic constituent of VITAMIN B12 (cobalamin) — the cobalt atom is at the centre of the corrin ring. Functions of Vitamin B12: (1) Methylmalonyl-CoA mutase cofactor → converts methylmalonyl-CoA → succinyl-CoA. (2) Methionine synthase cofactor → converts homocysteine → methionine (requires methylcobalamin). Deficiency: megaloblastic anaemia + subacute combined degeneration of spinal cord (posterior and lateral columns).\n\n📹 [VIDEO LINK]"},{"id":468,"s":"Vitamins and Minerals","t":"Trace Elements","u":"Molybdenum Xanthine Oxidase","q":"Which of the following enzymes of purine degradation requires molybdenum and produces uric acid as its end product?","o":["Cytochrome c oxidase","Ferroxidase","Glucose oxidase","Monoamine oxidase","Xanthine oxidase"],"a":4,"p":"XANTHINE OXIDASE is a MOLYBDENUM-containing enzyme that catalyses: hypoxanthine → xanthine → URIC ACID (final step of purine catabolism in humans). Xanthine oxidase inhibited by ALLOPURINOL (used to treat gout). Also molybdenum-containing: sulphite oxidase (sulphite → sulphate), aldehyde oxidase. Molybdenum deficiency is extremely rare — can cause sulphite toxicity.\n\n📹 [VIDEO LINK]"},{"id":469,"s":"Vitamins and Minerals","t":"Trace Elements","u":"Chromium Glucose Tolerance Factor","q":"The metallic constituent of the glucose tolerance factor, which potentiates insulin action, is which of the following?","o":["Chromium","Cobalt","Selenium","Sulphur","Zinc"],"a":0,"p":"CHROMIUM is the metallic constituent of GLUCOSE TOLERANCE FACTOR (GTF) — a chromium-containing oligopeptide that potentiates insulin signalling by enhancing insulin receptor tyrosine kinase activity → improved glucose uptake. Chromium deficiency (seen in total parenteral nutrition without chromium supplementation) → impaired glucose tolerance, insulin resistance. Trivalent chromium (Cr³⁺) is the biologically active form. Hexavalent chromium (Cr⁶⁺) is toxic and carcinogenic.\n\n📹 [VIDEO LINK]"},{"id":470,"s":"Vitamins and Minerals","t":"Trace Elements","u":"Fluoride Dental Caries","q":"Which of the following prevents the development of dental caries by incorporating into hydroxyapatite crystals making tooth enamel harder and more resistant to acid dissolution?","o":["Calcium","Fluorine","Phosphorus","Selenium","Zinc"],"a":1,"p":"FLUORINE incorporated into hydroxyapatite → FLUORAPATITE (Ca₁₀(PO₄)₆F₂) — harder and more acid-resistant than hydroxyapatite → protects tooth enamel from bacterial acid dissolution → PREVENTS DENTAL CARIES. Optimal fluoride concentration in drinking water = 0.7 ppm. EXCESS fluoride (>4 ppm) → DENTAL FLUOROSIS (mottled/brown teeth) → SKELETAL FLUOROSIS at very high doses. Fluoride also inhibits bacterial enzymes (enolase) → reduces acid production by oral bacteria.\n\n📹 [VIDEO LINK]"},{"id":471,"s":"Vitamins and Minerals","t":"Trace Elements","u":"Fluoride Optimal Concentration","q":"What is the recommended fluoride concentration in drinking water to balance dental benefits and minimise fluorosis risk?","o":["0.1 ppm","0.5 ppm","0.7 ppm","2.5 ppm","3.0 ppm"],"a":2,"p":"0.7 ppm (parts per million) = current WHO and US CDC recommendation for optimal fluoride in drinking water. Below 0.7 ppm = insufficient caries prevention. Above 4 ppm = risk of dental fluorosis (cosmetic) and skeletal fluorosis (structural damage). Previously recommended as 0.7-1.2 ppm; lowered to 0.7 ppm in 2015 as dietary fluoride intake from other sources increased. Fluoride prevention also achieved through fluoride toothpaste (1000-1450 ppm) applied topically.\n\n📹 [VIDEO LINK]"},{"id":472,"s":"Vitamins and Minerals","t":"Electrolytes","u":"Sodium Osmotic Pressure","q":"Sodium is the primary extracellular cation. Which of the following does sodium primarily maintain in the plasma?","o":["Diastolic pressure","Hydrostatic pressure","Oncotic pressure","Osmotic pressure","Systolic pressure"],"a":3,"p":"SODIUM and its accompanying anions (Cl⁻, HCO₃⁻) account for ~90% of plasma OSMOTIC PRESSURE (osmolality). Plasma osmolality ≈ 2 × [Na⁺] + [glucose/18] + [BUN/2.8]. Changes in Na⁺ detected by hypothalamic osmoreceptors → regulate ADH release → water balance. Compare: ONCOTIC PRESSURE = maintained primarily by ALBUMIN (pulls water back into capillaries). HYDROSTATIC PRESSURE = generated by cardiac output (pushes fluid out of capillaries). Sodium regulates OSMOTIC pressure; albumin regulates ONCOTIC pressure.\n\n📹 [VIDEO LINK]"},{"id":473,"s":"Vitamins and Minerals","t":"Electrolytes","u":"Osmoreceptors Hypothalamus","q":"A 70-year-old man with congestive heart failure is started on a low-sodium diet. Despite sodium restriction, his plasma sodium level remains stable. A specialised group of cells detects changes in the concentration of dissolved particles in his plasma and signals the posterior pituitary to adjust water retention accordingly. Where are these cells located?","o":["Adrenal cortex","Carotid sinus","Hypothalamus","Kidney collecting duct","Posterior pituitary"],"a":2,"p":"OSMORECEPTORS are located in the HYPOTHALAMUS (supraoptic and paraventricular nuclei) — they detect changes in plasma osmolality (primarily Na⁺ concentration) → stimulate or suppress ADH (vasopressin) secretion from the posterior pituitary → AQP2 inserted in renal collecting duct → water reabsorption → osmolality restored. Baroreceptors (carotid sinus, aortic arch) are volume/pressure sensors — secondary regulators of ADH.\n\n📹 [VIDEO LINK]"},{"id":474,"s":"Vitamins and Minerals","t":"Electrolytes","u":"Rapid Hyponatraemia Correction Demyelination","q":"A 45-year-old woman with severe hyperemesis gravidarum has had very low sodium levels of 112 mEq/L (normal 135-145) for the past 3 weeks. The emergency team corrects her sodium rapidly to 130 mEq/L over 4 hours. Which complication is most likely to develop?","o":["Cerebral haemorrhage","Cerebral oedema","Cerebral venous thrombosis","Osmotic demyelination syndrome","Subarachnoid haemorrhage"],"a":3,"p":"OSMOTIC DEMYELINATION SYNDROME (previously central pontine myelinolysis): chronic hyponatraemia → brain adapts by losing organic osmoles to prevent cerebral oedema. Rapid sodium correction → plasma suddenly hypertonic relative to adapted brain → water rapidly leaves brain cells → MYELIN SHEATH DESSICATION → demyelination → locked-in syndrome, quadriplegia, dysphagia, death. Rule: correct CHRONIC hyponatraemia at ≤8-10 mEq/L per day. CEREBRAL OEDEMA is the risk of UNDER-treating acute severe hyponatraemia — the opposite problem.\n\n📹 [VIDEO LINK]"},{"id":475,"s":"Vitamins and Minerals","t":"Electrolytes","u":"Metabolic Acidosis Hyperkalaemia","q":"A patient with metabolic acidosis develops hyperkalaemia. The biochemical reason is movement of potassium from intracellular fluid to extracellular fluid in exchange for which ion?","o":["Bicarbonate","Calcium","Chloride","Hydrogen ion","Sodium"],"a":3,"p":"Metabolic acidosis → excess H⁺ in extracellular fluid → H⁺ enters cells via H⁺/K⁺ exchanger → K⁺ exits cells → HYPERKALAEMIA. For every 0.1 decrease in pH → serum K⁺ rises approximately 0.6 mEq/L. Metabolic ALKALOSIS → opposite: H⁺ exits cells → K⁺ enters cells → HYPOKALAEMIA. Insulin drives K⁺ INTO cells (used to treat hyperkalaemia). Aldosterone increases renal K⁺ excretion.\n\n📹 [VIDEO LINK]"},{"id":476,"s":"Vitamins and Minerals","t":"Electrolytes","u":"Metabolic Acidosis Hyperkalaemia ABG","q":"A 45-year-old man presents with pH 7.3, serum HCO₃⁻ 22 mEq/L, and pCO₂ 40 mmHg. Which of the following electrolyte disturbances is he most likely to have?","o":["Hypercalcaemia","Hyperkalaemia","Hypernatraemia","Hyperphosphataemia","Hypophosphataemia"],"a":1,"p":"pH 7.3 (acidaemia) + normal HCO₃⁻ + normal pCO₂ = HIGH ANION GAP METABOLIC ACIDOSIS without respiratory compensation yet. Metabolic acidosis → H⁺/K⁺ exchange → K⁺ exits cells → HYPERKALAEMIA. Causes of high AG metabolic acidosis: MUDPILES (Methanol, Uraemia, DKA, Propylene glycol, INH/Iron, Lactic acidosis, Ethylene glycol, Salicylates).\n\n📹 [VIDEO LINK]"},{"id":477,"s":"Vitamins and Minerals","t":"Electrolytes","u":"Metabolic Alkalosis Hypokalaemia ABG","q":"A 45-year-old man presents with pH 7.5, serum HCO₃⁻ 38 mEq/L, and pCO₂ 48 mmHg. Which of the following electrolyte disturbances is he most likely to have?","o":["Hypercalcaemia","Hyperkalaemia","Hypernatraemia","Hyperphosphataemia","Hypokalaemia"],"a":4,"p":"pH 7.5 (alkalaemia) + elevated HCO₃⁻ + elevated pCO₂ (respiratory compensation — hypoventilation retains CO₂) = COMPENSATED METABOLIC ALKALOSIS. Expected pCO₂ = 0.7 × HCO₃⁻ + 21 ± 2 → for HCO₃⁻ 38 → expected pCO₂ ≈ 47.6 mmHg (consistent with 48 mmHg here). Metabolic alkalosis → H⁺ exits cells → K⁺ enters cells → HYPOKALAEMIA. Vomiting (common cause) → HCl loss → alkalaemia + direct K⁺ loss.\n\n📹 [VIDEO LINK]"},{"id":478,"s":"Vitamins and Minerals","t":"Electrolytes","u":"Insulin Transient Hypokalaemia","q":"A 40-year-old diabetic woman injects her usual insulin dose and is waiting 30 minutes before sitting down to enjoy her delicious meal. A blood test taken during this waiting period shows transient hypokalaemia. Which of the following mechanisms is responsible?","o":["Increased aldosterone secretion","Increased renal potassium excretion","Insulin-driven potassium shift into cells","Potassium loss through vomiting","Redistribution due to acidosis"],"a":2,"p":"Insulin → activates Na⁺/K⁺ ATPase → drives K⁺ INTO cells → serum K⁺ falls → TRANSIENT HYPOKALAEMIA in the period between insulin injection and meal absorption. This is why diabetics should not delay meals after insulin injection. Insulin is used therapeutically to treat life-threatening hyperkalaemia (given with glucose to prevent hypoglycaemia). In DKA: serum K⁺ may be HIGH initially (acidosis drives K⁺ out of cells) → when insulin started → K⁺ rapidly shifts into cells → MUST supplement K⁺ during DKA treatment.\n\n📹 [VIDEO LINK]"},{"id":479,"s":"Vitamins and Minerals","t":"Electrolytes","u":"Aldosterone Hyperkalaemia","q":"Secretion of which of the following hormones will be increased in hyperkalaemia?","o":["ADH","Aldosterone","Cortisol","Epinephrine","Renin"],"a":1,"p":"ALDOSTERONE secretion is directly stimulated by HYPERKALAEMIA (potassium acts directly on zona glomerulosa of adrenal cortex → ↑ aldosterone). Aldosterone → principal cells of collecting duct → ↑ Na⁺ reabsorption + ↑ K⁺ excretion → lowers serum K⁺. Aldosterone also stimulated by: angiotensin II (via RAAS) and ACTH (minor). In hyperkalaemia from renal failure → cannot excrete K⁺ despite high aldosterone → dangerous arrhythmias. Treatment: calcium gluconate (stabilises cardiac membrane), insulin + glucose (shifts K⁺ into cells), dialysis.\n\n📹 [VIDEO LINK]"},{"id":480,"s":"Vitamins","t":"Vitamin A","u":"Night Blindness Earliest Sign","q":"A 6-year-old boy living in an orphanage presents with difficulty seeing at night. He has been surviving on a monotonous diet of rice and bread for the past year with no animal products. Eye examination is normal in daylight. Which vitamin deficiency is most likely responsible?","o":["Vitamin A","Vitamin B12","Vitamin C","Vitamin D","Vitamin E"],"a":0,"p":"NIGHT BLINDNESS (nyctalopia) = EARLIEST sign of vitamin A deficiency. PRIMARY sources of vitamin A: animal products (liver, egg yolk, dairy, fatty fish) — contain PREFORMED vitamin A (retinol, retinyl esters). A diet of rice and bread with no animal products provides NO preformed vitamin A. Vitamin A deficiency → insufficient 11-cis retinal → impaired rhodopsin regeneration → rod cells fail in dim light → night blindness. Sequence of ocular signs: Night blindness → Conjunctival xerosis → Bitot's spots → Corneal xerosis → Keratomalacia.\n\n[Diagram: Eye anatomy cross-section showing retina. Rod cell enlarged — inside: opsin protein + 11-cis retinal = rhodopsin. Light: 11-cis retinal → all-trans retinal + opsin (bleached) → hyperpolarisation → nerve signal. Dark: all-trans retinal → (isomerase, requires vitamin A) → 11-cis retinal → rejoins opsin → rhodopsin regenerated. In vitamin A deficiency: regeneration blocked → rod cells fail in dim light → night blindness.]\n\n📹 [VIDEO LINK]"},{"id":481,"s":"Vitamins","t":"Vitamin A","u":"Bitots Spots","q":"A 5-year-old boy presents to a community eye camp. He has whitish, foamy, triangular patches on the temporal side of his conjunctiva. His parents report that he often struggles to see in dim light. These whitish foamy patches represent which stage of vitamin A deficiency?","o":["Bitot's spots","Conjunctival xerosis","Corneal xerosis","Keratomalacia","Nyctalopia"],"a":0,"p":"BITOT'S SPOTS = foamy, cheesy, triangular patches on the temporal conjunctiva → caused by keratinisation of conjunctival epithelium + overgrowth of Corynebacterium xerosis (gas-producing bacteria creates the foam). Third stage of vitamin A deficiency (after night blindness and conjunctival xerosis). Pathognomonic of vitamin A deficiency. Reversible with vitamin A treatment if cornea not yet involved.\n\n[Diagram: Eye diagram — temporal side of conjunctiva highlighted. Triangular foamy whitish patch shown at 3 o'clock position on the conjunctiva, pointing toward limbus. Label: Bitot's spot = keratinised conjunctival epithelium + Corynebacterium xerosis foam. Insert: histology of keratinised epithelium replacing normal columnar goblet cells. Note to editor: insert clinical photograph of Bitot's spots from net alongside diagram.]\n\n📹 [VIDEO LINK]"},{"id":482,"s":"Vitamins","t":"Vitamin A","u":"Corneal Xerosis","q":"A malnourished 4-year-old child presents with severe eye discomfort. Examination shows loss of corneal lustre and dryness extending over the corneal surface. This ocular finding represents which stage of vitamin A deficiency?","o":["Bitot's spots","Conjunctival xerosis","Corneal xerosis","Keratomalacia","Nyctalopia"],"a":2,"p":"CORNEAL XEROSIS = dryness and loss of lustre extending to the corneal surface → fourth stage of vitamin A deficiency. WARNING SIGN — if untreated, rapidly progresses to keratomalacia. Still reversible at this stage with prompt high-dose vitamin A. Sequence: Night blindness → Conjunctival xerosis → Bitot's spots → CORNEAL XEROSIS → Keratomalacia.\n\n[Note to editor: insert clinical photograph of corneal xerosis showing dull, hazy corneal surface from net.]\n\n📹 [VIDEO LINK]"},{"id":483,"s":"Vitamins","t":"Vitamin A","u":"Keratomalacia","q":"A 3-year-old boy with chronic diarrhoea and poor diet presents with cloudy cornea and purulent discharge. The corneal surface appears soft and ulcerated. This represents which sight-threatening stage of vitamin A deficiency?","o":["Bitot's spots","Conjunctival xerosis","Corneal xerosis","Keratomalacia","Nyctalopia"],"a":3,"p":"KERATOMALACIA = liquefactive necrosis of the corneal epithelium → ulceration, perforation, endophthalmitis, blindness — MOST SEVERE and IRREVERSIBLE stage. Caused by severe vitamin A depletion → corneal epithelium undergoes squamous metaplasia → keratin deposition → corneal softening and melting → permanent blindness. Requires immediate high-dose vitamin A. Vitamin A deficiency remains a leading cause of preventable childhood blindness worldwide.\n\n[Note to editor: insert clinical photograph of keratomalacia showing soft, melting, ulcerated cornea from net.]\n\n📹 [VIDEO LINK]"},{"id":484,"s":"Vitamins","t":"Vitamin A","u":"Conjunctival Xerosis","q":"A 10-year-old girl complains of gritty sensation in her eyes and difficulty blinking. The conjunctiva appears dry and wrinkled with loss of normal sheen. She has a diet poor in animal fats and dairy products. This ocular finding represents which stage of vitamin A deficiency?","o":["Bitot's spots","Conjunctival xerosis","Corneal xerosis","Keratomalacia","Nyctalopia"],"a":1,"p":"CONJUNCTIVAL XEROSIS = dryness and wrinkling of the conjunctiva with loss of normal glistening appearance → SECOND stage of vitamin A deficiency (after night blindness). Caused by loss of goblet cells → insufficient mucus production → dry, wrinkled, non-wettable conjunctiva. Gritty sensation = classic dry eye symptom. Reversible with vitamin A. Progresses to Bitot's spots if untreated.\n\n[Note to editor: insert clinical photograph of conjunctival xerosis showing dry, wrinkled conjunctiva with loss of sheen from net.]\n\n📹 [VIDEO LINK]"},{"id":485,"s":"Vitamins","t":"Vitamin A","u":"Rhodopsin 11-cis Retinal Dark State","q":"In rhodopsin, the visual pigment of rod cells in the retina, which of the following is bound to protein opsin in the dark state?","o":["11-cis retinal","11-cis retinol","All-trans retinal","All-trans retinol","Retinoic acid"],"a":0,"p":"In DARKNESS: 11-CIS RETINAL + opsin → RHODOPSIN. Light → 11-cis retinal isomerises → ALL-TRANS RETINAL → conformational change in opsin → activates TRANSDUCIN (Gαt) → phosphodiesterase activated → cGMP hydrolysed → CNG channels close → rod cell hyperpolarises → visual signal. All-trans retinal then released → reduced to all-trans retinol (NADH/NADPH) → isomerised back to 11-cis retinal (requires vitamin A supply) → cycle continues.\n\n[Diagram: GPCR phototransduction pathway — Rhodopsin (GPCR) in dark state: opsin + 11-cis retinal. Light → rhodopsin activated (metarhodopsin II) → activates TRANSDUCIN (Gαt-GDP → Gαt-GTP) → activates cGMP PHOSPHODIESTERASE → cGMP → 5'-GMP → ↓ cGMP → CNG CHANNELS CLOSE → Na⁺/Ca²⁺ entry stopped → membrane hyperpolarises → visual signal. Recovery: Rhodopsin kinase phosphorylates activated rhodopsin → arrestin binds → signal terminated.]\n\n📹 [VIDEO LINK]"},{"id":486,"s":"Vitamins","t":"Vitamin A","u":"G Protein Transducin Phototransduction","q":"A soldier asleep in darkness is awakened by a laser light. The nerve impulse in his optic nerve was triggered by activation of which G protein?","o":["G protein alpha-i","G protein alpha-q","G protein alpha-s","G protein Ras","G protein transducin"],"a":4,"p":"Phototransduction G protein = TRANSDUCIN (Gαt). Light → rhodopsin (GPCR) → activates TRANSDUCIN → Gαt activates phosphodiesterase → cGMP hydrolysed → cyclic nucleotide-gated (CNG) channels CLOSE → Na⁺/Ca²⁺ entry stopped → rod cell hyperpolarises → glutamate release stops → ON bipolar cells activated → optic nerve signal. Transducin is unique to photoreceptors. Compare: Gαs → adenylyl cyclase. Gαq → PLC. Gαi → inhibits adenylyl cyclase.\n\n📹 [VIDEO LINK]"},{"id":487,"s":"Vitamins","t":"Vitamin A","u":"Nyctalopia 11-cis Retinal Cannot Bind Opsin","q":"A 3-year-old child stranded in a flood relief camp for 9 months presents with nyctalopia. Which of the following compounds is unable to bind opsin in him?","o":["11-cis retinal","11-cis retinol","All-trans retinal","All-trans retinol","Retinoic acid"],"a":0,"p":"Only 11-CIS RETINAL binds opsin to form rhodopsin. In vitamin A deficiency: insufficient retinol → cannot regenerate 11-cis retinal → 11-cis retinal unavailable → cannot form rhodopsin → rod cells fail in dim light → nyctalopia. Retinoic acid cannot be converted back to retinal or retinol (irreversible oxidation) → cannot participate in visual cycle. 11-cis retinol is an intermediate but does NOT bind opsin directly — only 11-cis retinal (the aldehyde form) forms the Schiff base with lysine-296 of opsin.\n\n📹 [VIDEO LINK]"},{"id":488,"s":"Vitamins","t":"Vitamin A","u":"Retinoic Acid Acne Treatment","q":"A first-year medical student is advised a topical cream for acne. Which of the following compounds is likely to be present in this cream?","o":["11-cis retinal","All-trans retinal","Cholecalciferol","Retinoic acid","Retinol"],"a":3,"p":"RETINOIC ACID (all-trans retinoic acid / tretinoin) is used topically for acne — promotes keratinocyte differentiation, reduces sebaceous gland activity, decreases comedone formation. Binds RAR nuclear receptors → regulates gene expression → cell differentiation. ISOTRETINOIN (13-cis retinoic acid) = oral form for severe cystic acne — TERATOGENIC (Category X), absolutely contraindicated in pregnancy. Retinol supports reproduction and epithelial integrity. Retinal is specifically for the visual cycle.\n\n📹 [VIDEO LINK]"},{"id":489,"s":"Vitamins","t":"Vitamin A","u":"Vitamin A Teratogenicity Beef Liver","q":"A 28-year-old woman in the first trimester of pregnancy eats beef liver several times a week because she heard it is good for the baby. Her physician advises her to limit this food. Excessive intake of which of the following nutrients from her diet poses the greatest risk to the developing fetus?","o":["Folate","Iron","Vitamin A","Vitamin B12","Vitamin D"],"a":2,"p":"VITAMIN A (preformed retinol/retinyl esters from animal sources like beef liver) is TERATOGENIC in excess — especially during the first trimester → congenital malformations (cranial neural crest defects, cardiac malformations, ear defects). Mechanism: retinoic acid excess → disrupts HOX gene expression → developmental abnormalities. Beef liver = extremely rich in preformed vitamin A. BETA-CAROTENE (plant sources) is safe — conversion to retinol is regulated. Pregnant women should avoid liver and high-dose vitamin A supplements.\n\n📹 [VIDEO LINK]"},{"id":490,"s":"Vitamins","t":"Vitamin A","u":"Vitamin A Teratogenicity Pregnancy Measles","q":"A 25-year-old pregnant woman in her second trimester is diagnosed with measles. Her physician advises vitamin A-rich vegetables. What is the risk of HIGH-DOSE vitamin A supplementation (above 10,000 IU daily) in this patient?","o":["Antigenicity","Carcinogenicity","Immunogenicity","Mutagenicity","Teratogenicity"],"a":4,"p":"High-dose vitamin A (≥10,000 IU/day chronically) is TERATOGENIC — causes fetal craniofacial, cardiac, and neural tube defects by disrupting retinoic acid signalling during organogenesis. Short-term WHO-recommended doses for measles treatment are safe. Beta-carotene from plants is safe (conversion to retinol is self-limited). Key distinction: plant sources (beta-carotene) = SAFE in pregnancy. Preformed vitamin A (animal sources, supplements, isotretinoin) = TERATOGENIC in excess.\n\n📹 [VIDEO LINK]"},{"id":491,"s":"Vitamins","t":"Vitamin A","u":"Measles Vitamin A Bacterial Superinfection","q":"A 4-year-old boy presents with high fever, maculopapular rash starting behind the ears, Koplik spots on the buccal mucosa, and dry thickened conjunctiva. Measles is confirmed. Vitamin A supplementation is given as per WHO guidelines. Which complication of measles does vitamin A primarily help to prevent?","o":["Bacterial superinfection","Haemorrhagic rash","Hepatic failure","Neurological damage","Viral invasion"],"a":0,"p":"Vitamin A supplementation in measles primarily prevents BACTERIAL SUPERINFECTION by: (1) Maintaining epithelial integrity of respiratory and ocular mucosa → barrier function preserved. (2) Enhancing immune cell differentiation and function. (3) Reducing oxidative damage. WHO recommends high-dose vitamin A for ALL children with measles — reduces mortality up to 50%. Koplik spots = pathognomonic of measles — small white/bluish-white spots on bright red buccal mucosa opposite lower molars.\n\n[Diagram: Mouth interior view showing buccal mucosa. Small white spots (Koplik spots) shown on mucosa opposite lower molar teeth. Label: KOPLIK SPOTS — pathognomonic of measles, appear 1-2 days before maculopapular rash. Note to editor: insert clinical photograph of Koplik spots alongside diagram.]\n\n📹 [VIDEO LINK]"},{"id":492,"s":"Vitamins","t":"Vitamin A","u":"Retinyl Esters Chylomicron Transport","q":"Dietary vitamin A (retinyl esters) is absorbed in the intestine and transported to the liver. Which lipoprotein carrier transports retinyl esters from intestinal cells to the liver?","o":["Chylomicrons","HDL","IDL","LDL","VLDL"],"a":0,"p":"Dietary vitamin A absorption: Dietary retinyl esters → hydrolysed to retinol → absorbed into enterocyte → re-esterified to retinyl esters → packaged into CHYLOMICRONS → transported via LYMPHATICS → bloodstream → liver (hepatic stellate cells/Ito cells store vitamin A as retinyl palmitate). Transport from liver to peripheral tissues: retinol + RETINOL-BINDING PROTEIN 4 (RBP4) + TRANSTHYRETIN complex.\n\n[Diagram: Chylomicron metabolism — Enterocyte: retinyl esters packaged into CHYLOMICRON (apoB-48) → lymphatic duct → bloodstream. In capillaries: LIPOPROTEIN LIPASE cleaves TG → fatty acids enter muscle/adipose → CHYLOMICRON REMNANT (enriched in retinyl esters). Remnant → LIVER (LRP receptor) → retinyl esters stored in hepatic stellate cells (Ito cells). From liver: Retinol + RBP4 + Transthyretin → peripheral tissues.]\n\n📹 [VIDEO LINK]"},{"id":493,"s":"Vitamins","t":"Vitamin A","u":"Hypervitaminosis A","q":"A 40-year-old woman taking over-the-counter vitamins for 5 years presents with hepatomegaly, jaundice, dry pruritic skin, and frequent headaches. She is likely suffering from toxicity of which of the following vitamins?","o":["Vitamin A","Vitamin B1","Vitamin B2","Vitamin B3","Vitamin C"],"a":0,"p":"HYPERVITAMINOSIS A (chronic): hepatomegaly (vitamin A stored in hepatic stellate cells → accumulation → fibrosis), jaundice (liver damage), dry/pruritic/peeling skin, headache (↑ intracranial pressure — pseudotumour cerebri), bone pain, alopecia, teratogenicity. Vitamin A is FAT-SOLUBLE → stored in liver → toxicity accumulates with chronic excess. Water-soluble vitamins (B, C) are excreted renally — toxicity rare. Liver stores >90% of total body vitamin A in hepatic stellate cells (Ito cells).\n\n📹 [VIDEO LINK]"},{"id":494,"s":"Vitamins","t":"Vitamin A","u":"Blindness Retinoid Deficiency Orphan","q":"A 7-year-old orphan presents with blindness. Deficiency of which of the following is the most likely cause?","o":["Ascorbate","Biotin","Cobalamin","Folate","Retinoid"],"a":4,"p":"RETINOID (vitamin A) deficiency → keratomalacia → corneal ulceration → blindness. Vitamin A deficiency is the leading cause of preventable childhood blindness worldwide. Sequence: Night blindness → Conjunctival xerosis → Bitot's spots → Corneal xerosis → KERATOMALACIA (corneal melting → blindness). RETINOID = vitamin A and its derivatives (retinol, retinal, retinoic acid, retinyl esters).\n\n📹 [VIDEO LINK]"},{"id":495,"s":"Vitamins","t":"Vitamin A","u":"Earliest Sign Vitamin A Deficiency","q":"Which of the following best describes the earliest sign of vitamin A deficiency?","o":["Bitot's spots","Conjunctival xerosis","Corneal xerosis","Keratomalacia","Night blindness"],"a":4,"p":"NIGHT BLINDNESS (nyctalopia) = EARLIEST sign of vitamin A deficiency. Rod cells (dim-light vision) require 11-cis retinal to form rhodopsin. Deficiency → insufficient rhodopsin regeneration → rod cells fail before cone cells → dim-light vision impaired before daylight vision. Reversible with vitamin A supplementation. Later stages in order: Conjunctival xerosis → Bitot's spots → Corneal xerosis → Keratomalacia.\n\n📹 [VIDEO LINK]"},{"id":496,"s":"Vitamins","t":"Vitamin A","u":"Measles Vitamin A Susceptibility","q":"Vitamin A deficient children are particularly susceptible to complications of which of the following infectious diseases?","o":["Malaria","Measles","Mumps","Pneumonia","Tuberculosis"],"a":1,"p":"Vitamin A deficiency dramatically increases severity and mortality of MEASLES. Measles further depletes vitamin A → severe combined deficiency → loss of epithelial barrier integrity + impaired immune function → severe pneumonia, diarrhoea, keratoconjunctivitis, keratomalacia → blindness and death. WHO recommends high-dose vitamin A for ALL children with measles in developing countries. Reduces measles mortality by up to 50%.\n\n📹 [VIDEO LINK]"},{"id":497,"s":"Vitamins","t":"Vitamin A","u":"Vitamin A Forms Reproduction","q":"Which of the following forms of vitamin A is required for normal reproduction and spermatogenesis?","o":["11-cis retinal","All-trans retinal","Retinoic acid","Retinol and retinal","Retinol and retinoids"],"a":3,"p":"Different vitamin A forms have different functions: RETINAL = visual cycle. RETINOIC ACID = gene expression (nuclear receptors RAR/RXR) → epithelial differentiation, embryogenesis, immunity. RETINOL and RETINAL = required for REPRODUCTION (spermatogenesis, oogenesis). Key rule: retinoic acid supports ALL vitamin A functions EXCEPT vision and reproduction — these specifically require retinol/retinal.\n\n📹 [VIDEO LINK]"},{"id":498,"s":"Vitamins","t":"Vitamin A","u":"Retinoic Acid Epithelial Barrier Defence","q":"Which of the following vitamin A forms supports the body's barrier-based defence against pathogens by maintaining epithelial integrity?","o":["11-cis retinal","All-trans retinal","Retinoic acid","Retinol","Retinyl ester"],"a":2,"p":"RETINOIC ACID acts on nuclear receptors (RAR + RXR) → regulates gene expression → maintains normal EPITHELIAL DIFFERENTIATION → goblet cells produce mucus → mucus traps pathogens → barrier defence. Without retinoic acid → epithelial cells undergo squamous metaplasia → dry, keratinised epithelia → pathogen entry facilitated → infections in respiratory, GI, urinary, and ocular tracts. Retinal is for vision. Retinol supports reproduction and can be converted to retinoic acid.\n\n📹 [VIDEO LINK]"},{"id":499,"s":"Vitamins","t":"Vitamin A","u":"Beta-carotene Refugee Camp Gritty Eyes","q":"A 4-year-old boy living in a refugee camp has survived for the past 6 months on humanitarian aid consisting only of flour, rice, and cooking oil. He presents with dry skin and a gritty sensation in his eyes. Deficiency of which of the following vitamins is likely causing the gritty sensation in his eyes?","o":["Ascorbic acid","Cobalamin","Pyridoxine","Retinol","Thiamine"],"a":3,"p":"RETINOL (vitamin A) deficiency → loss of goblet cells in conjunctiva → insufficient mucus production → dry, non-wettable conjunctiva → GRITTY SENSATION = CONJUNCTIVAL XEROSIS (second stage of vitamin A deficiency). PRIMARY sources of preformed retinol = animal products (liver, egg yolk, dairy, fatty fish) — ALL absent in this diet. Beta-carotene (plant provitamin A) also absent (no vegetables or fruits). Gritty sensation specifically indicates conjunctival dryness from goblet cell loss.\n\n📹 [VIDEO LINK]"},{"id":500,"s":"Vitamins","t":"Vitamin A","u":"Measles Vitamin A WHO Protocol","q":"A 20-month-old male is brought to the emergency department with high-grade fever and a skin rash suggestive of measles. Which of the following vitamin supplementations should be considered?","o":["Vitamin A","Vitamin B12","Vitamin D","Vitamin E","Vitamin K"],"a":0,"p":"WHO PROTOCOL: High-dose Vitamin A for ALL children with measles: Age <6 months: 50,000 IU/day for 2 days. Age 6-11 months: 100,000 IU/day for 2 days. Age ≥12 months: 200,000 IU/day for 2 days. Measles depletes vitamin A → deficiency → loss of mucosal barrier integrity + impaired immunity → severe complications. Supplementation restores epithelial barriers, enhances immune response, reduces mortality by up to 50%, prevents pneumonia, diarrhoea, and keratomalacia.\n\n📹 [VIDEO LINK]"},{"id":501,"s":"Vitamins","t":"Vitamin A","u":"Vitamin A Storage Liver","q":"Vitamin A is stored mainly in which of the following organs?","o":["Bone","Kidney","Liver","Lung","Spleen"],"a":2,"p":"LIVER stores >90% of total body vitamin A, primarily in HEPATIC STELLATE CELLS (Ito cells) as RETINYL PALMITATE (retinyl esters). The liver releases retinol bound to RETINOL-BINDING PROTEIN 4 (RBP4) + TRANSTHYRETIN complex → delivered to peripheral tissues. Normal liver stores last 6-12 months. In chronic hypervitaminosis A: excessive retinyl ester accumulation in stellate cells → stellate cell activation → hepatic fibrosis → cirrhosis.\n\n📹 [VIDEO LINK]"},{"id":502,"s":"Vitamins","t":"Vitamin D","u":"Rickets ECM Hydroxyapatite Indoors","q":"A toddler who lives mostly indoors with his purdah-observing mother and does not like milk products presents at 18 months with waddling gait, bowing of legs, and swellings at the costochondral junctions. Which component of his bone extracellular matrix is primarily reduced?","o":["Collagen fibrils","Elastin fibres","Fibronectin","Glycosaminoglycans","Hydroxyapatite crystals"],"a":4,"p":"Three compounding risk factors: (1) Purdah-observing mother → infant kept indoors → minimal UV exposure → ↓ cholecalciferol synthesis in skin. (2) Darker skin → ↓ UV penetration. (3) No milk products → no dietary vitamin D or calcium. All three → vitamin D deficiency → impaired calcium absorption → decreased mineralisation → insufficient HYDROXYAPATITE CRYSTAL deposition → soft bones → rickets. Osteoid (organic matrix) is present but unmineralised.\n\n📹 [VIDEO LINK]"},{"id":503,"s":"Vitamins","t":"Vitamin D","u":"PTH Phosphaturia Rickets Kidney","q":"A 14-month-old exclusively breastfed boy presents with bowing of legs, frontal bossing, and beading at the costochondral junctions. His serum calcium is low, serum PTH is elevated, and serum vitamin D is markedly deficient. The elevated PTH is actively working to maintain serum calcium. How is this elevated PTH influencing the kidney to compensate?","o":["Calcium and phosphate excretion","Calcium and phosphate reabsorption","Calcium excretion and phosphate reabsorption","Calcium reabsorption and phosphate excretion","Phosphate reabsorption only"],"a":3,"p":"In rickets: vitamin D deficiency → ↓ intestinal calcium absorption → hypocalcaemia → secondary ↑ PTH (compensatory response). PTH acts on kidney: CALCIUM REABSORPTION (distal tubule — activates TRPV5 channels and calbindin → calcium saved) + PHOSPHATE EXCRETION (proximal tubule — internalises NaPi cotransporters → phosphaturia). Net: calcium retained + phosphate lost → serum calcium rises. Formula-fed infants are protected because formula is vitamin D-fortified.\n\n📹 [VIDEO LINK]"},{"id":504,"s":"Vitamins","t":"Vitamin D","u":"PTH Calcium Reabsorption Rickets","q":"A 2-year-old girl with nutritional rickets has low serum calcium and elevated PTH. The elevated PTH is working to restore serum calcium by increasing reabsorption of which mineral in the kidney?","o":["Calcium","Chloride","Magnesium","Phosphate","Sodium"],"a":0,"p":"PTH increases CALCIUM REABSORPTION in the distal convoluted tubule by activating TRPV5 calcium channels and calbindin. This saves calcium that would otherwise be lost in urine → helps raise serum calcium. Simultaneously PTH causes phosphate excretion. Together: calcium retained + phosphate excreted. This compensatory response in rickets is the parathyroid gland's attempt to restore serum calcium in the face of vitamin D deficiency.\n\n📹 [VIDEO LINK]"},{"id":505,"s":"Vitamins","t":"Vitamin D","u":"PTH 1-Alpha Hydroxylase Kidney","q":"A girl with nutritional rickets has secondary hyperparathyroidism. The elevated PTH activates which enzyme in the proximal tubule of the kidney to increase production of active vitamin D?","o":["1-alpha hydroxylase","24-hydroxylase","25-hydroxylase","Alpha hydroxylase","Hydroxymethylase"],"a":0,"p":"PTH activates 1-ALPHA HYDROXYLASE (CYP27B1) in the kidney proximal tubule → converts calcidiol (25-OH D3) → calcitriol (1,25-OH D3) = active vitamin D → increased intestinal calcium and phosphate absorption. 25-hydroxylase is in the LIVER and is NOT regulated by PTH. 24-hydroxylase is activated by HIGH calcitriol (degradation pathway). The PTH → 1-alpha hydroxylase → calcitriol axis is the critical link between parathyroid gland and vitamin D activation.\n\n📹 [VIDEO LINK]"},{"id":506,"s":"Vitamins","t":"Vitamin D","u":"PTH Bone Demineralisation Rickets","q":"A toddler with rickets and secondary hyperparathyroidism. The elevated PTH is acting on bones. Which of the following best describes this effect?","o":["Bone calcification","Bone demineralisation","Bone metastasis","Bone mineralisation","Bone ossification"],"a":1,"p":"Elevated PTH → stimulates OSTEOCLAST activity + inhibits osteoblast activity → BONE DEMINERALISATION → releases calcium and phosphate from bone → compensatory attempt to restore serum calcium. In rickets: SECONDARY hyperparathyroidism — the parathyroid gland responds appropriately to low calcium but causes bone loss as a consequence. Chronic secondary hyperparathyroidism → severe bone demineralisation → pathological fractures.\n\n📹 [VIDEO LINK]"},{"id":507,"s":"Vitamins","t":"Vitamin D","u":"Calcitriol Bone Mineralisation Healthy Infant","q":"A healthy breastfed infant receives vitamin D drops as per paediatric guidelines. Calcitriol in this healthy infant promotes which of the following in bone?","o":["Bone demineralisation","Bone metastasis","Bone mineralisation","Bone resorption","Osteoclast activation"],"a":2,"p":"In a HEALTHY infant with adequate vitamin D: calcitriol → increased intestinal calcium and phosphate absorption → adequate serum levels → BONE MINERALISATION → hydroxyapatite crystal deposition into osteoid → strong bones. Contrast with RICKETS: insufficient calcitriol → inadequate calcium/phosphate → bone demineralisation. Calcitriol binds VDR (Type 2 nuclear receptor) → gene transcription → calbindin synthesis → calcium absorption.\n\n📹 [VIDEO LINK]"},{"id":508,"s":"Vitamins","t":"Vitamin D","u":"Vitamin D Deficiency Dark Skin Scotland","q":"A dark-skinned infant born in Scotland to a South Asian family develops bowing of legs and frontal bossing at 14 months. He is exclusively breastfed. Which vitamin is deficient in him?","o":["Vitamin A","Vitamin B12","Vitamin D","Vitamin E","Vitamin K"],"a":2,"p":"VITAMIN D DEFICIENCY — three compounding risk factors: (1) Dark skin (melanin blocks UV-B → ↓ cholecalciferol synthesis). (2) Northern latitude Scotland (low UV-B, especially in winter). (3) Exclusively breastfed (breast milk contains very little vitamin D). All three → severe vitamin D deficiency → rickets. UK NICE recommends all infants receive 400 IU/day vitamin D supplementation. Dark-skinned infants in northern latitudes are the highest-risk group.\n\n📹 [VIDEO LINK]"},{"id":509,"s":"Vitamins","t":"Vitamin D","u":"Renal Osteodystrophy 1-Alpha Hydroxylase CKD","q":"An adult patient with chronic kidney disease (CKD stage 4) presents with bone pain. Despite adequate sun exposure and normal diet, serum calcitriol is markedly low. Which enzyme is malfunctioning in this patient?","o":["1-alpha hydroxylase","24-hydroxylase","25-hydroxylase","Dehydrogenase","Oxidase"],"a":0,"p":"CKD → loss of renal tubular mass → 1-ALPHA HYDROXYLASE deficient → cannot convert calcidiol → calcitriol → active vitamin D deficiency despite adequate sun and normal liver function → hypocalcaemia → secondary hyperparathyroidism → renal osteodystrophy. Calcidiol (25-OH D3) may be normal (liver functioning normally). The defect is specifically in the second hydroxylation step (kidney). Treatment: give CALCITRIOL directly (bypasses deficient kidney step).\n\n📹 [VIDEO LINK]"},{"id":510,"s":"Vitamins","t":"Vitamin D","u":"Alcohol Hypocalcaemia Magnesium Refractory","q":"A homeless alcoholic man presents with recurrent muscle cramps and perioral tingling. Serum calcium is 6.8 mg/dL (normal 8.5–10.5). PTH is 19 pg/mL (normal 10–60). His symptoms do not improve despite calcium and vitamin D supplementation for 2 weeks. What is the best next investigation?","o":["Serum albumin","Serum calcitriol","Serum creatinine","Serum magnesium","Serum phosphate"],"a":3,"p":"REFRACTORY HYPOCALCAEMIA despite calcium and vitamin D supplementation + inappropriately low/normal PTH + alcohol use disorder = HYPOMAGNESAEMIA. Magnesium deficiency impairs: (1) PTH SECRETION from parathyroid glands. (2) PTH ACTION on target tissues. Without magnesium → PTH cannot be secreted normally → calcium cannot be restored. Best next step = SERUM MAGNESIUM. Treatment: IV magnesium → PTH restored → calcium normalises.\n\n📹 [VIDEO LINK]"},{"id":511,"s":"Vitamins","t":"Vitamin D","u":"Hypervitaminosis D OTC Supplements","q":"A 55-year-old health-conscious athlete presents with fatigue, nausea, polyuria, and bone pain for 3 months. He has been taking over-the-counter supplements purchased from a pharmacy. He has no known malignancy and no history of granulomatous disease. Serum calcium is elevated and serum PTH is suppressed. Which of the following is the most likely diagnosis?","o":["Hyperparathyroidism","Hypervitaminosis D","Malignancy","Sarcoidosis","Vitamin D deficiency"],"a":1,"p":"Elevated calcium + SUPPRESSED PTH + over-the-counter supplement use + no malignancy or granulomatous disease = HYPERVITAMINOSIS D. Excess calcitriol → excessive intestinal calcium absorption + bone resorption → hypercalcaemia → PTH suppressed by negative feedback. Polyuria = nephrogenic diabetes insipidus (calcium inhibits AQP2 insertion). Symptoms: fatigue, nausea, vomiting, polyuria, bone pain, nephrocalcinosis. Treatment: stop supplements, increase fluids, corticosteroids in severe cases.\n\n📹 [VIDEO LINK]"},{"id":512,"s":"Vitamins","t":"Vitamin D","u":"Osteoporosis T-Score Fracture","q":"A sedentary 75-year-old woman trips on a step while getting off a bus and fractures her hip. DEXA scan shows T-score of -2.8 SD at the femoral neck. Serum calcium, phosphate, and ALP are all normal. Which condition is the most likely precipitating factor?","o":["Osteoarthritis","Osteomalacia","Osteopenia","Osteopetrosis","Osteoporosis"],"a":4,"p":"T-score ≤ -2.5 = OSTEOPOROSIS. SERUM CALCIUM IS NORMAL IN OSTEOPOROSIS — reduced QUANTITY of normally mineralised bone → calcium balance maintained at expense of bone mass → serum calcium remains normal. Unlike osteomalacia: ↓ Ca + ↓ phosphate + ↑ ALP (mineralisation defect). Osteoporosis = quantity defect, not mineralisation defect. Normal calcium + normal phosphate + normal ALP + low T-score = osteoporosis.\n\n📹 [VIDEO LINK]"},{"id":513,"s":"Vitamins","t":"Vitamin D","u":"Postmenopausal Osteoporosis Osteoclast","q":"A 65-year-old oestrogen-deficient woman who takes no medications fractures her wrist after a minor fall. DEXA scan confirms osteoporosis. Which cell type is overactive in her bones?","o":["Chondroblast","Chondrocyte","Fibroblast","Osteoblast","Osteoclast"],"a":4,"p":"Oestrogen deficiency → loss of RANKL suppression → ↑ RANKL → OSTEOCLAST activation → ↑ bone resorption → osteoporosis. Oestrogen normally promotes osteoprotegerin (OPG) which inhibits osteoclastogenesis. Without oestrogen → OPG falls → RANKL activates osteoclasts unopposed. Treatment: bisphosphonates (inhibit osteoclasts), denosumab (anti-RANKL), teriparatide (intermittent PTH — anabolic).\n\n📹 [VIDEO LINK]"},{"id":514,"s":"Vitamins","t":"Vitamin D","u":"Calcitonin Inhibits Osteoclasts","q":"A patient with medullary thyroid carcinoma has markedly elevated serum calcitonin. Which of the following best describes the action of calcitonin on bone?","o":["Activates osteoclasts","Activates osteoblasts","Inhibits osteoblasts","Inhibits osteoclasts","Stimulates bone resorption"],"a":3,"p":"CALCITONIN inhibits OSTEOCLAST activity → decreases bone resorption → lowers serum calcium. Produced by parafollicular C cells of thyroid. Physiological antagonist of PTH on bone. Clinical uses: Paget's disease, hypercalcaemia of malignancy, osteoporosis (second-line). Medullary thyroid carcinoma = C cell tumour → extremely elevated calcitonin = tumour marker.\n\n📹 [VIDEO LINK]"},{"id":515,"s":"Vitamins","t":"Vitamin D","u":"Osteomalacia Anticonvulsants","q":"A 50-year-old woman on long-term phenytoin for epilepsy presents with bone pain and proximal muscle weakness. Laboratory tests show low calcium, low phosphate, and elevated alkaline phosphatase. Bone biopsy shows excess unmineralised osteoid. Which of the following is the correct diagnosis?","o":["Osteoarthritis","Osteomalacia","Osteopenia","Osteopetrosis","Osteoporosis"],"a":1,"p":"OSTEOMALACIA = deficient mineralisation of newly formed bone matrix (osteoid) in adults. Lab: ↓ Ca + ↓ phosphate + ↑ ALP. Unmineralised osteoid on biopsy = hallmark. Phenytoin causes osteomalacia by inducing hepatic CYP450 → accelerated degradation of vitamin D → vitamin D deficiency. Compare: OSTEOPOROSIS = normal Ca + normal phosphate + normal/mildly ↑ ALP (quantity defect, not mineralisation defect).\n\n📹 [VIDEO LINK]"},{"id":516,"s":"Vitamins","t":"Vitamin D","u":"Liver Disease Calcidiol Formation Impaired","q":"A 60-year-old man with autoimmune hepatitis presents with bone pain. He has adequate sun exposure and takes standard vitamin D supplements but his vitamin D levels remain low. Which biochemical step is most likely impaired?","o":["1-alpha hydroxylation","1,25 hydroxylation","Calcidiol formation","Calcitriol formation","Cholecalciferol formation"],"a":2,"p":"Standard vitamin D supplements provide CHOLECALCIFEROL. Cholecalciferol → 25-HYDROXYLASE (liver) → CALCIDIOL (first hydroxylation step). Autoimmune hepatitis → hepatocyte loss → 25-hydroxylase deficient → CALCIDIOL FORMATION impaired → no substrate for kidney → calcitriol also falls → vitamin D deficiency despite adequate sun and supplementation. Treatment: give CALCIDIOL directly (bypasses liver). Compare: CKD → 1-alpha hydroxylation fails → give CALCITRIOL directly.\n\n[Diagram: Vitamin D synthesis pathway — Skin: 7-dehydrocholesterol → (UV light) → Cholecalciferol (working normally, green tick). LIVER: Cholecalciferol → (25-hydroxylase) → Calcidiol (25-OH D3) — RED X: 25-hydroxylase deficient → CALCIDIOL FORMATION BLOCKED. KIDNEY: Calcidiol → (1-alpha-hydroxylase) → Calcitriol — dotted arrow (no substrate). Standard supplement: enters at cholecalciferol → blocked at liver. Calcidiol supplement: bypasses liver. Compare: Renal disease → 1-alpha hydroxylation blocked → give calcitriol.]\n\n📹 [VIDEO LINK]"},{"id":517,"s":"Vitamins","t":"Vitamin D","u":"Vitamin D K2 Co-supplementation Vascular Calcification","q":"A 36-year-old woman on high-dose vitamin D supplements starts vitamin K₂ supplementation on her physician's advice. Co-supplementation of vitamin K₂ with vitamin D best prevents which of the following complications?","o":["Hepatic failure","Neural tube defects","Peripheral neuropathy","Renal calculi","Vascular calcification"],"a":4,"p":"Vitamin D → ↑ intestinal calcium absorption → elevated serum calcium. Without sufficient Vitamin K₂: calcium may deposit in soft tissues and blood vessels → VASCULAR CALCIFICATION. Vitamin K₂ (menaquinone) activates OSTEOCALCIN and MATRIX GLA PROTEIN (MGP) — both require gamma-carboxylation → carboxylated osteocalcin binds calcium INTO bone. Carboxylated MGP INHIBITS arterial calcification. Co-supplementation of Vitamin D + K₂ ensures increased calcium absorbed under vitamin D is directed INTO BONE, not into arteries.\n\n📹 [VIDEO LINK]"},{"id":518,"s":"Vitamins","t":"Vitamin D","u":"Ergocalciferol Plants","q":"Which of the following forms of vitamin D is found in plants and fungi?","o":["7-dehydrocholesterol","Calcidiol","Calcitriol","Cholecalciferol","Ergocalciferol"],"a":4,"p":"ERGOCALCIFEROL (Vitamin D2) = plant/fungal form. Produced when ERGOSTEROL (plant/fungal sterol) is exposed to UV light. CHOLECALCIFEROL (Vitamin D3) = animal/human form → produced in skin from 7-dehydrocholesterol upon UV-B exposure. Both D2 and D3 are hydroxylated in liver (25-hydroxylase → calcidiol) and kidney (1-alpha-hydroxylase → calcitriol). D3 is generally considered more potent than D2 at raising serum 25-OH vitamin D levels.\n\n📹 [VIDEO LINK]"},{"id":519,"s":"Vitamins","t":"Vitamin D","u":"Calcidiol Storage Form","q":"The storage form of vitamin D in the body, measured clinically to assess vitamin D status, is which of the following?","o":["7-dehydrocholesterol","Calcidiol","Calcitriol","Cholecalciferol","Ergocalciferol"],"a":1,"p":"CALCIDIOL (25-OH vitamin D3) = major CIRCULATING and STORAGE form of vitamin D. Half-life ~3 weeks (vs calcitriol = ~4-6 hours). Calcidiol = the form MEASURED clinically to assess vitamin D status (normal: 20-40 ng/mL). CALCITRIOL (1,25-OH D3) = ACTIVE form (binds VDR) but present in much lower concentrations. Distinction: calcidiol = storage/measurement form. Calcitriol = active form. In liver disease → calcidiol formation impaired → supplement with calcidiol.\n\n📹 [VIDEO LINK]"},{"id":520,"s":"Vitamins","t":"Vitamin E","u":"Vitamin E Natural Antioxidant","q":"The most important natural antioxidant in cell membranes and plasma lipoproteins is which of the following?","o":["Vitamin A","Vitamin B12","Vitamin D","Vitamin E","Vitamin K"],"a":3,"p":"VITAMIN E (alpha-tocopherol) = most important natural CHAIN-BREAKING antioxidant. Mechanism: vitamin E donates a hydrogen atom to peroxyl radicals → peroxyl radical converted to stable lipid hydroperoxide → TOCOPHEROXYL RADICAL formed (relatively stable → does not propagate chain). Vitamin E protects: (1) PUFAs in cell membranes from peroxidation. (2) LDL from oxidation (ox-LDL → atherosclerosis). Vitamin C REGENERATES tocopherol from tocopheroxyl radical → vitamin C and E work together as antioxidant pair.\n\n📹 [VIDEO LINK]"},{"id":521,"s":"Vitamins","t":"Vitamin E","u":"Tocopheroxyl Radical Chain Breaking","q":"Chain-breaking, free-radical trapping antioxidant action of vitamin E in cell membranes leads to the formation of which of the following?","o":["Hydroxyl radical","Hydroxyperoxy radical","Peroxyradical","Superoxide radical","Tocopheroxyl radical"],"a":4,"p":"Lipid peroxidation chain: PUFA → (free radical) → PUFA radical → (O₂) → peroxyl radical (ROO•) — chain-carrying species. Vitamin E intercepts: tocopherol + ROO• → lipid hydroperoxide (ROOH) + TOCOPHEROXYL RADICAL (Toc•). Tocopheroxyl radical is relatively stable → does NOT propagate chain → chain BROKEN. Vitamin C then reduces Toc• → regenerates active tocopherol.\n\n📹 [VIDEO LINK]"},{"id":522,"s":"Vitamins","t":"Vitamin E","u":"Vitamin E PUFA Requirement","q":"The requirement of vitamin E is increased with greater dietary intake of which of the following?","o":["Long-chain fatty acids","Monounsaturated fatty acids","Polyunsaturated fatty acids","Saturated fatty acids","Very long-chain fatty acids"],"a":2,"p":"POLYUNSATURATED FATTY ACIDS (PUFAs) have multiple double bonds → highly susceptible to lipid peroxidation by free radicals. More PUFA in diet and cell membranes → greater oxidation risk → greater vitamin E requirement. SATURATED fatty acids have no double bonds → not susceptible to peroxidation → no increased vitamin E requirement. High PUFA diets require higher vitamin E intake — the two must be balanced.\n\n📹 [VIDEO LINK]"},{"id":523,"s":"Vitamins","t":"Vitamin E","u":"Vitamin C Regenerates Vitamin E","q":"Vitamin C regenerates active vitamin E (tocopherol) from tocopheroxyl radical by which of the following reactions?","o":["Carboxylation","Hydroxylation","Methylation","Oxidation","Reduction"],"a":4,"p":"After vitamin E traps a peroxyl radical → TOCOPHEROXYL RADICAL (Toc•) formed. Vitamin C (ascorbate) REDUCES Toc• → regenerates active TOCOPHEROL. Ascorbate → MONODEHYDROASCORBATE RADICAL (MDHA) → MDHA further reduced back to ascorbate by NADH-dependent enzyme (monodehydroascorbate reductase). Antioxidant network: Vitamin E (fat-soluble, membrane) → traps peroxyl radicals. Vitamin C (water-soluble) → regenerates vitamin E at membrane-cytosol interface. Glutathione → regenerates vitamin C.\n\n📹 [VIDEO LINK]"},{"id":524,"s":"Vitamins","t":"Vitamin K","u":"Vitamin K Gamma Carboxylation Cofactor","q":"Which of the following reactions requires vitamin K as a cofactor?","o":["Alpha carboxylation","Beta oxidation","Gamma carboxylation","Hydroxylation","Omega oxidation"],"a":2,"p":"Vitamin K is the cofactor for GAMMA-GLUTAMYL CARBOXYLASE → converts GLUTAMATE residues → GAMMA-CARBOXYGLUTAMATE (Gla) residues. Gla residues chelate Ca²⁺ → proteins bind phospholipid surfaces → coagulation cascade activated. Vitamin K-dependent proteins: CLOTTING FACTORS II, VII, IX, X. ANTICOAGULANT proteins C, S, Z. BONE proteins: osteocalcin, matrix Gla protein (MGP).\n\n📹 [VIDEO LINK]"},{"id":525,"s":"Vitamins","t":"Vitamin K","u":"Vitamin K Glutamate Residues","q":"Vitamin K activates clotting factors by gamma carboxylation of which amino acid residue?","o":["Aspartate residues","Butyrate residues","Fumarate residues","Glutamate residues","Malate residues"],"a":3,"p":"Vitamin K-dependent gamma-glutamyl carboxylase converts GLUTAMATE (Glu) residues → GAMMA-CARBOXYGLUTAMATE (Gla) residues. The extra carboxyl group on the gamma carbon allows Gla residues to chelate TWO Ca²⁺ ions → clotting factors undergo conformational change → bind to phosphatidylserine on platelet surfaces → coagulation cascade proceeds. Without vitamin K (or with warfarin) → glutamate residues not carboxylated → clotting factors cannot bind phospholipid surfaces → anticoagulation.\n\n📹 [VIDEO LINK]"},{"id":526,"s":"Vitamins","t":"Vitamin K","u":"Vitamin K Clotting Factors II VII IX X","q":"Which of the following are the clotting factors linked to vitamin K?","o":["Factors I, III, IV, X","Factors II, III, IV, X","Factors II, VII, VIII, X","Factors II, VII, IX, X","Factors II, VII, VIII, XI"],"a":3,"p":"Vitamin K-dependent clotting factors: II (prothrombin), VII, IX, X — mnemonic: 1972 (19=IX, 7=VII, 2=II, add X). Also: Proteins C, S, Z (anticoagulants). Factor VIII = NOT vitamin K-dependent (deficient in haemophilia A). Warfarin blocks vitamin K epoxide reductase → blocks recycling of KH₂ → all 4 factors + proteins C and S fail → anticoagulation. PT/INR reflects factors II, VII, X → used to monitor warfarin.\n\n📹 [VIDEO LINK]"},{"id":527,"s":"Vitamins","t":"Vitamin K","u":"Neonatal Bleeding Post-translational Modification","q":"A term infant is born at home, does well with breastfeeding, but bleeds from the umbilical cord and nostrils on day 2. Impairment of which of the following processes is the most likely cause?","o":["Co-translational modifications","Co-transcriptional modifications","Post-transcriptional modifications","Post-translational modifications","Translational modifications"],"a":3,"p":"Neonatal vitamin K deficiency bleeding: Vitamin K-dependent gamma-carboxylation of clotting factors II, VII, IX, X is a POST-TRANSLATIONAL MODIFICATION — occurs AFTER the protein is fully synthesised on ribosomes, in the ER lumen. Why NOT co-translational: co-translational modifications occur WHILE the protein is being synthesised (e.g. signal peptide cleavage, N-glycosylation). Gamma-carboxylation requires the completed polypeptide in the ER lumen → definitively post-translational. Neonatal intestines are sterile → no bacterial K2. Breast milk contains very little vitamin K. Prevention: vitamin K1 injection at birth.\n\n📹 [VIDEO LINK]"},{"id":528,"s":"Vitamins","t":"Vitamin K","u":"Warfarin Vitamin K Epoxide Cannot Be Reduced","q":"In the presence of warfarin, which of the following cannot be reduced back to its active hydroquinone form?","o":["Menaquinone","Phylloquinone","Vitamin K epoxide","Vitamin K hydroquinone","Vitamin K quinone"],"a":2,"p":"Vitamin K cycle: Active KH₂ (hydroquinone) → gamma-carboxylase uses it → K EPOXIDE formed (spent form) → VITAMIN K EPOXIDE REDUCTASE (VKORC1) normally reduces K epoxide → K quinone → KH₂ (recycled). WARFARIN inhibits VKORC1 → VITAMIN K EPOXIDE ACCUMULATES (cannot be reduced) → KH₂ not regenerated → gamma-carboxylase has no substrate → clotting factors II, VII, IX, X + proteins C, S cannot be gamma-carboxylated → anticoagulation.\n\n📹 [VIDEO LINK]"},{"id":529,"s":"Vitamins","t":"Vitamin K","u":"Cefamandole Vitamin K Depletion Bleeding","q":"A 60-year-old woman with septicaemia receiving cefamandole develops oozing from her IV line site. Depletion of which of the following vitamins is responsible?","o":["Vitamin A","Vitamin B","Vitamin D","Vitamin E","Vitamin K"],"a":4,"p":"Cefamandole (and other N-methylthiotetrazole-containing cephalosporins: cefoperazone, moxalactam) → inhibits vitamin K epoxide reductase → same mechanism as warfarin → vitamin K deficiency → impaired gamma-carboxylation of clotting factors → BLEEDING. Also: broad-spectrum antibiotics → kill intestinal bacteria → reduce bacterial vitamin K2 production → worsens deficiency. The combination of gut flora suppression + direct VKORC1 inhibition makes these cephalosporins prone to causing bleeding.\n\n📹 [VIDEO LINK]"},{"id":530,"s":"Vitamins","t":"Vitamin K","u":"Fetal Warfarin Syndrome Gla Proteins","q":"A 28-year-old woman with epilepsy taking phenytoin is prescribed warfarin for DVT during pregnancy. Impaired gamma carboxylation of which of the following proteins is the major concern for fetal bone development?","o":["Fetal apolipoprotein B","Fetal clotting factors","Fetal Gla proteins","Fetal haemoglobin","Maternal clotting factors"],"a":2,"p":"FETAL WARFARIN SYNDROME (warfarin embryopathy): warfarin crosses the placenta → inhibits gamma-carboxylation of FETAL GLA PROTEINS — specifically MATRIX GLA PROTEIN (MGP) and OSTEOCALCIN. MGP normally INHIBITS ectopic calcification in cartilage — without carboxylated MGP → STIPPLED EPIPHYSES + NASAL HYPOPLASIA. Osteocalcin guides calcium into bone → abnormal bone formation without it. These bone defects are DISTINCT from fetal haemorrhage (impaired clotting factors) and fetal thrombosis.\n\n📹 [VIDEO LINK]"},{"id":531,"s":"Vitamins","t":"Vitamin K","u":"Hypervitaminosis K Menadione Haemolytic Anaemia","q":"Hypervitaminosis K in neonates caused specifically by menadione (synthetic vitamin K₃) may result in which of the following?","o":["Aplastic anaemia","Haemolytic anaemia","Iron deficiency anaemia","Megaloblastic anaemia","Sideroblastic anaemia"],"a":1,"p":"MENADIONE (vitamin K₃, synthetic) in neonates → generates REACTIVE OXYGEN SPECIES → oxidative damage to RBC membranes → HEINZ BODY HAEMOLYTIC ANAEMIA → hyperbilirubinaemia (jaundice). This is why menadione is NO LONGER used for neonatal vitamin K prophylaxis. Natural forms (phylloquinone K1, menaquinone K2) do NOT cause haemolytic anaemia. All neonates now receive PHYLLOQUINONE (K1) injection at birth. Particularly dangerous in G6PD-deficient neonates.\n\n📹 [VIDEO LINK]"},{"id":532,"s":"Vitamins","t":"Vitamin C","u":"Scurvy Tea Toast Diet","q":"A 68-year-old man living alone on a tea and toast diet for 2 months presents with fatigue, bleeding gums, perifollicular haemorrhages, and corkscrew hairs over his lower limbs. Haemoglobin is 10.5 g/dL, MCV 78 fL. Deficiency of which of the following vitamins is responsible?","o":["Vitamin B1","Vitamin B6","Vitamin C","Vitamin D","Vitamin K"],"a":2,"p":"SCURVY (Vitamin C deficiency): perifollicular haemorrhages + corkscrew/swan-neck hairs + bleeding/spongy gums = CLASSIC tetrad. Mechanism: vitamin C required for prolyl hydroxylase and lysyl hydroxylase (Fe²⁺ maintenance) → without vitamin C → collagen triple helix unstable → weak blood vessels → haemorrhage. Corkscrew hairs = follicular hyperkeratosis with coiled hairs embedded. Tea and toast diet = classic nutritional history for elderly scurvy.\n\n📹 [VIDEO LINK]"},{"id":533,"s":"Vitamins","t":"Vitamin C","u":"Vitamin C Ferrireductase Iron Absorption","q":"Vitamin C is a coenzyme for ferrireductase in the duodenum. This function of vitamin C facilitates iron absorption by converting iron to which form that can be transported by DMT1?","o":["Fe²⁺ (ferrous)","Fe³⁺ (ferric)","Fe⁴⁺","Ferritin-bound iron","Transferrin-bound iron"],"a":0,"p":"Dietary iron comes mainly as Fe³⁺ (ferric) in non-haem form. DMT1 ONLY transports Fe²⁺ (ferrous). Vitamin C acts as electron donor for DUODENAL CYTOCHROME b (DCYTB/ferrireductase) → reduces Fe³⁺ → Fe²⁺ → enters enterocyte via DMT1. Also: vitamin C maintains acidic environment → iron stays soluble.\n\n[Diagram: Intestinal lumen cross-section. Lumen side: dietary Fe³⁺ → DCYTB (requires Vitamin C as electron donor) → Fe²⁺ → enters enterocyte via DMT1 (apical membrane). Inside enterocyte: Fe²⁺ stored as FERRITIN or exported via FERROPORTIN (basolateral). Hephaestin (copper) oxidises Fe²⁺ → Fe³⁺ → binds TRANSFERRIN in portal blood. Vitamin C molecule shown donating electron to DCYTB. HEPCIDIN shown degrading ferroportin. Inhibitors in red: phytates, tannins, antacids block Fe³⁺ reduction step.]\n\n📹 [VIDEO LINK]"},{"id":534,"s":"Vitamins","t":"Vitamin C","u":"Ascorbic Acid Highest in Adrenal","q":"In the human body, the highest concentration of ascorbic acid is found in which of the following glands?","o":["Adrenal gland","Exocrine gland","Parathyroid gland","Pituitary gland","Thyroid gland"],"a":0,"p":"ADRENAL GLAND has the highest concentration of ascorbic acid. Functions: (1) ADRENAL CORTEX: required for hydroxylation reactions in steroidogenesis (CYP450-dependent). (2) ADRENAL MEDULLA: required for DOPAMINE BETA-HYDROXYLASE → converts dopamine → noradrenaline. Stress → ACTH → adrenal stimulation → rapid ascorbic acid depletion. This is why vitamin C is described as an anti-stress vitamin — high demand during stress response.\n\n📹 [VIDEO LINK]"},{"id":535,"s":"Vitamins","t":"Vitamin C","u":"Vitamin C Aspartate Hydroxylase Anticoagulant","q":"Anticoagulant role of vitamin C depends upon its coenzyme function for which of the following hydroxylases?","o":["Aspartate hydroxylase","Dopamine beta-hydroxylase","Lysyl hydroxylase","Peptidylglycine hydroxylase","Prolyl hydroxylase"],"a":0,"p":"ASPARTATE BETA-HYDROXYLASE requires vitamin C as cofactor → hydroxylates aspartate residues in the EGF-like domains of PROTEIN C and PROTEIN S (anticoagulant proteins) → hydroxylated aspartate residues are essential for the anticoagulant activity of Protein C and Protein S. Without vitamin C → aspartate hydroxylase impaired → Protein C and Protein S cannot be fully activated → anticoagulant pathway impaired → mild prothrombotic tendency in scurvy. Per Lippincott's Biochemistry.\n\n📹 [VIDEO LINK]"},{"id":536,"s":"Vitamins","t":"Thiamine — B1","u":"Wernicke Encephalopathy Oxidative Decarboxylation","q":"A 54-year-old chronic alcoholic is brought to the emergency department with confusion, horizontal nystagmus, and ataxia. Which biochemical process is most likely impaired?","o":["Carboxylation","Deamination","Hydroxylation","Oxidative decarboxylation","Transamination"],"a":3,"p":"Wernicke encephalopathy = THIAMINE DEFICIENCY. Thiamine pyrophosphate (TPP) is the cofactor for OXIDATIVE DECARBOXYLATION reactions: (1) Pyruvate dehydrogenase (pyruvate → acetyl-CoA). (2) Alpha-ketoglutarate dehydrogenase (alpha-KG → succinyl-CoA). (3) Branched-chain alpha-keto acid dehydrogenase (BCAA catabolism). (4) Transketolase (pentose phosphate pathway). Without TPP → pyruvate and alpha-KG accumulate → citric acid cycle cannot run → energy failure → NEUROLOGICAL DAMAGE. Give THIAMINE BEFORE glucose.\n\n📹 [VIDEO LINK]"},{"id":537,"s":"Vitamins","t":"Thiamine — B1","u":"Korsakoff Psychosis Irreversible","q":"Which of the following features of Wernicke-Korsakoff syndrome is irreversible?","o":["Ataxia","Korsakoff psychosis","Nystagmus","Ophthalmoplegia","Wernicke confusion"],"a":1,"p":"WERNICKE ENCEPHALOPATHY (acute): ophthalmoplegia, nystagmus, ataxia, confusion — partially REVERSIBLE with thiamine. KORSAKOFF PSYCHOSIS (chronic): anterograde + retrograde amnesia, confabulation — IRREVERSIBLE (bilateral mammillary body destruction). 80% of untreated Wernicke → progresses to Korsakoff. Only 25% of Korsakoff patients recover even with thiamine. Confabulation = patients confidently fabricate memories to fill memory gaps — neurological impairment, not deception.\n\n📹 [VIDEO LINK]"},{"id":538,"s":"Vitamins","t":"Thiamine — B1","u":"Thiamine Before Glucose Alcoholic","q":"A comatose 40-year-old chronic alcoholic is brought to the emergency department. His blood glucose is 2.1 mmol/L. Which vitamin must be administered BEFORE giving glucose?","o":["Biotin","Niacin","Pyridoxine","Riboflavin","Thiamine"],"a":4,"p":"THIAMINE must be given BEFORE glucose in alcoholics. Glucose administration → metabolises glucose using remaining thiamine → rapidly consumes remaining thiamine → precipitates ACUTE WERNICKE ENCEPHALOPATHY. Protocol: give IV THIAMINE first → then glucose. Chronic alcoholics: poor dietary thiamine intake + impaired intestinal absorption + impaired liver storage → marginal thiamine. Any increase in carbohydrate metabolism → thiamine depletion → Wernicke. Applies to ALL malnourished patients being resuscitated with glucose.\n\n📹 [VIDEO LINK]"},{"id":539,"s":"Vitamins","t":"Thiamine — B1","u":"Thiamine Polished Rice Citric Acid Cycle","q":"Dr. Eijkman observed fatigue in chickens fed on polished rice, reversed by adding thiamine-rich rice bran. Inhibition of which of the following pathways caused the fatigue?","o":["Amino acid cycle","Cori's cycle","Citric acid cycle","Glucose alanine cycle","Urea cycle"],"a":2,"p":"Polished white rice = thiamine removed during milling. Thiamine (TPP) required for: (1) PYRUVATE DEHYDROGENASE → acetyl-CoA cannot be formed → CITRIC ACID CYCLE cannot run. (2) ALPHA-KETOGLUTARATE DEHYDROGENASE → TCA cycle blocked at another step. Result: acetyl-CoA deficiency → citric acid cycle stops → ATP deficiency → FATIGUE. Pyruvate accumulates → lactic acidosis. Eijkman's observation in 1897 led to discovery of vitamins.\n\n📹 [VIDEO LINK]"},{"id":540,"s":"Vitamins","t":"Thiamine — B1","u":"Wet Beri Beri High Output Cardiac Failure","q":"A 32-year-old man returning from a remote work camp where he ate almost entirely polished white rice presents with tachycardia, elevated JVP, warm extremities, and bilateral pitting oedema. Echocardiogram shows high-output cardiac failure. Deficiency of which vitamin is responsible?","o":["Biotin","Niacin","Pantothenic acid","Riboflavin","Thiamine"],"a":4,"p":"WET BERI BERI = THIAMINE deficiency with cardiovascular involvement. Mechanism: thiamine deficiency → ATP deficiency in vascular smooth muscle → peripheral VASODILATION → compensatory ↑ cardiac output → HIGH-OUTPUT cardiac failure → oedema, elevated JVP, tachycardia, warm extremities. Compare: DRY BERI BERI = peripheral neuropathy. SHOSHIN = fulminant cardiovascular collapse (acute severe form). Both from same thiamine deficiency.\n\n📹 [VIDEO LINK]"},{"id":541,"s":"Vitamins","t":"Thiamine — B1","u":"Dry Beri Beri Peripheral Neuropathy","q":"A 46-year-old man presents with progressive foot drop, bilateral sensory loss, and inability to walk. He is a labourer who eats mostly polished white rice and tea. Ankle reflexes are absent. Cardiac exam is unremarkable. Which vitamin deficiency best explains his condition?","o":["Cobalamin","Niacin","Pyridoxine","Riboflavin","Thiamine"],"a":4,"p":"DRY BERI BERI = THIAMINE deficiency with peripheral nervous system involvement. Features: peripheral neuropathy (bilateral, symmetric, ascending), foot drop, sensory loss (stocking-glove distribution), areflexia, muscle wasting. WITHOUT cardiac features = dry beri beri. Polished rice + labourer demographics = classic scenario. Compare: Cobalamin deficiency = subacute combined degeneration (posterior AND lateral columns, not predominantly peripheral neuropathy).\n\n📹 [VIDEO LINK]"},{"id":542,"s":"Vitamins","t":"Thiamine — B1","u":"Thiamine Branched Chain Amino Acids","q":"Patients with thiamine deficiency cannot properly catabolise which of the following groups of amino acids?","o":["Acidic amino acids","Aromatic amino acids","Basic amino acids","Branched-chain amino acids","Polar amino acids"],"a":3,"p":"BRANCHED-CHAIN AMINO ACIDS (leucine, isoleucine, valine) catabolism requires BRANCHED-CHAIN ALPHA-KETO ACID DEHYDROGENASE (BCKAD) — uses THIAMINE PYROPHOSPHATE (TPP) as cofactor. Without thiamine → BCKAD blocked → branched-chain alpha-keto acids accumulate. Also: transketolase (pentose phosphate pathway) requires TPP → NADPH production impaired. Thiamine TPP is required for ALL oxidative decarboxylation reactions involving alpha-keto acids.\n\n📹 [VIDEO LINK]"},{"id":543,"s":"Vitamins","t":"Thiamine — B1","u":"Thiamine Emergency Alcoholic","q":"Which of the following vitamins should be added to the emergency treatment of a comatose 40-year-old chronic alcoholic?","o":["Biotin","Niacin","Pyridoxine","Riboflavin","Thiamine"],"a":4,"p":"THIAMINE (Vitamin B1) must be given to ALL comatose alcoholic patients BEFORE glucose. Alcoholism → poor thiamine intake + impaired intestinal absorption + impaired hepatic storage → marginal thiamine. Any glucose load → precipitates Wernicke encephalopathy. Standard emergency protocol: IV THIAMINE 100-500 mg BEFORE any glucose infusion. Wernicke encephalopathy triad: ophthalmoplegia + ataxia + confusion — partially reversible. Korsakoff psychosis = irreversible if not caught early.\n\n📹 [VIDEO LINK]"},{"id":544,"s":"Vitamins","t":"Thiamine — B1","u":"Thiamine Chronic Alcoholic Disoriented","q":"A 47-year-old drunk and disoriented chronic alcoholic presents with hypoglycaemia. Which vitamin supplementation should be added to his treatment plan?","o":["Vitamin B1","Vitamin B2","Vitamin B3","Vitamin B6","Vitamin B9"],"a":0,"p":"THIAMINE (Vitamin B1) must be given to ALL malnourished/alcoholic patients presenting with hypoglycaemia BEFORE glucose is administered. Glucose metabolism consumes remaining thiamine → precipitates Wernicke encephalopathy if thiamine not given first. Hypoglycaemia in alcoholics = impaired gluconeogenesis (alcohol inhibits NADH-dependent gluconeogenic steps) + impaired glycogenolysis. Protocol: IV thiamine → then glucose resuscitation.\n\n📹 [VIDEO LINK]"},{"id":545,"s":"Vitamins","t":"Riboflavin — B2","u":"Cheilosis Succinate Dehydrogenase","q":"An 85-year-old woman in a nursing home presents with fissuring at the angles of the mouth (cheilosis). Which enzyme of energy metabolism is most likely malfunctioning?","o":["Acetyl-CoA carboxylase","HMG-CoA reductase","Pyruvate kinase","Succinate dehydrogenase","Xanthine oxidase"],"a":3,"p":"Cheilosis (angular stomatitis) = RIBOFLAVIN DEFICIENCY. Riboflavin forms FAD and FMN. SUCCINATE DEHYDROGENASE (Complex II) contains FAD as prosthetic group → oxidises succinate → fumarate → FADH₂ → electrons to ubiquinone. Riboflavin deficiency → impaired succinate dehydrogenase → impaired electron transport chain → reduced ATP. Features: cheilosis, glossitis (magenta tongue), seborrheic dermatitis, corneal vascularisation, photophobia.\n\n📹 [VIDEO LINK]"},{"id":546,"s":"Vitamins","t":"Riboflavin — B2","u":"Riboflavin Succinate to Fumarate TCA","q":"In the citric acid cycle, riboflavin is required at which of the following steps?","o":["Alpha-ketoglutarate to succinate","Citrate to isocitrate","Malate to oxaloacetate","Oxaloacetate to citrate","Succinate to fumarate"],"a":4,"p":"SUCCINATE DEHYDROGENASE catalyses succinate → fumarate → requires FAD (riboflavin derivative) as prosthetic group. FAD accepts 2H → FADH₂. This is the ONLY TCA cycle step requiring riboflavin (FAD). It is also the ONLY TCA enzyme embedded in the inner mitochondrial membrane (Complex II). Compare: pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase also contain FAD (in their dihydrolipoyl dehydrogenase component) — but these are not TCA steps per se.\n\n📹 [VIDEO LINK]"},{"id":547,"s":"Vitamins","t":"Riboflavin — B2","u":"FMN NADH Dehydrogenase Complex I","q":"FMN is associated with which of the following enzymes in the electron transport chain?","o":["Alpha-ketoglutarate dehydrogenase","Branched-chain alpha-keto acid dehydrogenase","NADH dehydrogenase","Pyruvate dehydrogenase","Succinate dehydrogenase"],"a":2,"p":"FMN (Flavin Mononucleotide) = prosthetic group of NADH DEHYDROGENASE (Complex I). NADH → Complex I → FMN accepts electrons → FMNH₂ → electrons transferred to ubiquinone (CoQ) → proton pumping → ATP synthesis. FAD = prosthetic group of SUCCINATE DEHYDROGENASE (Complex II) and dihydrolipoyl dehydrogenase (in PDH, AKDH, BCKAD complexes). Both FMN and FAD are riboflavin (B2) derivatives. FMN = Complex I. FAD = Complex II.\n\n📹 [VIDEO LINK]"},{"id":548,"s":"Vitamins","t":"Riboflavin — B2","u":"Riboflavin Phototherapy Neonates","q":"Which of the following vitamins should be given to infants receiving phototherapy for neonatal jaundice?","o":["Vitamin B1","Vitamin B2","Vitamin B3","Vitamin B5","Vitamin B12"],"a":1,"p":"RIBOFLAVIN (Vitamin B2) is PHOTOSENSITIVE — phototherapy light (wavelength 420-490 nm) degrades riboflavin in the blood and skin of neonates → supplementation during phototherapy prevents deficiency. Riboflavin photodegradation also explains why riboflavin-containing solutions must be protected from light. Phototherapy converts bilirubin to more water-soluble photoisomers for excretion.\n\n📹 [VIDEO LINK]"},{"id":549,"s":"Vitamins","t":"Riboflavin — B2","u":"Riboflavin Deficiency Redox Reactions","q":"A 33-year-old woman presents with painful fissures at the corners of her mouth, a magenta-coloured tongue, photophobia, and seborrheic dermatitis around the nose. Laboratory tests show decreased activity of succinate dehydrogenase. Which of the following best describes the impaired biochemical reactions?","o":["Carboxylation reactions","Decarboxylation reactions","Methylation reactions","Redox reactions","Transamination reactions"],"a":3,"p":"Riboflavin (as FAD and FMN) mediates REDOX REACTIONS — accepts and donates electrons in oxidation-reduction reactions. FAD → FADH₂ (reduced). FMN → FMNH₂ (reduced). Key redox reactions requiring riboflavin: succinate → fumarate (Complex II, FAD), NADH → CoQ (Complex I, FMN), beta-oxidation (acyl-CoA dehydrogenase, FAD), amino acid catabolism, drug metabolism. Cheilosis + magenta tongue + seborrheic dermatitis = classic riboflavin deficiency triad.\n\n📹 [VIDEO LINK]"},{"id":550,"s":"Vitamins","t":"Riboflavin — B2","u":"Riboflavin Magenta Tongue Cheilosis","q":"An 89-year-old widower living alone on tea and toast presents with painful cracks at the corners of his mouth, a magenta-coloured tongue, and redness with vascularisation of the cornea. Which vitamin deficiency best explains these findings?","o":["Niacin","Pantothenic acid","Pyridoxal phosphate","Riboflavin","Thiamine"],"a":3,"p":"RIBOFLAVIN DEFICIENCY: cheilosis (cracks at angles of mouth) + glossitis (magenta/beefy red tongue) + corneal vascularisation (neovascularisation of cornea) + seborrheic dermatitis = classic riboflavin deficiency triad. FAD and FMN are essential for redox reactions including succinate dehydrogenase (Complex II) and NADH dehydrogenase (Complex I). Tea and toast diet with no dairy, meat, or eggs = classic riboflavin deficiency history in elderly.\n\n📹 [VIDEO LINK]"},{"id":551,"s":"Vitamins","t":"Niacin — B3","u":"Pellagra Casal Necklace Corn Diet","q":"A 55-year-old farmer from an area where corn is the staple diet presents with diarrhoea, Casal's necklace, and progressive dementia. Which of the following best explains his condition?","o":["Dry beri beri","Korsakoff psychosis","Pellagra","Scurvy","Wet beri beri"],"a":2,"p":"PELLAGRA = NIACIN deficiency. The 4 Ds: DERMATITIS (photosensitive — Casal's necklace = symmetrical hyperpigmented rash encircling the neck like a necklace on sun-exposed skin), DIARRHOEA, DEMENTIA, DEATH. Corn-based diets: niacin in corn = niacytin (bound, not bioavailable) + corn is low in tryptophan. Carcinoid syndrome, isoniazid, Hartnup disease also cause pellagra-like symptoms.\n\n[Diagram: Silhouette of person showing Casal's necklace — symmetrical hyperpigmented, scaly rash in a collar distribution around the neck and upper chest. Label: CASAL'S NECKLACE = photosensitive dermatitis of pellagra — symmetrical, hyperpigmented, sharply demarcated at sun-exposed vs covered skin border. Also shown: back of hands (another sun-exposed area). Note to editor: insert clinical photograph alongside diagram.]\n\n📹 [VIDEO LINK]"},{"id":552,"s":"Vitamins","t":"Niacin — B3","u":"Isoniazid Pellagra Pyridoxine","q":"A 45-year-old man on isoniazid therapy for tuberculosis presents with diarrhoea, dementia, and a rash on sun-exposed areas. Which of the following vitamins required for niacin synthesis is inhibited by isoniazid?","o":["Biotin","Cobalamin","Pyridoxine","Riboflavin","Thiamine"],"a":2,"p":"ISONIAZID inhibits PYRIDOXAL KINASE → blocks conversion of pyridoxine → pyridoxal phosphate (PLP). PLP is required for KYNURENINASE → converts kynurenine → alanine + anthranilate (in tryptophan → niacin pathway). Without PLP → kynureninase fails → tryptophan cannot be converted to niacin → PELLAGRA-LIKE SYMPTOMS in isoniazid users. Treatment: co-prescribe PYRIDOXINE (B6) with isoniazid. Isoniazid also causes sideroblastic anaemia (PLP deficiency → impairs ALA synthase).\n\n📹 [VIDEO LINK]"},{"id":553,"s":"Vitamins","t":"Niacin — B3","u":"Carcinoid Pellagra Like Symptoms","q":"Which of the following may lead to pellagra-like symptoms due to diversion of tryptophan away from niacin synthesis?","o":["Carcinoid syndrome","Cushing's syndrome","Down's syndrome","Irritable bowel syndrome","Metabolic syndrome"],"a":0,"p":"CARCINOID SYNDROME: carcinoid tumour → excess SEROTONIN production from tryptophan → tryptophan diverted to serotonin synthesis → LESS tryptophan available for NIACIN synthesis → pellagra-like symptoms (dermatitis, diarrhoea, dementia) ON TOP OF carcinoid features (flushing, diarrhoea, bronchospasm). 60 mg tryptophan = 1 mg niacin. Any condition diverting tryptophan away from the niacin synthesis pathway → functional niacin deficiency.\n\n📹 [VIDEO LINK]"},{"id":554,"s":"Vitamins","t":"Niacin — B3","u":"Niacin Inhibits Hormone Sensitive Lipase","q":"Niacin has lipid-lowering action because it inhibits which of the following enzymes in adipose tissue?","o":["Colipase","Hormone-sensitive lipase","Lingual lipase","Lipoprotein lipase","Pancreatic lipase"],"a":1,"p":"Niacin activates GPR109A receptor on adipocytes → Gi signalling → INHIBITS HORMONE-SENSITIVE LIPASE (HSL) in adipose tissue → ↓ free fatty acid MOBILISATION from adipose stores → ↓ free fatty acid flux to liver → LESS substrate for triglyceride synthesis in liver → ↓ VLDL ASSEMBLY → ↓ VLDL secretion → ↓ LDL. IMPORTANT: mobilised free fatty acids in fasting go to BETA-OXIDATION for energy, NOT to VLDL synthesis. Niacin's effect works by reducing fatty acid flux to the liver which under post-absorptive conditions contributes to hepatic triglyceride synthesis. Secondary effects: enhances lipoprotein lipase (↑ VLDL clearance) + inhibits HDL catabolism (↑ HDL). Side effect: FLUSHING (prostaglandin D2 mediated — prevented by aspirin pre-treatment).\n\n📹 [VIDEO LINK]"},{"id":555,"s":"Vitamins","t":"Niacin — B3","u":"Niacin Inhibits HDL Catabolism","q":"Niacin has lipid-lowering action because it inhibits catabolism of which of the following?","o":["HDL","IDL","LDL","Oxidised LDL","VLDL"],"a":0,"p":"Niacin inhibits HDL CATABOLISM by reducing hepatic expression of SR-B1 and apolipoprotein A-I catabolism → HDL particles persist longer in circulation → ↑ HDL. Niacin is the MOST EFFECTIVE drug for raising HDL cholesterol. In addition: ↓ VLDL synthesis (via HSL inhibition → ↓ free fatty acids → ↓ hepatic VLDL assembly) → ↓ VLDL → ↓ IDL → ↓ LDL.\n\n📹 [VIDEO LINK]"},{"id":556,"s":"Vitamins","t":"Niacin — B3","u":"Niacin Calcium Intracellular","q":"In response to neurotransmitters, niacin increases intracellular concentration of which of the following?","o":["Calcium","Potassium","Selenium","Sodium","Zinc"],"a":0,"p":"NIACIN non-coenzyme action: NAD⁺ is the substrate for CYCLIC ADP-RIBOSE (cADPR) synthesis by CD38 (ADP-ribosyl cyclase). cADPR → second messenger → activates ryanodine receptors on endoplasmic reticulum → releases Ca²⁺ into cytosol → INCREASES INTRACELLULAR CALCIUM → regulates muscle contraction, hormone secretion, neurotransmitter release. Also: NAD⁺ used by POLY-ADP-RIBOSE POLYMERASE (PARP) in DNA repair. SIRTUINS use NAD⁺ → deacetylation → gene regulation.\n\n📹 [VIDEO LINK]"},{"id":557,"s":"Vitamins","t":"Niacin — B3","u":"Niacin Non-Coenzyme Ribosylation","q":"Which of the following is the non-coenzyme action of niacin?","o":["Carboxylation","Dehydrogenation","Hydroxylation","Oxidation","Ribosylation"],"a":4,"p":"RIBOSYLATION (ADP-ribosylation) = NON-COENZYME role of niacin/NAD⁺. NAD⁺ used as ADP-RIBOSYL DONOR by: (1) PARP (poly-ADP-ribose polymerase) — DNA repair. (2) Bacterial toxins (cholera toxin ADP-ribosylates Gαs, pertussis toxin ADP-ribosylates Gαi). (3) CD38 → cyclic ADP-ribose → calcium signalling. (4) SIRTUINS → NAD⁺-dependent protein deacetylases. Compare: coenzyme role of niacin = redox reactions (dehydrogenation, oxidation) as NAD⁺/NADH and NADP⁺/NADPH.\n\n📹 [VIDEO LINK]"},{"id":558,"s":"Vitamins","t":"Pantothenic Acid — B5","u":"Burning Feet Syndrome WWII Prisoner","q":"A prisoner of war in the Pacific theatre during World War II has been surviving for 8 months on minimal rations of white rice and water with no meat, eggs, or dairy. He develops an agonising burning sensation in both feet and hands, making walking unbearable. Deficiency of which of the following vitamins is most likely responsible?","o":["Vitamin B1","Vitamin B3","Vitamin B5","Vitamin B6","Vitamin B7"],"a":2,"p":"BURNING FEET SYNDROME = the specific named nutritional deficiency manifestation of PANTOTHENIC ACID (B5) deficiency. First described in World War II prisoners of war in Japanese camps surviving on severely restricted rations — burning, agonising paresthesias of feet and hands, relieved by pantothenic acid supplementation. Pantothenic acid = component of CoA and ACP → required for fatty acid synthesis, TCA cycle, steroid synthesis, acetylcholine synthesis. DISTINCTION: Vitamin B12 also causes peripheral neuropathy with burning sensations but the SPECIFIC named syndrome burning feet = B5 (pantothenic acid) per Harper's Biochemistry.\n\n📹 [VIDEO LINK]"},{"id":559,"s":"Vitamins","t":"Pantothenic Acid — B5","u":"Pantothenic Acid Part of CoA","q":"Which of the following vitamins is part of Coenzyme A (CoA)?","o":["Vitamin B1","Vitamin B3","Vitamin B5","Vitamin B6","Vitamin B7"],"a":2,"p":"COENZYME A = pantothenic acid (B5) + beta-mercaptoethylamine + adenosine 3',5'-bisphosphate. The reactive thiol group (–SH) forms thioester bonds with acyl groups. Functions: acetyl-CoA (TCA cycle, fatty acid synthesis, cholesterol synthesis), succinyl-CoA (haem synthesis, TCA cycle), malonyl-CoA (fatty acid synthesis), propionyl-CoA, HMG-CoA. ACYL CARRIER PROTEIN (ACP) also contains a phosphopantetheine group derived from B5. B5 = required for adrenal cortex steroid synthesis.\n\n📹 [VIDEO LINK]"},{"id":560,"s":"Vitamins","t":"Pyridoxine — B6","u":"Pyridoxal Phosphate Active Form","q":"Which of the following is the biologically active form of vitamin B6?","o":["Pyridoxal","Pyridoxal phosphate","Pyridoxamine","Pyridoxamine phosphate","Pyridoxine"],"a":1,"p":"PYRIDOXAL PHOSPHATE (PLP) = active coenzyme form of vitamin B6. Required for: (1) TRANSAMINASES — amino acid catabolism and synthesis. (2) ALA SYNTHASE — first step of haem synthesis. (3) DECARBOXYLASES — amino acid → neurotransmitter (DOPA → dopamine, 5-HTP → serotonin, histidine → histamine, glutamate → GABA). (4) CYSTATHIONINE SYNTHASE — homocysteine catabolism. (5) GLYCOGEN PHOSPHORYLASE — glycogen breakdown. Isoniazid blocks pyridoxal kinase → PLP deficiency → sideroblastic anaemia + peripheral neuropathy.\n\n📹 [VIDEO LINK]"},{"id":561,"s":"Vitamins","t":"Pyridoxine — B6","u":"Isoniazid Sideroblastic Anaemia Pyridoxine","q":"A 42-year-old man with pulmonary tuberculosis on isoniazid for 6 months presents with microcytic hypochromic anaemia. Bone marrow shows ring sideroblasts. Which vitamin deficiency is responsible?","o":["Vitamin B1","Vitamin B2","Vitamin B6","Vitamin B9","Vitamin B12"],"a":2,"p":"Isoniazid inhibits PYRIDOXAL KINASE → blocks conversion of pyridoxine → PYRIDOXAL PHOSPHATE (PLP). PLP is the cofactor for ALA SYNTHASE (rate-limiting, first step of haem synthesis). Without PLP → ALA synthase impaired → haem synthesis blocked → iron accumulates in mitochondria → RING SIDEROBLASTS on bone marrow biopsy → SIDEROBLASTIC ANAEMIA (microcytic hypochromic, iron overload in marrow). Treatment: pyridoxine supplementation (100 mg/day) alongside isoniazid.\n\n📹 [VIDEO LINK]"},{"id":562,"s":"Vitamins","t":"Pyridoxine — B6","u":"Pyridoxal Phosphate Transaminases Cofactor","q":"The vitamin required as coenzyme for the action of transaminases (aminotransferases) is which of the following?","o":["Niacin","Pantothenic acid","Pyridoxal phosphate","Riboflavin","Thiamine"],"a":2,"p":"PYRIDOXAL PHOSPHATE (PLP) is the coenzyme for ALL TRANSAMINASES (aminotransferases). PLP acts as the amino group carrier: accepts amino group from alpha-amino acid → PLP becomes pyridoxamine phosphate (PMP) → transfers amino group to alpha-keto acid → new amino acid formed. Key transaminases: AST (liver marker), ALT (liver marker). Both require PLP. B6 deficiency → impaired amino acid catabolism and interconversion.\n\n📹 [VIDEO LINK]"},{"id":563,"s":"Vitamins","t":"Biotin — B7","u":"Biotin Carboxylation Reactions","q":"Biotin is required in which of the following reactions?","o":["Acetylation","Carboxylation","Deamination","Decarboxylation","Transamination"],"a":1,"p":"BIOTIN = cofactor for all CARBOXYLATION reactions. Four biotin-dependent carboxylases: (1) ACETYL-CoA CARBOXYLASE — acetyl-CoA → malonyl-CoA (rate-limiting step of fatty acid synthesis). (2) PYRUVATE CARBOXYLASE — pyruvate → oxaloacetate (gluconeogenesis). (3) PROPIONYL-CoA CARBOXYLASE — propionyl-CoA → methylmalonyl-CoA. (4) METHYLCROTONYL-CoA CARBOXYLASE — leucine catabolism. Biotin acts as CO₂ carrier. Active intermediate = 1-N-carboxybiocytin formed from bicarbonate + ATP.\n\n📹 [VIDEO LINK]"},{"id":564,"s":"Vitamins","t":"Biotin — B7","u":"Avidin Raw Egg Biotin Deficiency","q":"A 22-year-old bodybuilder has been consuming a dozen raw eggs daily for 3 months as part of his protein diet. He presents with thinning hair, loss of hair colour, and a red scaly rash around his nose, mouth, and eyes. Which of the following proteins from raw eggs is causing his symptoms?","o":["Albumin","Avidin","Lysozyme","Ovalbumin","Ovomucin"],"a":1,"p":"RAW EGG WHITE contains AVIDIN — a glycoprotein that binds vitamin B7 (biotin) with extremely high affinity (Kd ~10⁻¹⁵ M) → biotin CANNOT be absorbed → biotin deficiency → impaired carboxylation reactions → alopecia (hair loss + colour change), seborrheic dermatitis (red scaly rash around nose, mouth, eyes). COOKING denatures avidin → cannot bind biotin → cooked eggs safe. Egg yolk is rich in biotin. Albumin (ovalbumin) = main storage protein. Lysozyme = antimicrobial enzyme. Ovomucin = gives viscosity. Only avidin binds and inactivates a vitamin.\n\n📹 [VIDEO LINK]"},{"id":565,"s":"Vitamins","t":"Biotin — B7","u":"Biotinidase Deficiency Recycling B7","q":"Deficiency of which of the following enzymes can affect endogenous recycling of vitamin B7 from dietary protein, leading to its deficiency with neurological and dermatological presentation?","o":["Biotinidase","Carboxylase","Chelatase","Dehydrogenase","Oxidase"],"a":0,"p":"BIOTINIDASE cleaves vitamin B7 from biocytin (B7-lysine conjugate — formed during protein digestion and endogenous protein turnover) → releases FREE vitamin B7 for recycling and reuse. Without biotinidase → vitamin B7 cannot be recycled → functional deficiency despite adequate dietary intake → all four B7-dependent carboxylases impaired → organic acidaemia, seizures, hypotonia, hearing loss, optic atrophy, seborrheic dermatitis, alopecia. HOLOCARBOXYLASE SYNTHETASE deficiency = separate condition where vitamin B7 cannot be attached to apocarboxylase → all carboxylases inactive from birth.\n\n📹 [VIDEO LINK]"},{"id":566,"s":"Vitamins","t":"Biotin — B7","u":"Biotin Vitamin H","q":"Which of the following vitamins is also known as vitamin H?","o":["Vitamin B1","Vitamin B2","Vitamin B3","Vitamin B5","Vitamin B7"],"a":4,"p":"BIOTIN = VITAMIN H (from German 'Haut' meaning skin — reflecting its role in skin health). Also called vitamin B7. Coenzyme for all four carboxylation reactions. Found in eggs (yolk), liver, nuts, legumes. Recommended daily intake = 30 μg/day (adults). High-dose biotin (100-300 mg/day) used experimentally in multiple sclerosis — promotes remyelination by activating acetyl-CoA carboxylase (rate-limiting step of myelin fatty acid synthesis).\n\n📹 [VIDEO LINK]"},{"id":567,"s":"Vitamins","t":"Biotin — B7","u":"Biotin MS Carboxylation Myelin","q":"A 30-year-old woman with relapsing-remitting multiple sclerosis is prescribed high-dose biotin. Which rate-limiting step of myelin synthesis does biotin facilitate?","o":["Carboxylation","Isomerisation","Oxidation","Reduction","Translocation"],"a":0,"p":"High-dose biotin in MS: activates ACETYL-CoA CARBOXYLASE → acetyl-CoA → malonyl-CoA → fatty acid synthesis → MYELIN synthesis. This is the RATE-LIMITING CARBOXYLATION step of fatty acid/myelin synthesis. Also: activates pyruvate carboxylase → ↑ oxaloacetate → ↑ TCA cycle → ↑ ATP for myelinating oligodendrocytes → reduces AXONAL HYPOXIA. High-dose biotin for MS = 100-300 mg/day (vs normal RDA of 30 μg/day = 3000-10,000 fold higher).\n\n📹 [VIDEO LINK]"},{"id":568,"s":"Vitamins","t":"Biotin — B7","u":"Biotin TPN Deficiency Urinary Marker","q":"A 65-year-old man on total parenteral nutrition for 6 weeks develops an erythematous rash around his mouth and nose, ataxia, and lethargy. Biotin deficiency is suspected. Which of the following tests will best confirm this diagnosis?","o":["Serum biotin","Serum propionic acid","Serum valeric acid","Urine biotin","Urinary 3-hydroxyisovaleric acid"],"a":4,"p":"URINARY 3-HYDROXYISOVALERIC ACID (3-HIA) = most sensitive functional marker of biotin deficiency. Biotin deficiency → methylcrotonyl-CoA carboxylase impaired → methylcrotonyl-CoA accumulates → converted to 3-hydroxyisovaleric acid → excreted in urine. Elevated 3-HIA = early functional biotin deficiency even before clinical symptoms. Compare: serum biotin levels = poor sensitivity. TPN without biotin supplementation = iatrogenic biotin deficiency.\n\n📹 [VIDEO LINK]"},{"id":569,"s":"Vitamins","t":"Folate — B9","u":"Folate NTDs First Trimester","q":"A 30-year-old pregnant woman presents at 6 months for her first antenatal visit. An anomaly scan reveals spina bifida and anencephaly. Deficiency of which nutrient during the first trimester is most likely responsible?","o":["Ascorbate","Biotin","Folate","Niacin","Riboflavin"],"a":2,"p":"FOLATE deficiency in the FIRST TRIMESTER (especially peri-conceptionally) → impaired one-carbon metabolism → reduced methylation of DNA → HYPOMETHYLATION → neural tube closure fails → NEURAL TUBE DEFECTS (anencephaly, spina bifida). Folate supplementation 400 μg/day starting AT LEAST 1 month BEFORE conception and through first trimester → reduces NTD risk by >70%. NTD occurs at 3-4 weeks gestation (neural tube closes by day 28) — anomaly scan at 6 months is too late for prevention.\n\n📹 [VIDEO LINK]"},{"id":570,"s":"Vitamins","t":"Folate — B9","u":"Methotrexate Folate DHFR","q":"A 26-year-old woman is administered methotrexate for ectopic pregnancy. Which of the following vitamins is primarily affected by this treatment?","o":["Vitamin B1","Vitamin B3","Vitamin B5","Vitamin B9","Vitamin B12"],"a":3,"p":"Methotrexate inhibits DIHYDROFOLATE REDUCTASE (DHFR) → prevents regeneration of tetrahydrofolate (THF) from dihydrofolate (DHF) → impairs FOLATE (B9) metabolism → thymidylate synthesis blocked → DNA synthesis inhibited → rapidly dividing cells (trophoblast) die → terminates ectopic pregnancy. High-dose methotrexate → rescue with LEUCOVORIN (folinic acid, 5-formyl THF) → bypasses DHFR block.\n\n📹 [VIDEO LINK]"},{"id":571,"s":"Vitamins","t":"Folate — B9","u":"B12 Deficiency Functional Folate Deficiency Methyl Trap","q":"A 32-year-old strict vegetarian woman presents with macrocytic anaemia and tingling in her hands and feet. Vitamin B₁₂ deficiency is confirmed. She is at risk of developing functional deficiency of which of the following vitamins?","o":["Biotin","Folate","Niacin","Pyridoxine","Thiamine"],"a":1,"p":"B12 deficiency → METHYL-TRAP HYPOTHESIS: B12 (methylcobalamin) required for METHIONINE SYNTHASE → converts methylTHF + homocysteine → methionine + THF. Without B12 → methylTHF cannot donate its methyl group → folate TRAPPED as methylTHF → cannot be converted to other THF forms needed for purine and thymidylate synthesis → FUNCTIONAL FOLATE DEFICIENCY despite normal serum folate. Serum folate = normal (folate present but trapped). Functional folate = deficient (cannot participate in DNA synthesis) → megaloblastic anaemia.\n\n📹 [VIDEO LINK]"},{"id":572,"s":"Vitamins","t":"Folate — B9","u":"Neural Tube Defects Hypomethylation","q":"Which of the following forms the biochemical basis for neural tube defects in folate deficiency?","o":["Hypermethylation of DNA","Hypomethylation of DNA","Hypoacetylation of histones","Hypomyelination","Oxidative DNA damage"],"a":1,"p":"Folate deficiency → ↓ THF → ↓ 5,10-methyleneTHF → ↓ SAM (S-adenosylmethionine, universal methyl donor) → HYPOMETHYLATION of DNA → impaired epigenetic regulation of developmental genes → neural tube closure fails → NTDs. Also: ↓ thymidylate synthesis → ↓ DNA synthesis → impaired cell division in rapidly proliferating neural fold cells. Folate-responsive NTDs: spina bifida, anencephaly, meningocele. 70% prevention with peri-conceptional folate supplementation.\n\n📹 [VIDEO LINK]"},{"id":573,"s":"Vitamins","t":"Cobalamin — B12","u":"Pernicious Anaemia Anti-IF Antibodies","q":"A 58-year-old woman presents with smooth beefy red tongue, pallor, and decreased vibration and position sense in lower limbs. Anti-intrinsic factor antibodies are positive. Which of the following is the most likely diagnosis?","o":["Anaemia of chronic disease","Folate deficiency","Iron deficiency anaemia","Pernicious anaemia","Thalassaemia"],"a":3,"p":"PERNICIOUS ANAEMIA = autoimmune destruction of gastric parietal cells → ↓ INTRINSIC FACTOR → cannot absorb vitamin B12 in terminal ileum → B12 deficiency → macrocytic megaloblastic anaemia + subacute combined degeneration of spinal cord (posterior columns + lateral corticospinal tracts → loss of vibration + position sense + UMN signs). Smooth beefy red tongue (glossitis) + hyper-segmented neutrophils on smear + anti-intrinsic factor antibodies = pathognomonic. Treatment: lifelong intramuscular B12.\n\n📹 [VIDEO LINK]"},{"id":574,"s":"Vitamins","t":"Cobalamin — B12","u":"B12 Deficiency Methylmalonic Acid","q":"A 42-year-old woman presents with tiredness and tingling. Blood work shows macrocytic anaemia and elevated homocysteine. One would also expect elevated levels of which of the following metabolites?","o":["Acetic acid","Formic acid","Methylmalonic acid","Oxalic acid","Propionic acid"],"a":2,"p":"B12 deficiency → METHYLMALONYL-CoA MUTASE (requires adenosylcobalamin) impaired → methylmalonyl-CoA cannot be converted → succinyl-CoA → METHYLMALONIC ACID accumulates → elevated in urine and serum. METHYLMALONIC ACID elevation = SPECIFIC for B12 deficiency (NOT elevated in folate deficiency — key distinguishing test). Elevated HOMOCYSTEINE = seen in BOTH B12 AND folate deficiency. Diagnostic algorithm: elevated MMA → B12 deficiency. Normal MMA + elevated homocysteine → folate deficiency.\n\n📹 [VIDEO LINK]"},{"id":575,"s":"Vitamins","t":"Cobalamin — B12","u":"B12 Methylation Reaction","q":"Vitamin B12 is required for which of the following biochemical reactions?","o":["Carboxylation","Deamination","Hydroxylation","Methylation","Transamination"],"a":3,"p":"B12 (methylcobalamin) required for METHYLATION: METHIONINE SYNTHASE → methylTHF + homocysteine → methionine + THF. B12 (adenosylcobalamin) required for ISOMERISATION: METHYLMALONYL-CoA MUTASE → methylmalonyl-CoA → succinyl-CoA. The methylation reaction is critical for: methionine synthesis → SAM → methylation of DNA, RNA, myelin basic protein. Failure of myelin basic protein methylation → SUBACUTE COMBINED DEGENERATION.\n\n📹 [VIDEO LINK]"},{"id":576,"s":"Vitamins","t":"Cobalamin — B12","u":"Myelin Basic Protein Methylation Arginine SCD","q":"Failure of methylation of which of the following amino acid residues in myelin basic protein leads to irreversible degeneration of the spinal cord in B12 deficiency?","o":["Alanine","Arginine","Asparagine","Glutamine","Serine"],"a":1,"p":"METHYLATION OF ARGININE RESIDUES in myelin basic protein (MBP) by protein arginine methyltransferase (PRMT) → stabilises MBP → maintains myelin compaction → prevents demyelination. B12 deficiency → ↓ SAM (methyl donor) → ↓ arginine methylation in MBP → myelin destabilisation → SUBACUTE COMBINED DEGENERATION of the spinal cord. Posterior columns (loss of vibration, position sense, Romberg positive) and lateral corticospinal tracts (UMN signs) affected. Irreversible if not treated early.\n\n📹 [VIDEO LINK]"},{"id":577,"s":"Vitamins","t":"Cobalamin — B12","u":"Gastrectomy B12 Deficiency IF","q":"A 55-year-old man with gastrectomy for gastric carcinoma presents with megaloblastic anaemia. Which of the following is responsible for this anaemia?","o":["Vitamin B6 deficiency","Vitamin B9 deficiency","Vitamin B12 deficiency","Vitamin C deficiency","Vitamin E deficiency"],"a":2,"p":"Gastrectomy removes PARIETAL CELLS → loss of INTRINSIC FACTOR → cannot absorb dietary B12 in terminal ileum → B12 deficiency → methyl-trap → functional folate deficiency → megaloblastic anaemia + potential SACD. Folate serum levels may be NORMAL (folate present but trapped as methylTHF). Treatment: lifelong intramuscular or high-dose oral B12 (passive absorption without IF at very high doses). Total gastrectomy = definite IF deficiency.\n\n📹 [VIDEO LINK]"},{"id":578,"s":"Vitamins","t":"Cobalamin — B12","u":"B12 RDA 2.4 mcg","q":"The RDA for vitamin B12 in adults is which of the following?","o":["0.4 μg/day","1.4 μg/day","2.4 μg/day","4.4 μg/day","10.4 μg/day"],"a":2,"p":"RDA for Vitamin B12 = 2.4 μg/day (adults). Pregnancy = 2.6 μg/day. Lactation = 2.8 μg/day. Stored in liver for 3-5 years → deficiency develops slowly. Groups at risk: strict vegetarians/vegans, elderly (atrophic gastritis → impaired IF secretion), pernicious anaemia, gastrectomy, ileal disease/resection, metformin use. Compare: RDA folate = 400 μg/day (adults) / 600 μg/day (pregnancy). B12 is in micrograms; folate is in micrograms but 400 times more needed.\n\n📹 [VIDEO LINK]"},{"id":579,"s":"Vitamins","t":"Cobalamin — B12","u":"Isoniazid B6 Optic Neuritis Blurred Vision","q":"A 35-year-old woman being treated for tuberculosis with isoniazid develops blurred vision. Which vitamin should be co-prescribed?","o":["Vitamin B1","Vitamin B3","Vitamin B5","Vitamin B6","Vitamin B7"],"a":3,"p":"Isoniazid inhibits pyridoxal kinase → blocks conversion of pyridoxine → PYRIDOXAL PHOSPHATE (PLP). PLP required for: kynureninase (niacin synthesis), ALA synthase (haem synthesis → anaemia), decarboxylases (neurotransmitter synthesis → neurological symptoms including OPTIC NEURITIS → blurred vision). Blurred vision from isoniazid = OPTIC NEURITIS due to PLP deficiency → impaired dopamine and serotonin synthesis in optic nerve. Treatment: PYRIDOXINE (B6) 25-50 mg/day co-prescribed with isoniazid.\n\n📹 [VIDEO LINK]"},{"id":580,"s":"Vitamins","t":"Pantothenic Acid — B5","u":"ACP Pantothenic Acid Fatty Acid Synthesis","q":"Which of the following vitamins is part of acyl carrier protein (ACP) used in fatty acid synthesis?","o":["Vitamin B1","Vitamin B3","Vitamin B5","Vitamin B6","Vitamin B7"],"a":2,"p":"ACYL CARRIER PROTEIN (ACP) contains a 4'-PHOSPHOPANTETHEINE prosthetic group derived from PANTOTHENIC ACID (B5). The reactive thiol group (–SH) of ACP carries acyl intermediates during fatty acid synthesis (acetyl-ACP, malonyl-ACP, palmitoyl-ACP). CoA also contains pantothenic acid. B5 therefore essential for: fatty acid synthesis (ACP), fatty acid oxidation (CoA), TCA cycle (acetyl-CoA, succinyl-CoA), steroid synthesis (acetyl-CoA), acetylcholine synthesis (acetyl-CoA).\n\n📹 [VIDEO LINK]"},{"id":581,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"Monosaccharide Cannot Be Hydrolysed","q":"Which of the following carbohydrates cannot be hydrolysed into simpler sugars?","o":["Cellulose","Lactose","Maltose","Ribose","Sucrose"],"a":3,"p":"MONOSACCHARIDES (e.g. glucose, fructose, galactose, ribose) = simplest carbohydrates — cannot be hydrolysed further. DISACCHARIDES (lactose, maltose, sucrose) = two monosaccharides joined by glycosidic bonds → can be hydrolysed. POLYSACCHARIDES (cellulose, starch, glycogen) = many monosaccharides → can be hydrolysed to component sugars. Ribose = a pentose monosaccharide (5 carbons) — component of RNA, ATP, NADH, FADH₂, coenzymes. Glucose = the most important monosaccharide in human metabolism.\n\n📹 [VIDEO LINK]"},{"id":582,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"Monosaccharides Classification","q":"Glucose, fructose, and galactose are examples of which of the following?","o":["Disaccharides","Monosaccharides","Oligosaccharides","Pentosaccharides","Polysaccharides"],"a":1,"p":"MONOSACCHARIDES = single sugar units — the building blocks of all carbohydrates. Cannot be further hydrolysed. Classified by: (1) Number of carbons: triose (3C), tetrose (4C), pentose (5C), hexose (6C). (2) Functional group: aldose (aldehyde, e.g. glucose) vs ketose (keto, e.g. fructose). Glucose (aldohexose), fructose (ketohexose), galactose (aldohexose) = the three most important hexoses. All share molecular formula C₆H₁₂O₆ but differ in arrangement → structural isomers.\n\n📹 [VIDEO LINK]"},{"id":583,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"Polysaccharides Classification","q":"Which of the following is the correct classification of starch, glycogen, and cellulose?","o":["Disaccharides","Monosaccharides","Oligosaccharides","Polysaccharides","Trisaccharides"],"a":3,"p":"POLYSACCHARIDES = many monosaccharide units (>10) linked by glycosidic bonds. HOMOPOLYSACCHARIDES: all same monosaccharide — starch (amylose + amylopectin, plant glucose storage), glycogen (animal glucose storage, more branched than starch), cellulose (structural, beta 1→4 linkages, dietary fibre), dextran (bacterial glucose storage), inulin (plant fructan, GFR marker). HETEROPOLYSACCHARIDES: different monosaccharides — glycosaminoglycans (hyaluronate, chondroitin sulfate, heparin, etc.).\n\n📹 [VIDEO LINK]"},{"id":584,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"Non-Reducing Sugar Diabetes","q":"A 55-year-old man with type 2 diabetes has persistent hyperglycaemia. His physician orders a urine dipstick using glucose oxidase — specific for glucose and does NOT detect other reducing sugars. Which of the following is a reducing sugar that would be MISSED by this specific test but detected by Benedict's reagent?","o":["Cellulose","Fructose","Glycogen","Starch","Sucrose"],"a":1,"p":"REDUCING SUGARS have a FREE ANOMERIC CARBON → can donate electrons → reduce Cu²⁺ (Benedict's reagent → brick red). All monosaccharides are reducing: GLUCOSE, FRUCTOSE, GALACTOSE. Benedict's test detects ALL reducing sugars but cannot distinguish between them. Glucose oxidase dipstick = SPECIFIC for glucose only. Clinical implication: urine may contain fructose (excess fructose intake) or galactose (galactosaemia) — both give positive Benedict's but NEGATIVE glucose oxidase dipstick. SUCROSE is NON-REDUCING (both anomeric carbons occupied) → negative Benedict's.\n\n📹 [VIDEO LINK]"},{"id":585,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"Negative Benedicts Test Non-Reducing","q":"A laboratory technician uses Benedict's reagent to test urine for reducing sugars. Which of the following would give a NEGATIVE Benedict's test?","o":["Fructose","Galactose","Glucose","Lactose","Sucrose"],"a":4,"p":"SUCROSE is the only NON-REDUCING sugar among the options — both anomeric carbons (C1 of glucose, C2 of fructose) are involved in the alpha 1→2 glycosidic bond → NO FREE ANOMERIC CARBON → cannot reduce Cu²⁺ → NEGATIVE Benedict's test. All other options are REDUCING: glucose, fructose, galactose (monosaccharides — always reducing), lactose (free anomeric C1 of glucose moiety). Clinical: lactosuria in pregnancy/lactation → positive Benedict's but negative glucose oxidase dipstick → important distinction.\n\n📹 [VIDEO LINK]"},{"id":586,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"Mutarotation Anomers","q":"Mutarotation is a property of reducing sugars. Which of the following best describes mutarotation?","o":["Aldose to ketose conversion","Enantiomer formation","Interconversion of alpha and beta anomers","Isomerisation at non-anomeric carbon","Rotation of polarisation plane only"],"a":2,"p":"MUTAROTATION = spontaneous INTERCONVERSION between ALPHA and BETA ANOMERS of a reducing sugar in solution, passing through the open-chain form. Alpha-D-glucose (specific rotation +112°) ⇌ open chain ⇌ beta-D-glucose (specific rotation +18.7°) → equilibrium mixture (specific rotation +52.7°). ANOMERS differ only at the anomeric carbon (C1 in aldoses): alpha = OH axial (below ring in Haworth), beta = OH equatorial (above ring). EPIMERISM = differing at any non-anomeric carbon.\n\n📹 [VIDEO LINK]"},{"id":587,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"Epimers C4 Glucose Galactose","q":"Glucose and galactose have the same molecular formula (C₆H₁₂O₆) but differ only in the configuration at carbon-4. They are therefore classified as which of the following?","o":["Anomers","Diastereomers","Enantiomers","Epimers","Isomers"],"a":3,"p":"EPIMERS = two sugars that differ in configuration at ONE specific non-anomeric carbon. Glucose and galactose = C4 EPIMERS (differ only at C4). Glucose and mannose = C2 EPIMERS. Compare: ANOMERS = differ at the anomeric carbon (C1) only. ENANTIOMERS = mirror images (D vs L — all carbons inverted). Clinical: GALACTOSAEMIA — GALT enzyme deficiency → galactose-1-phosphate accumulates → liver damage, cataracts, intellectual disability.\n\n[Diagram: Haworth projections of glucose and galactose side by side. Glucose: ring structure with OH at C4 pointing DOWN (equatorial). Galactose: identical except OH at C4 pointing UP (axial). Red circle highlighting C4 in both. Arrow between them labelled: C4 EPIMERS — differ ONLY at C4. Interconversion by UDP-galactose-4-epimerase. Clinical box: Galactosaemia = deficient GALT → galactose toxic → cataracts + liver damage + intellectual disability.]\n\n📹 [VIDEO LINK]"},{"id":588,"s":"Carbohydrate Chemistry and Metabolism","t":"Disaccharides","u":"Lactose Neonate Breast Milk Beta 1-4","q":"A neonate is thriving well on his mother's feed. As he is suckling, his intestinal enzymes are busy breaking the disaccharide in breast milk. Which of the following glycosidic linkages are these enzymes cleaving?","o":["Alpha 1→2","Alpha 1→4","Beta 1→2","Beta 1→4","Beta 1→6"],"a":3,"p":"Breast milk contains LACTOSE → LACTASE (beta-galactosidase) on intestinal brush border cleaves the BETA 1→4 glycosidic bond between galactose (C1) and glucose (C4) → free glucose and galactose → absorbed via SGLT1 → enterocyte → portal blood. Lactase activity is highest at birth → declines after weaning in most of the world's population (lactase non-persistence → adult lactose intolerance). Persistent lactase activity in adulthood is a genetic trait common in Northern Europeans.\n\n📹 [VIDEO LINK]"},{"id":589,"s":"Carbohydrate Chemistry and Metabolism","t":"Disaccharides","u":"Lactose Monosaccharide Components","q":"A neonate suffering from diarrhoea is advised a lactose-free formula milk. A disaccharide containing which of the following two monosaccharides is absent in the formula milk?","o":["Galactose and fructose","Glucose and fructose","Glucose and galactose","Glucose and glucose","Glucose and mannose"],"a":2,"p":"LACTOSE (milk sugar) = GLUCOSE + GALACTOSE linked by BETA 1→4 glycosidic bond. Hydrolysed by LACTASE (beta-galactosidase) on intestinal brush border → absorbed monosaccharides. Lactase deficiency → lactose reaches colon → bacterial fermentation → gas (bloating, flatulence) + organic acids + osmotic water retention → OSMOTIC DIARRHOEA. Lactose-free formula substitutes lactose with other carbohydrates.\n\n📹 [VIDEO LINK]"},{"id":590,"s":"Carbohydrate Chemistry and Metabolism","t":"Disaccharides","u":"Lactulose Beta 1-4 Not Hydrolysed","q":"A 45-year-old woman with cirrhosis presents with constipation and worsening confusion. Her physician prescribes lactulose. Intestinal enzymes will NOT be able to break which of the following linkages in lactulose?","o":["Alpha 1→2","Alpha 1→4","Beta 1→2","Beta 1→4","Beta 1→6"],"a":3,"p":"LACTULOSE = synthetic disaccharide (fructose + galactose, beta 1→4 linkage). Humans lack the enzyme to hydrolyse lactulose → passes UNDIGESTED to colon → fermented by bacteria → organic acids → acidifies colon → NH₃ → NH₄⁺ (trapped) → excreted → reduces hepatic encephalopathy. Also acts as osmotic laxative. The key pharmacological property of lactulose is precisely that it is NOT broken down by intestinal enzymes — this is essential for its mechanism of action.\n\n📹 [VIDEO LINK]"},{"id":591,"s":"Carbohydrate Chemistry and Metabolism","t":"Disaccharides","u":"Sucrose Alpha 1-2 Linkage Tea","q":"A student is sipping tea sweetened with table sugar (sucrose). His intestinal enzymes are busy hydrolyzing which of the following linkages in the sucrose?","o":["Alpha 1→2","Alpha 1→4","Beta 1→2","Beta 1→4","Beta 1→6"],"a":0,"p":"SUCROSE = glucose (alpha C1) + fructose (beta C2) connected by ALPHA 1→2 glycosidic bond. Cleaved by SUCRASE (invertase) on the intestinal brush border → glucose + fructose (INVERT SUGAR). Sucrose is NON-REDUCING because both anomeric carbons (C1 glucose, C2 fructose) are occupied in the bond → no free anomeric carbon → cannot reduce Cu²⁺ → negative Benedict's test. Sucrase-isomaltase deficiency → sucrose intolerance → osmotic diarrhoea.\n\n📹 [VIDEO LINK]"},{"id":592,"s":"Carbohydrate Chemistry and Metabolism","t":"Polysaccharides","u":"Inulin GFR Gold Standard","q":"Which of the following carbohydrates is used as the gold standard for measuring glomerular filtration rate?","o":["Cellulose","Fructose","Glucose","Inulin","Sucrose"],"a":3,"p":"INULIN = polysaccharide of fructose → ideal GFR marker because freely filtered at glomerulus, NOT reabsorbed, NOT secreted, NOT metabolised → inulin clearance = GFR (gold standard). GFR = (Urine inulin × Urine flow rate) / Plasma inulin. Not used routinely (requires IV infusion + continuous urine collection) → replaced clinically by creatinine clearance and eGFR. Inulin is also a prebiotic dietary fibre → fermented by gut bacteria → promotes bowel health.\n\n📹 [VIDEO LINK]"},{"id":593,"s":"Carbohydrate Chemistry and Metabolism","t":"Polysaccharides","u":"Cellulose Beta 1-4 Buffalo Rumen","q":"Buffaloes are large, well-nourished animals despite feeding almost entirely on grass and straw. Which of the following linkages in cellulose are their rumen bacteria able to break down, providing them with nutritional energy?","o":["Alpha 1→2","Alpha 1→4","Beta 1→2","Beta 1→4","Beta 1→6"],"a":3,"p":"Cellulose = polymer of glucose with BETA 1→4 linkages → forms rigid, insoluble structural fibres. Buffaloes (and other ruminants) have RUMEN bacteria and protozoa that produce CELLULASE → breaks BETA 1→4 linkages → releases glucose → absorbed and metabolised. Humans and most animals LACK cellulase → cannot digest cellulose → passes as dietary fibre. The beta 1→4 linkage creates a flat, rigid conformation in cellulose (ideal for structural support) unlike the alpha 1→4 linkage in starch which creates a helical structure (ideal for energy storage).\n\n📹 [VIDEO LINK]"},{"id":594,"s":"Carbohydrate Chemistry and Metabolism","t":"Polysaccharides","u":"Cellulose Beta 1-4 Human Cannot Digest","q":"A slim, healthy man from the Mediterranean region enjoys a diet rich in vegetables, legumes, and whole grains. Despite consuming foods rich in glucose units, he remains slim because he cannot extract energy from structural carbohydrates. Which of the following linkages can he NOT hydrolyse?","o":["Alpha 1→2","Alpha 1→4","Beta 1→2","Beta 1→4","Beta 1→6"],"a":3,"p":"Humans LACK CELLULASE → cannot break BETA 1→4 glycosidic bonds in CELLULOSE → cellulose passes through the gut undigested as dietary FIBRE. Although cellulose contains only glucose (same as starch), the beta 1→4 linkage makes it completely indigestible for humans. Benefits of dietary fibre: reduces bowel transit time, reduces colorectal cancer risk, lowers cholesterol, regulates blood glucose, prevents constipation. Compare: alpha 1→4 (starch, glycogen) = digestible by alpha-amylase → energy source.\n\n📹 [VIDEO LINK]"},{"id":595,"s":"Carbohydrate Chemistry and Metabolism","t":"Polysaccharides","u":"Glycogen Branching Rapid Glucose Release","q":"Glycogen is more highly branched than amylopectin in starch. Which of the following best explains the physiological significance of glycogen's high degree of branching?","o":["Decreases solubility","Prevents amylase degradation","Rapid glucose release","Reduces glucose units stored","Slows glucose release"],"a":2,"p":"GLYCOGEN's HIGH BRANCHING (alpha 1→6 branches every 8-10 glucose units) → MORE NON-REDUCING ENDS → glycogen phosphorylase acts ONLY at non-reducing ends → more ends = faster glucose release when demand is high (exercise, hypoglycaemia). Amylopectin branches every 24-30 glucose units → fewer non-reducing ends → slower glucose release. Amylose = unbranched → slowest release. Glycogen: alpha 1→4 backbone + alpha 1→6 branches. Branching enzyme creates branches. Debranching enzyme removes them during glycogenolysis.\n\n📹 [VIDEO LINK]"},{"id":596,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycosaminoglycans","u":"GAG Structure Negatively Charged","q":"A 62-year-old woman with DVT is given heparin. The highly negative charge of heparin that allows it to bind antithrombin III is due to which structural feature of glycosaminoglycans?","o":["Branched arrangement","Homopolysaccharide backbone","Phosphate groups","Short chain length","Sulfate and carboxylate groups"],"a":4,"p":"GLYCOSAMINOGLYCANS = long, UNBRANCHED, LINEAR HETEROPOLYSACCHARIDES of repeating disaccharide units — hexuronic acid (contributes COO⁻ carboxylate groups) + hexosamine (carries SO₄²⁻ sulfate groups). The COMBINATION of sulfate + carboxylate groups makes GAGs the most highly negatively charged biological polymers. This negative charge allows heparin to bind positively charged residues in antithrombin III → conformational change → AT III becomes 1000-fold more active → anticoagulation. Exception: hyaluronic acid = NOT sulfated (carboxylate only).\n\n[Diagram: Repeating disaccharide unit of a GAG — hexuronic acid (COO⁻ shown in red) + hexosamine (SO₄²⁻ shown in red) connected by glycosidic bond. Multiple negative charges highlighted. Long unbranched chain shown with repeating units. Annotation: negative charge allows binding to positively charged antithrombin III residues.]\n\n📹 [VIDEO LINK]"},{"id":597,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycosaminoglycans","u":"Hyaluronate Not Covalently Linked to Protein","q":"Which of the following glycosaminoglycans is the ONLY one not covalently attached to a protein core?","o":["Chondroitin sulfate","Dermatan sulfate","Heparan sulfate","Hyaluronate","Keratan sulfate"],"a":3,"p":"HYALURONATE (hyaluronic acid) is the ONLY GAG not covalently linked to a protein core. It exists as a FREE polysaccharide chain. Instead, proteoglycans attach NON-COVALENTLY through LINK PROTEINS to hyaluronate creating large aggregan complexes in cartilage. All other GAGs (chondroitin sulfate, dermatan sulfate, heparan sulfate, keratan sulfate, heparin) are covalently attached to protein cores forming proteoglycans. Hyaluronate is also the ONLY non-sulfated major GAG. Functions: joint lubrication, vitreous humour, wound healing, embryogenesis.\n\n📹 [VIDEO LINK]"},{"id":598,"s":"Carbohydrate Chemistry and Metabolism","t":"Proteoglycans","u":"GAG Linked to Serine Residue","q":"GAGs are covalently linked to their protein cores in proteoglycans through which amino acid residue?","o":["Alanine","Glutamine","Proline","Serine","Threonine"],"a":3,"p":"Most GAGs are linked to protein cores via O-GLYCOSIDIC BOND to SERINE residues. The linkage tetrasaccharide (glucuronate-galactose-galactose-xylose) bridges the GAG chain to the serine hydroxyl group. EXCEPTION: KERATAN SULFATE TYPE I (corneal KS) is linked via N-GLYCOSIDIC BOND to ASPARAGINE residues. Keratan Sulfate TYPE II (cartilage KS) is linked via O-glycosidic bond to serine/threonine. The initial glycosylation (xylose attachment to serine) occurs in the ROUGH ENDOPLASMIC RETICULUM.\n\n📹 [VIDEO LINK]"},{"id":599,"s":"Carbohydrate Chemistry and Metabolism","t":"Proteoglycans","u":"Keratan Sulfate I Linked to Asparagine","q":"Keratan Sulfate Type I in the cornea is linked to its protein core through which of the following amino acid residues?","o":["Alanine","Asparagine","Glutamine","Proline","Serine"],"a":1,"p":"KERATAN SULFATE TYPE I (corneal KS-I) is linked to the protein core via N-GLYCOSIDIC BOND to ASPARAGINE residues (N-linked glycosylation). Keratan Sulfate TYPE II (cartilage KS-II) is linked via O-GLYCOSIDIC BOND to SERINE/THREONINE. This distinction is clinically important for understanding Macular Corneal Dystrophy — a condition caused by defective synthesis of corneal KS-I proteoglycans. Keratan sulfate provides CORNEAL TRANSPARENCY — its uniform spacing prevents light scattering. Hexuronic acid component of keratan sulfate = GALACTOSE (NOT glucuronate — unique among GAGs).\n\n📹 [VIDEO LINK]"},{"id":600,"s":"Carbohydrate Chemistry and Metabolism","t":"Proteoglycans","u":"Proteoglycan Protein Core Synthesised in RER","q":"The protein component of proteoglycans is synthesised in which organelle?","o":["Golgi apparatus","Lysosomes","Peroxisome","Rough endoplasmic reticulum","Smooth endoplasmic reticulum"],"a":3,"p":"Proteoglycan synthesis pathway: (1) ROUGH ER → protein core synthesised on ribosomes → translocated into RER lumen → initial glycosylation begins (xylose attached to serine). (2) GOLGI APPARATUS → further sugar additions, epimerisation (glucuronate → iduronate), sulfation (using PAPS as sulfate donor) → complete GAG chains formed. (3) SECRETORY VESICLES → proteoglycans transported to ECM. Degradation: LYSOSOMES → lysosomal hydrolases degrade GAG chains. Deficient lysosomal degradation → MUCOPOLYSACCHARIDOSES.\n\n📹 [VIDEO LINK]"},{"id":601,"s":"Carbohydrate Chemistry and Metabolism","t":"Proteoglycans","u":"Initial Glycosylation in RER","q":"In proteoglycan synthesis, initial glycosylation of the protein core occurs in which organelle?","o":["Golgi apparatus","Lysosomes","Mitochondria","Rough endoplasmic reticulum","Smooth endoplasmic reticulum"],"a":3,"p":"INITIAL GLYCOSYLATION (attachment of the first sugar — xylose — to serine residues of the protein core) occurs in the ROUGH ENDOPLASMIC RETICULUM. This is the rate-limiting committed step of GAG chain synthesis. Subsequent additions (galactose, galactose, glucuronate forming the linker tetrasaccharide) continue in the RER. Further elongation, epimerisation of glucuronate to iduronate, and SULFATION all occur in the GOLGI apparatus.\n\n📹 [VIDEO LINK]"},{"id":602,"s":"Carbohydrate Chemistry and Metabolism","t":"Proteoglycans","u":"Final Glycosylation Sulfation in Golgi","q":"Final glycosylation and sulfation of proteoglycans occur in which organelle?","o":["Golgi apparatus","Lysosomes","Mitochondria","Rough endoplasmic reticulum","Smooth endoplasmic reticulum"],"a":0,"p":"The GOLGI APPARATUS is where the following final modifications occur: (1) Elongation of GAG chains. (2) EPIMERISATION — glucuronic acid → iduronic acid (by epimerase). (3) SULFATION — sulfate groups added using PAPS (3'-phosphoadenosine-5'-phosphosulfate) as sulfate donor. After processing in Golgi → proteoglycans packaged into secretory vesicles → transported to ECM (final destination) or remain at cell surface.\n\n📹 [VIDEO LINK]"},{"id":603,"s":"Carbohydrate Chemistry and Metabolism","t":"Proteoglycans","u":"Epimerase Glucuronate to Iduronate","q":"Glucuronic acid is converted to iduronic acid in proteoglycan synthesis by the action of which of the following enzymes?","o":["Epimerase","Isomerase","Kinase","Mutase","Racemase"],"a":0,"p":"EPIMERASE converts glucuronic acid → IDURONIC ACID by inverting the configuration at C5 → occurs in the GOLGI during proteoglycan processing. Iduronic acid is found in: heparin, heparan sulfate, and DERMATAN SULFATE. Distinction: EPIMERASE (changes configuration at one carbon), ISOMERASE (converts aldose ↔ ketose), MUTASE (moves phosphate group), RACEMASE (inverts all stereocentres).\n\n📹 [VIDEO LINK]"},{"id":604,"s":"Carbohydrate Chemistry and Metabolism","t":"Proteoglycans","u":"Proteoglycans Non-Covalently Attached to Hyaluronate","q":"Proteoglycans in the extracellular matrix are non-covalently attached through link proteins to which of the following?","o":["Chondroitin sulfate","Dermatan sulfate","Heparan sulfate","Hyaluronate","Keratan sulfate"],"a":3,"p":"PROTEOGLYCANS attach NON-COVALENTLY to HYALURONATE through LINK PROTEINS → creates large AGGRECAN AGGREGATES (hyaluronate as backbone, proteoglycan monomers as bristles). This aggrecan-hyaluronate complex is the major structural component of articular cartilage — provides compressive strength and water retention. The proteoglycan monomers (e.g. aggrecan) have many chondroitin sulfate and keratan sulfate chains covalently attached.\n\n[Diagram: Aggrecan aggregate — central hyaluronate backbone. Link proteins at regular intervals connecting proteoglycan monomers to hyaluronate. Each proteoglycan monomer shown as a protein core with multiple GAG chains (chondroitin sulfate + keratan sulfate) branching outward like a bottle brush. Label: NON-COVALENT attachment via link proteins to hyaluronate. COVALENT attachment of GAG chains to protein core via serine residues.]\n\n📹 [VIDEO LINK]"},{"id":605,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycosaminoglycans","u":"Hyaluronate Cell Migration Wound Healing","q":"Which of the following glycosaminoglycans is involved in cell migration during embryogenesis, morphogenesis, and wound healing?","o":["Chondroitin sulfate","Dermatan sulfate","Heparan sulfate","Heparin","Hyaluronate"],"a":4,"p":"HYALURONATE plays KEY ROLES in: (1) EMBRYOGENESIS and MORPHOGENESIS — creates space for cell migration by its high water-binding capacity (one molecule binds up to 1000 water molecules). (2) WOUND HEALING — accumulates in healing tissue → provides scaffold for cell migration → later replaced by collagen. (3) JOINT LUBRICATION — component of synovial fluid. (4) VITREOUS HUMOUR — fills eye. Hyaluronidase (in sperm) cleaves hyaluronate → facilitates sperm penetration of ovum.\n\n📹 [VIDEO LINK]"},{"id":606,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycosaminoglycans","u":"Hyaluronate Joint Lubrication","q":"Which of the following glycosaminoglycans provides joint lubrication and is the biochemical basis for smooth functioning of joints?","o":["Chondroitin sulfate","Dermatan sulfate","Heparin","Heparan sulfate","Hyaluronate"],"a":4,"p":"HYALURONATE in SYNOVIAL FLUID provides JOINT LUBRICATION. Mechanism: hyaluronate's highly negative charge → attracts water → creates viscous, elastic fluid → VISCOELASTIC PROPERTIES → absorbs shock during compression → provides lubrication during movement. Under compression: water squeezed out → GAG chains resist compression (negative charges repel each other → RESILIENCE). Intra-articular hyaluronate injections (viscosupplementation) used in osteoarthritis treatment.\n\n📹 [VIDEO LINK]"},{"id":607,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycosaminoglycans","u":"GAG Resilience Repulsion Negative Charges","q":"Which of the following is the biochemical mechanism responsible for the resilience and cushioning properties of glycosaminoglycans in synovial fluid and cartilage?","o":["Attraction of negative charges","Attraction of opposite charges","Attraction of positive charges","Repulsion of negative charges","Repulsion of positive charges"],"a":3,"p":"GAGs are heavily sulfated and contain uronic acids → HIGHLY NEGATIVELY CHARGED. LIKE CHARGES REPEL → when cartilage or synovial fluid is compressed → GAG chains resist compression by REPULSION OF NEGATIVE CHARGES → springs back to original shape = RESILIENCE. The negative charges also attract Na⁺ and water → DONNAN EFFECT → high water content (cartilage is 65-80% water) → further cushioning. This mechanism allows cartilage to withstand compressive forces up to 5× body weight.\n\n📹 [VIDEO LINK]"},{"id":608,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycosaminoglycans","u":"Heparin Binds Antithrombin III DVT","q":"A 62-year-old woman with DVT is prescribed heparin. The anticoagulant action of heparin involves binding and activating which of the following proteins in her blood?","o":["Antithrombin I","Antithrombin III","Factor VII","Protein C","Thrombomodulin"],"a":1,"p":"HEPARIN → binds ANTITHROMBIN III (AT III) → conformational change in AT III → AT III becomes 1000-fold more active → rapidly inhibits THROMBIN (IIa) and FACTOR Xa → anticoagulation. Heparin acts as a CATALYST — accelerates the AT III reaction then dissociates and can bind more AT III. LMWH (enoxaparin) preferentially inhibits Factor Xa. Heparin is a highly sulfated heparan sulfate — most negatively charged biological molecule known. Monitored by APTT.\n\n📹 [VIDEO LINK]"},{"id":609,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycosaminoglycans","u":"Heparin Anticoagulant Binds Antithrombin","q":"Which of the following glycosaminoglycans has anticoagulant activity by binding antithrombin III?","o":["Chondroitin sulfate","Dermatan sulfate","Heparin","Hyaluronate","Keratan sulfate"],"a":2,"p":"HEPARIN = most highly sulfated GAG → ANTICOAGULANT. Mechanisms: (1) Binds ANTITHROMBIN III → inhibits thrombin (IIa) and factor Xa. (2) RELEASES LIPOPROTEIN LIPASE from capillary walls → clears plasma triglycerides. (3) Binds growth factors (FGF, VEGF) → modulates angiogenesis. Produced by MAST CELLS and BASOPHILS. Found in liver, lungs, mast cells. Clinically used: unfractionated heparin (UFH), LMWH, fondaparinux.\n\n📹 [VIDEO LINK]"},{"id":610,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycosaminoglycans","u":"Heparin Releases Lipoprotein Lipase","q":"Heparin has lipid-clearing activity because it causes release of which of the following enzymes from capillary walls?","o":["Gastric lipase","Hepatic lipase","Hormone-sensitive lipase","Lipoprotein lipase","Pancreatic lipase"],"a":3,"p":"Heparin releases LIPOPROTEIN LIPASE (LPL) from its heparan sulfate proteoglycan anchors on capillary endothelial cells → LPL enters circulation → hydrolyses triglycerides in VLDL and chylomicrons → LIPID-CLEARING EFFECT → post-heparin lipolytic activity (PHLA). LPL is normally anchored to endothelial cell surface heparan sulfate proteoglycans. Heparin displaces LPL from these anchors.\n\n📹 [VIDEO LINK]"},{"id":611,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycosaminoglycans","u":"Keratan Sulfate Corneal Transparency","q":"Which of the following glycosaminoglycans is present in cornea and is responsible for corneal transparency?","o":["Chondroitin 4-sulfate","Chondroitin 6-sulfate","Dermatan sulfate","Heparan sulfate","Keratan sulfate I"],"a":4,"p":"KERATAN SULFATE TYPE I (KS-I) = found in CORNEA → linked to protein core via N-glycosidic bond (asparagine). KS-I provides CORNEAL TRANSPARENCY by maintaining uniform spacing between collagen fibrils in the stroma → regular array → minimal light scattering → clear vision. Defective KS-I synthesis → Macular Corneal Dystrophy. Hexuronic acid component of keratan sulfate = GALACTOSE (NOT glucuronate — unique among GAGs). KS-II = CARTILAGE (linked via O-glycosidic bond to serine/threonine).\n\n📹 [VIDEO LINK]"},{"id":612,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycosaminoglycans","u":"Chondroitin Sulfate Cartilage","q":"Chondroitin sulfate proteoglycans have a role in which of the following?","o":["Anticoagulation","Cartilage formation","Cell migration","Cornea transparency","Skin fibroblasts"],"a":1,"p":"CHONDROITIN SULFATE = most abundant GAG in the body. Roles: (1) CARTILAGE formation — major component of aggrecan in articular cartilage → compressive strength. (2) BONE formation — in bone matrix proteoglycans. (3) CORNEA formation — chondroitin 4-sulfate and 6-sulfate in corneal stroma. Contains glucuronate + N-acetylgalactosamine. Glucosamine supplements (for osteoarthritis) = chondroitin sulfate and glucosamine are the main components.\n\n📹 [VIDEO LINK]"},{"id":613,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycosaminoglycans","u":"Dermatan Sulfate Skin Fibroblasts LDL","q":"Which of the following glycosaminoglycans is a component of skin fibroblasts and binds LDL to plasma vessel walls?","o":["Chondroitin sulfate","Dermatan sulfate","Heparan sulfate","Heparin","Hyaluronate"],"a":1,"p":"DERMATAN SULFATE (DS) = contains IDURONIC ACID + N-acetylgalactosamine. Functions: (1) SKIN FIBROBLASTS — component of decorin proteoglycan in skin dermis → regulates collagen fibril assembly. (2) LDL BINDING to plasma vessel walls — DS proteoglycans (biglycan, decorin) bind and retain LDL in arterial intima → contributes to atherosclerosis. (3) COAGULATION — dermatan sulfate activates heparin cofactor II. Elevated dermatan sulfate in urine = marker of mucopolysaccharidoses (Hunter, Hurler syndromes).\n\n📹 [VIDEO LINK]"},{"id":614,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycosaminoglycans","u":"Heparan Sulfate Cell Surfaces Aorta","q":"Which of the following glycosaminoglycans is commonly found on cell surfaces and in the aortic wall?","o":["Chondroitin sulfate","Dermatan sulfate","Heparan sulfate","Heparin","Hyaluronate"],"a":2,"p":"HEPARAN SULFATE = structurally similar to heparin but LESS sulfated. Found predominantly on CELL SURFACES (as heparan sulfate proteoglycans — syndecan, glypican) and in BASEMENT MEMBRANES and AORTIC WALL. Functions: (1) Cell surface receptor binding (growth factors FGF, VEGF). (2) LIPOPROTEIN LIPASE anchoring on endothelial cells. (3) Filtration barrier in glomerular basement membrane — negative charges prevent albumin passage. Heparan sulfate → heparin (mast cell modification by further sulfation and epimerisation).\n\n📹 [VIDEO LINK]"},{"id":615,"s":"Carbohydrate Chemistry and Metabolism","t":"Mucopolysaccharidoses","u":"MPS Defect in Degradation","q":"Mucopolysaccharidoses are inherited storage diseases caused by which of the following?","o":["Defects in degradation of proteoglycans","Defects in synthesis of proteoglycans","Increased rate of synthesis","Synthesis of abnormally small protein cores","Synthesis of structurally altered polysaccharides"],"a":0,"p":"MUCOPOLYSACCHARIDOSES (MPS) = lysosomal storage diseases caused by DEFICIENT LYSOSOMAL ENZYMES that degrade GAG chains. Without these enzymes → GAG chains ACCUMULATE in LYSOSOMES → progressive cellular dysfunction → coarse facial features, organomegaly, skeletal deformities, neurological deterioration. GAG synthesis is NORMAL — the problem is FAILURE OF DEGRADATION. Different MPS types have different enzyme deficiencies affecting different GAG types.\n\n[Note to editor: Insert diagram of lysosomal GAG degradation pathway showing all enzymes in sequence. Circle the deficient enzyme in each MPS type in red. Pathway shows: Heparan sulfate + Dermatan sulfate breakdown → Hunter (iduronate sulfatase, step 2 circled) → Hurler (alpha-L-iduronidase, step 1 circled) → Sanfilippo A (heparan sulfatase circled) → Sly (beta-glucuronidase circled). Insert clinical photographs of gargoyle facies from net.]\n\n📹 [VIDEO LINK]"},{"id":616,"s":"Carbohydrate Chemistry and Metabolism","t":"Mucopolysaccharidoses","u":"Hunter Syndrome Iduronate Sulfatase","q":"A mentally retarded child with unusual facial features, enlarged tongue and tonsils, is diagnosed with Hunter syndrome. Which of the following enzyme is deficient?","o":["Alpha-L-iduronidase","Beta-glucuronidase","Iduronate sulfatase","N-acetylgalactosamine sulfatase","N-acetylglucosaminidase"],"a":2,"p":"HUNTER SYNDROME (MPS II): IDURONATE SULFATASE deficiency → accumulation of HEPARAN SULFATE + DERMATAN SULFATE. X-LINKED RECESSIVE (only MPS that is X-linked — affects males predominantly). Features: coarse facial features, enlarged tongue/tonsils/adenoids, hepatosplenomegaly, intellectual disability, recurrent respiratory infections, valvular heart disease. Key distinguishing feature from Hurler: NO CORNEAL CLOUDING in Hunter.\n\n[Note to editor: Insert clinical photograph of Hunter syndrome facial features from net. Insert lysosomal degradation pathway diagram with iduronate sulfatase circled in red.]\n\n📹 [VIDEO LINK]"},{"id":617,"s":"Carbohydrate Chemistry and Metabolism","t":"Mucopolysaccharidoses","u":"Hurler Syndrome Alpha-L-Iduronidase Corneal Clouding","q":"A child presents with corneal clouding, developmental disability, coarse facial features, and recurrent ear infections. Alpha-L-iduronidase is deficient. Which condition does this child have?","o":["Hunter syndrome","Hurler syndrome","Morquio syndrome","Sanfilippo syndrome","Sly syndrome"],"a":1,"p":"HURLER SYNDROME (MPS I): ALPHA-L-IDURONIDASE deficiency → accumulation of HEPARAN SULFATE + DERMATAN SULFATE. AUTOSOMAL RECESSIVE. Features: CORNEAL CLOUDING (key distinguishing feature from Hunter), coarse gargoyle-like facies, macroglossia, hepatosplenomegaly, intellectual disability, recurrent ear infections, skeletal deformities, valvular heart disease. Treatment: haematopoietic stem cell transplantation + enzyme replacement therapy (laronidase).\n\n[Note to editor: Insert clinical photograph of Hurler syndrome showing corneal clouding and facial features from net. Insert lysosomal degradation pathway with alpha-L-iduronidase circled in red.]\n\n📹 [VIDEO LINK]"},{"id":618,"s":"Carbohydrate Chemistry and Metabolism","t":"Mucopolysaccharidoses","u":"Hunter Syndrome GAG Accumulation No Corneal Clouding","q":"A gargoyle-like child with enlarged abdomen due to hepatosplenomegaly, valvular heart disease, and NO corneal clouding has Hunter's syndrome. Which of the following GAGs accumulates?","o":["Chondroitin sulfate and dermatan sulfate","Dermatan sulfate and keratan sulfate","Heparan sulfate and chondroitin sulfate","Heparan sulfate and dermatan sulfate","Keratan sulfate and heparan sulfate"],"a":3,"p":"HUNTER SYNDROME (MPS II): iduronate sulfatase deficiency → HEPARAN SULFATE + DERMATAN SULFATE accumulate. NO corneal clouding = classic distinguishing feature of Hunter vs Hurler. Both heparan sulfate and dermatan sulfate detected in urine of Hunter patients. Same GAGs accumulate in Hurler — but Hurler has corneal clouding (different enzyme). X-linked recessive pattern → family history in maternal uncles.\n\n📹 [VIDEO LINK]"},{"id":619,"s":"Carbohydrate Chemistry and Metabolism","t":"Mucopolysaccharidoses","u":"Hurler Syndrome GAG Accumulation Corneal Clouding","q":"A child with corneal clouding, coarse facial features, progressive joint stiffening, and deficiency of alpha-L-iduronidase. Which of the following GAGs accumulates?","o":["Chondroitin sulfate and dermatan sulfate","Dermatan sulfate and keratan sulfate","Heparan sulfate and chondroitin sulfate","Heparan sulfate and dermatan sulfate","Heparan sulfate and keratan sulfate"],"a":3,"p":"HURLER SYNDROME (MPS I): ALPHA-L-IDURONIDASE deficiency → HEPARAN SULFATE + DERMATAN SULFATE accumulate. Alpha-L-iduronidase cleaves the alpha-L-iduronic acid residues from both heparan sulfate and dermatan sulfate chains. Without it → both GAG chains accumulate in lysosomes → progressive multi-organ dysfunction. CORNEAL CLOUDING present in Hurler (absent in Hunter) — this is the key clinical distinguishing feature.\n\n📹 [VIDEO LINK]"},{"id":620,"s":"Carbohydrate Chemistry and Metabolism","t":"Mucopolysaccharidoses","u":"Sanfilippo A Heparan Sulfatase","q":"A 5-year-old boy presents with coarse facial features and intellectual disability. Enzyme assay shows decreased activity of heparan sulfatase. What is the most likely diagnosis?","o":["Hunter syndrome","Hurler syndrome","Morquio syndrome","Sanfilippo syndrome A","Sly syndrome"],"a":3,"p":"SANFILIPPO SYNDROME A (MPS IIIA): HEPARAN N-SULFATASE (heparan sulfatase) deficiency → accumulation of HEPARAN SULFATE ONLY. Features: severe INTELLECTUAL DISABILITY and BEHAVIOURAL PROBLEMS (predominantly neurological) with relatively mild somatic features. Four types (A-D) based on enzyme deficiency but all accumulate only heparan sulfate. Sanfilippo A = heparan sulfatase. Sanfilippo B = N-acetylglucosaminidase.\n\n[Note to editor: Insert clinical photograph of Sanfilippo syndrome from net. Insert lysosomal degradation pathway with heparan sulfatase circled in red.]\n\n📹 [VIDEO LINK]"},{"id":621,"s":"Carbohydrate Chemistry and Metabolism","t":"Mucopolysaccharidoses","u":"Sly Syndrome Beta Glucuronidase","q":"A 4-year-old child with coarse facial features and hepatosplenomegaly is diagnosed with Sly syndrome. Which enzyme is deficient?","o":["Alpha-L-iduronidase","Beta-glucuronidase","Heparan sulfatase","Hyaluronidase","N-acetylglucosaminidase"],"a":1,"p":"SLY SYNDROME (MPS VII): BETA-GLUCURONIDASE deficiency → accumulation of DERMATAN SULFATE, HEPARAN SULFATE, and CHONDROITIN SULFATE (all contain glucuronic acid requiring beta-glucuronidase for degradation). Rarest MPS. Features: coarse facial features, hepatosplenomegaly, skeletal dysplasia, intellectual disability.\n\n[Note to editor: Insert clinical photograph of Sly syndrome from net. Insert lysosomal degradation pathway with beta-glucuronidase circled in red.]\n\n📹 [VIDEO LINK]"},{"id":622,"s":"Carbohydrate Chemistry and Metabolism","t":"Mucopolysaccharidoses","u":"Hurler Syndrome Diagnosis Corneal Clouding","q":"An infant presented with corneal clouding, developmental disability, and coarse facial features. Lab diagnosis showed dermatan sulfate and heparan sulfate in urine. The deficient enzyme is alpha-L-iduronidase. What is the most likely diagnosis?","o":["Hunter syndrome","Hurler syndrome","Morquio syndrome","Sanfilippo syndrome","Sly syndrome"],"a":1,"p":"CORNEAL CLOUDING + dermatan sulfate + heparan sulfate in urine + alpha-L-iduronidase deficiency = HURLER SYNDROME (MPS I). Corneal clouding distinguishes Hurler from Hunter (same GAG accumulation, different enzyme, different inheritance). Hurler = autosomal recessive. Hunter = X-linked recessive. Both accumulate heparan sulfate + dermatan sulfate. Treatment of Hurler: haematopoietic stem cell transplantation (early, before 2 years) + enzyme replacement therapy (laronidase).\n\n📹 [VIDEO LINK]"},{"id":623,"s":"Carbohydrate Chemistry and Metabolism","t":"Mucopolysaccharidoses","u":"Hunter Syndrome Clear Cornea Diagnosis","q":"A 4-year-old boy presents with mental retardation, skeletal deformity, coarse gargoyle-like facial features, and CLEAR CORNEA. Lab investigations show accumulation of dermatan sulfate and heparan sulfate in urine. What is the most likely diagnosis?","o":["Hunter syndrome","Hurler syndrome","Morquio syndrome","Sanfilippo syndrome","Sly syndrome"],"a":0,"p":"CLEAR CORNEA + dermatan sulfate + heparan sulfate in urine = HUNTER SYNDROME (MPS II). CLEAR CORNEA is the KEY distinguishing feature: Hunter has NO corneal clouding (unlike Hurler which has corneal clouding). Both Hunter and Hurler accumulate the same GAGs (heparan sulfate + dermatan sulfate) but different enzyme deficiencies and different inheritance. Hunter = iduronate sulfatase deficiency, X-linked recessive. Hurler = alpha-L-iduronidase deficiency, autosomal recessive.\n\n📹 [VIDEO LINK]"},{"id":624,"s":"Carbohydrate Chemistry and Metabolism","t":"Mucopolysaccharidoses","u":"Hurler Deficient Enzyme Dermatan Heparan","q":"A child with Hurler syndrome has dermatan sulfate and heparan sulfate in urine along with corneal clouding and coarse facial features. Which enzyme is deficient?","o":["Alpha-L-iduronidase","Beta-glucuronidase","Hyaluronidase","N-acetylglucosaminidase","Neuraminidase"],"a":0,"p":"HURLER SYNDROME (MPS I) = ALPHA-L-IDURONIDASE deficiency. Alpha-L-iduronidase cleaves alpha-L-iduronic acid residues from non-reducing ends of DERMATAN SULFATE and HEPARAN SULFATE. Without this enzyme → both GAGs accumulate → lysosomal storage → progressive multi-organ damage. Corneal clouding present in Hurler. Coarse facial features, hepatosplenomegaly, intellectual disability, joint stiffening, recurrent infections = classic Hurler features.\n\n📹 [VIDEO LINK]"},{"id":625,"s":"Carbohydrate Chemistry and Metabolism","t":"Blood Glucose Regulation","u":"DKA Kussmaul Breathing Metabolic Acidosis","q":"A 6-year-old girl presents in comatose state with Kussmaul's respiration, ketonuria, and glucosuria. She is diagnosed with diabetic ketoacidosis. Kussmaul's breathing in this child is a compensatory response to which of the following?","o":["Hypocalcaemia","Hypoglycaemia","Metabolic acidosis","Metabolic alkalosis","Respiratory alkalosis"],"a":2,"p":"KUSSMAUL'S BREATHING = deep, rapid, laboured breathing — a COMPENSATORY RESPONSE to METABOLIC ACIDOSIS in DKA. Mechanism: ↑ ketone bodies (acetoacetate, beta-hydroxybutyrate) → metabolic acidosis → ↓ blood pH → medullary chemoreceptors activated → hyperventilation → exhale CO₂ → ↓ pCO₂ → partially corrects pH. The breath smells of KETONES (fruity/acetone smell) from exhaled acetone. DKA pH typically <7.3, HCO₃ <15 mEq/L, high anion gap.\n\n📹 [VIDEO LINK]"},{"id":626,"s":"Carbohydrate Chemistry and Metabolism","t":"Blood Glucose Regulation","u":"Type 2 DM Polydipsia Hyperosmolarity","q":"A 55-year-old man with long-standing uncontrolled type 2 diabetes presents with a 3-month history of increased urination and excessive thirst. His fasting blood glucose is 18 mmol/L. The biochemical basis of his excessive thirst is which of the following?","o":["Hypercalcaemia","Hyperkalaemia","Hypernatraemia","Hyperosmolarity","Lactic acidosis"],"a":3,"p":"POLYDIPSIA in type 2 DM: uncontrolled hyperglycaemia → ↑ plasma osmolality → HYPEROSMOLARITY → OSMORECEPTORS in hypothalamus detect ↑ osmolality → THIRST CENTRE activated → polydipsia. Also: glucosuria → osmotic diuresis → dehydration → further ↑ osmolality → more thirst. Type 2 DM does NOT typically present with DKA (residual insulin prevents ketogenesis). Compare: TYPE 1 DM — absolute insulin deficiency → unchecked lipolysis → free fatty acids → ketogenesis → DKA → Kussmaul's breathing → acute presentation.\n\n📹 [VIDEO LINK]"},{"id":627,"s":"Carbohydrate Chemistry and Metabolism","t":"Blood Glucose Regulation","u":"Osmotic Diuresis Glucosuria Renal Threshold","q":"A 55-year-old man with uncontrolled type 2 diabetes has blood glucose of 18 mmol/L and passes large volumes of urine. Which of the following best explains osmotic diuresis in this patient?","o":["ADH suppression","Glucosuria","Hypernatraemia","Ketonuria","Tubular damage"],"a":1,"p":"OSMOTIC DIURESIS: blood glucose exceeds RENAL THRESHOLD (10 mmol/L / ~180 mg/dL) → SGLT2 in proximal tubule becomes SATURATED → glucose CANNOT be fully reabsorbed → GLUCOSURIA → glucose in tubular fluid is osmotically active → retains water in tubular lumen → OSMOTIC DIURESIS → POLYURIA. Each gram of unabsorbed glucose carries approximately 15 mL of water. SGLT2 inhibitors (gliflozins) deliberately lower the renal threshold → increase glucosuria → reduce blood glucose.\n\n📹 [VIDEO LINK]"},{"id":628,"s":"Carbohydrate Chemistry and Metabolism","t":"Blood Glucose Regulation","u":"Impaired Fasting Glucose Criteria","q":"A 40-year-old man with type 2 diabetes has fasting blood glucose of 6.8 mmol/L on two separate occasions. Which of the following correctly describes the diagnostic criteria for his condition?","o":["Fasting glucose 5.1–5.5 mmol/L","Fasting glucose 5.6–6.9 mmol/L","Fasting glucose 7.0 mmol/L or above","Fasting glucose 7.8–11.0 mmol/L","Random glucose 11.1 mmol/L or above"],"a":1,"p":"WHO/ADA diagnostic criteria: NORMAL fasting = <5.6 mmol/L. IMPAIRED FASTING GLUCOSE (IFG): 5.6–6.9 mmol/L = pre-diabetes. DIABETES MELLITUS: fasting ≥7.0 mmol/L on two separate occasions OR random ≥11.1 mmol/L with symptoms. IMPAIRED GLUCOSE TOLERANCE (IGT): 2-hour OGTT glucose 7.8–11.0 mmol/L. IFG and IGT = pre-diabetes states → high risk of progression to T2DM → lifestyle intervention reduces risk by 58%.\n\n📹 [VIDEO LINK]"},{"id":629,"s":"Carbohydrate Chemistry and Metabolism","t":"Minor Pathways of Glucose","u":"Sorbitol Cataract Diabetes Aldose Reductase","q":"A 60-year-old man with poorly controlled diabetes presents with progressive blurring of vision. Examination confirms bilateral cataracts. Which of the following is responsible for cataract formation in him?","o":["Fructose","Gluconic acid","Glucuronic acid","Mannitol","Sorbitol"],"a":4,"p":"SORBITOL PATHWAY: hyperglycaemia → excess glucose in lens cells (insulin-independent uptake) → ALDOSE REDUCTASE (requires NADPH) → glucose → SORBITOL. Lens cells LACK SORBITOL DEHYDROGENASE → SORBITOL ACCUMULATES → osmotically active → attracts water → OSMOTIC SWELLING of lens fibres → CATARACT FORMATION. Same mechanism occurs in peripheral nerves (diabetic neuropathy — sorbitol accumulates in Schwann cells) and retinal pericytes (diabetic retinopathy). Aldose reductase inhibitors (epalrestat) used to prevent these complications.\n\n📹 [VIDEO LINK]"},{"id":630,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycogen Storage Diseases","u":"Galactosaemia GALT Deficiency","q":"A neonate develops jaundice, vomiting, and cataracts within days of starting breast milk. Blood galactose is markedly elevated. Deficiency of which enzyme is responsible?","o":["Aldose reductase","Galactokinase","Galactose-1-phosphate uridylyltransferase","Hexokinase","Phosphoglucomutase"],"a":2,"p":"GALACTOSAEMIA (classic): GALACTOSE-1-PHOSPHATE URIDYLYLTRANSFERASE (GALT) deficiency → galactose-1-phosphate accumulates → toxic to liver (jaundice, cirrhosis), brain (intellectual disability), lens (cataracts via galactitol). Treatment: eliminate galactose from diet (no breast milk, no dairy). Detected on newborn screening. Galactokinase deficiency → milder (cataracts only, no intellectual disability) — galactitol accumulates but not galactose-1-phosphate.\n\n📹 [VIDEO LINK]"},{"id":631,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycogen Storage Diseases","u":"Hereditary Fructose Intolerance Aldolase B","q":"A 6-month-old infant develops vomiting, hypoglycaemia, and progressive liver damage when weaned from breast milk to fruit purees and sweetened foods. He is found to have absent aldolase B activity. Which of the following accumulates and causes the liver toxicity?","o":["Fructose","Fructose-1,6-bisphosphate","Fructose-1-phosphate","Fructose-6-phosphate","Glucose-6-phosphate"],"a":2,"p":"HEREDITARY FRUCTOSE INTOLERANCE: ALDOLASE B deficiency → FRUCTOSE-1-PHOSPHATE ACCUMULATES → toxic to liver (inhibits glycogen phosphorylase → hypoglycaemia; inhibits aldolase A → blocks glycolysis; direct hepatocyte toxicity → liver damage → jaundice, cirrhosis). Also causes renal tubular acidosis. Symptoms on introduction of fruit/fructose/sucrose. Patients develop AVERSION TO SWEET FOODS (protective instinct). Treatment: eliminate fructose, sucrose, sorbitol from diet.\n\n📹 [VIDEO LINK]"},{"id":632,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycogen Storage Diseases","u":"Von Gierke Glucose-6-Phosphatase Lactic Acidosis","q":"A 2-year-old boy presents with severe fasting hypoglycaemia, massive hepatomegaly, lactic acidosis, and a doll-like face with truncal obesity. Which enzyme is deficient?","o":["Alpha-1,6-glucosidase","Glucose-6-phosphatase","Glycogen phosphorylase","Glycogen synthase","Phosphoglucomutase"],"a":1,"p":"VON GIERKE DISEASE (GSD Type I): GLUCOSE-6-PHOSPHATASE deficiency → glucose-6-phosphate cannot be converted to free glucose → SEVERE FASTING HYPOGLYCAEMIA. G6P diverted to glycolysis → pyruvate → LACTIC ACIDOSIS. Also → HYPERURICAEMIA (G6P → pentose phosphate → purines) + HYPERLIPIDAEMIA. Glycogen and triglycerides accumulate → MASSIVE HEPATOMEGALY, renomegaly, doll-like face, truncal obesity. Treatment: frequent feeds, uncooked cornstarch. Glucose-6-phosphatase located in smooth endoplasmic reticulum.\n\n📹 [VIDEO LINK]"},{"id":633,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycogen Storage Diseases","u":"McArdle Muscle Glycogen Phosphorylase","q":"A 25-year-old man experiences severe muscle cramps and myoglobinuria after intense exercise. At rest he is completely asymptomatic. Forearm ischaemic exercise test shows no rise in blood lactate. Which enzyme is deficient in his muscle?","o":["Aldolase B","Glucose-6-phosphatase","Glycogen phosphorylase","Glycogen synthase","Phosphofructokinase"],"a":2,"p":"McARDLE DISEASE (GSD Type V): MUSCLE GLYCOGEN PHOSPHORYLASE deficiency → cannot break down muscle glycogen → no glucose-1-phosphate → energy failure during exercise → MUSCLE CRAMPS, MYOGLOBINURIA (rhabdomyolysis), exercise intolerance. Forearm ischaemic exercise test: normal → blood lactate rises. McArdle → NO RISE IN LACTATE. Blood ammonia rises (purine nucleotide cycle intact). SECOND WIND PHENOMENON — symptoms improve after a few minutes as fatty acid mobilisation compensates. No liver involvement.\n\n📹 [VIDEO LINK]"},{"id":634,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"ABO Blood Groups Oligosaccharides H Antigen","q":"ABO blood group antigens on the surface of red blood cells are determined by oligosaccharide chains. The H antigen is expressed on which blood group types?","o":["All blood groups","Blood group A only","Blood group AB only","Blood group B only","Blood group O only"],"a":0,"p":"H antigen = fucose-containing oligosaccharide → present on ALL blood types. Blood group A: N-acetylgalactosamine added to H antigen by A transferase. Blood group B: galactose added by B transferase. Blood group AB: both modifications present. Blood group O: H antigen only — neither transferase adds a sugar → H antigen exposed. Bombay blood group: no H antigen → no A, B, or H — ultra-rare. ABO incompatibility → immediate haemolytic transfusion reaction (IgM anti-A or anti-B → complement activation → intravascular haemolysis).\n\n📹 [VIDEO LINK]"},{"id":635,"s":"Carbohydrate Chemistry and Metabolism","t":"Blood Glucose Regulation","u":"Glycemic Index Definition","q":"A 45-year-old man with type 2 diabetes is counselled about diet. Which of the following best describes the glycemic index?","o":["Caloric content","Carbohydrate quantity","Fibre content","Glucose response ratio","Sugar concentration"],"a":3,"p":"GLYCEMIC INDEX (GI) = measure of how rapidly a carbohydrate food raises BLOOD GLUCOSE RELATIVE TO PURE GLUCOSE (GI = 100) — a glucose response ratio. Low GI (<55): oats, legumes, lentils, pasta, most fruits → gradual glucose release → better glycaemic control. High GI (>70): white bread, white rice, corn flakes → rapid glucose surge → high insulin demand. Low GI diet → reduces postprandial hyperglycaemia → reduces HbA1c → reduces cardiovascular risk. Glycaemic load = GI × grams of carbohydrate per serving / 100.\n\n📹 [VIDEO LINK]"},{"id":636,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"Sialic Acid RBC Negative Charge","q":"N-acetylneuraminic acid (sialic acid) is found on the surface of red blood cells and contributes to their negative surface charge. This negative surface charge prevents which of the following?","o":["Haemoglobin release","Oxygen binding","RBC agglutination","RBC deformability","Transferrin binding"],"a":2,"p":"N-ACETYLNEURAMINIC ACID (NANA, sialic acid) = 9-carbon sugar — most abundant terminal sugar on cell surface glycoproteins and gangliosides. On RBC surface: sialic acid contributes NEGATIVE CHARGE → RBCs REPEL EACH OTHER → PREVENTS AGGLUTINATION and ROULEAUX formation. Neuraminidase (sialidase) cleaves sialic acid → removes negative charge → RBC agglutination. Influenza virus haemagglutinin binds sialic acid on respiratory epithelium → viral entry. Neuraminidase = oseltamivir target.\n\n📹 [VIDEO LINK]"},{"id":637,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"Anomers Alpha Beta Glucose","q":"In carbohydrate chemistry, alpha-D-glucose and beta-D-glucose are classified as which of the following?","o":["Anomers","Diastereomers","Enantiomers","Epimers","Isomers"],"a":0,"p":"ANOMERS = two cyclic forms of a sugar that differ ONLY in the configuration at the ANOMERIC CARBON (C1 in aldoses, C2 in ketoses). Alpha-D-glucose (OH axial at C1) and beta-D-glucose (OH equatorial at C1) are ANOMERS. They interconvert via MUTAROTATION (through open chain form) in solution. Enzymes are stereospecific: alpha-glucosidase cleaves alpha linkages (starch). Beta-galactosidase cleaves beta linkages (lactose). Compare: EPIMERS = differ at one non-anomeric carbon (glucose vs galactose = C4 epimers).\n\n📹 [VIDEO LINK]"},{"id":638,"s":"Lipid Chemistry","t":"Lipid Classification","u":"Simple Lipids Esters of Fatty Acids","q":"Which of the following may be defined as esters of fatty acids with alcohol?","o":["Compound lipids","Derived lipids","Phospholipids","Simple lipids","Sphingolipids"],"a":3,"p":"SIMPLE LIPIDS = esters of fatty acids with alcohol. Types: (1) TRIACYLGLYCEROLS (glycerol + 3 fatty acids) — energy storage. (2) WAXES (long-chain alcohol + fatty acid) — structural/protective. COMPOUND LIPIDS = simple lipid + additional group (phospholipids, glycolipids, lipoproteins). DERIVED LIPIDS = products of hydrolysis of simple and compound lipids — fatty acids, fatty aldehydes, sterols, fat-soluble vitamins.\n\n📹 [VIDEO LINK]"},{"id":639,"s":"Lipid Chemistry","t":"Lipid Classification","u":"Tripalmitin Simple Triacylglycerol","q":"Which of the following best describes tripalmitin?","o":["Compound lipid","Mixed triacylglycerol","Phospholipid","Simple lipid","Simple triacylglycerol"],"a":4,"p":"TRIPALMITIN = glycerol + THREE PALMITIC ACID molecules = SIMPLE TRIACYLGLYCEROL (all three positions identical fatty acids). MIXED TRIACYLGLYCEROL = different fatty acids at different positions (most naturally occurring TAGs). COMPOUND LIPID = TAG + additional group (e.g. phospholipid). Tripalmitin is a pure laboratory standard — naturally occurring TAGs are almost always mixed. Simple TAGs raise LDL-C when the fatty acid is palmitic (16:0).\n\n📹 [VIDEO LINK]"},{"id":640,"s":"Lipid Chemistry","t":"Lipid Classification","u":"Plasmalogen Unsaturated Vinyl Ether C1","q":"Which of the following is the glycerophospholipid in which the fatty acid at carbon 1 is replaced by an unsaturated alkyl group?","o":["Cardiolipin","Lecithin","Plasmalogen","Platelet activating factor","Sphingomyelin"],"a":2,"p":"PLASMALOGEN (phosphatidal choline/ethanolamine) = glycerophospholipid where C1 position has an UNSATURATED ALKYL GROUP in VINYL ETHER linkage (enol ether, -O-CH=CH-R) instead of an ester-linked fatty acid. Found abundantly in BRAIN, HEART, and SKELETAL MUSCLE. Plasmalogens are important for membrane structure and cell signalling. Deficient in ZELLWEGER SYNDROME (peroxisomal disorder — plasmalogen synthesis is peroxisome-dependent).\n\n📹 [VIDEO LINK]"},{"id":641,"s":"Lipid Chemistry","t":"Lipid Classification","u":"PAF Saturated Alkyl Ether C1","q":"Which of the following is the glycerophospholipid in which the fatty acid at carbon 1 is replaced by a saturated alkyl group in ether linkage?","o":["Cardiolipin","Lecithin","Plasmalogen","Platelet activating factor","Sphingomyelin"],"a":3,"p":"PLATELET ACTIVATING FACTOR (PAF) = glycerophospholipid where C1 has a SATURATED ALKYL GROUP in ETHER linkage + ACETYL group at C2 (instead of long-chain fatty acid) + phosphocholine head group. Produced by: platelets, basophils, neutrophils, endothelium. Actions: activates platelets (PAF receptor → IP3/Ca²⁺), triggers inflammation, bronchoconstriction (at low concentrations), vasodilation. Potent inflammatory mediator in anaphylaxis and asthma.\n\n📹 [VIDEO LINK]"},{"id":642,"s":"Lipid Chemistry","t":"Lipid Classification","u":"Fatty Aldehydes Derived Lipids","q":"Fatty aldehydes are classified as which of the following?","o":["Compound lipids","Derived lipids","Phospholipids","Simple lipids","Sphingolipids"],"a":1,"p":"DERIVED LIPIDS = products obtained from hydrolysis of simple and compound lipids. Include: fatty acids, fatty aldehydes, sterols (cholesterol), fat-soluble vitamins (A, D, E, K), ketone bodies. FATTY ALDEHYDES are released from PLASMALOGENS upon hydrolysis (the vinyl ether linkage releases a fatty aldehyde) → classified as derived lipids. Fatty aldehydes are not stored as intact lipids but are produced as metabolic intermediates.\n\n📹 [VIDEO LINK]"},{"id":643,"s":"Lipid Chemistry","t":"Fatty Acid Nomenclature","u":"Alpha Carbon Refsum Disease","q":"A child with Refsum disease cannot perform alpha oxidation of branched-chain fatty acids. Oxidation at which of the following carbon atoms is impaired in him, when carbon 1 is the carboxyl carbon and carbon 5 is the methyl carbon?","o":["Carbon 1","Carbon 2","Carbon 3","Carbon 4","Carbon 5"],"a":1,"p":"ALPHA OXIDATION = oxidation at CARBON 2 (alpha carbon). Required for BRANCHED-CHAIN FATTY ACIDS (e.g. phytanic acid) that have a methyl group at C3 blocking beta-oxidation. Alpha oxidation removes C1 as CO₂ → shortens chain by one carbon → allows subsequent beta-oxidation. REFSUM DISEASE: phytanic acid alpha-hydroxylase deficiency → phytanic acid accumulates → peripheral neuropathy, ataxia, retinitis pigmentosa.\n\n[Diagram: Horizontal fatty acid chain. Five carbons labelled left to right: C1 = COOH (carboxyl, red), C2 = ALPHA carbon (green, circled — ALPHA OXIDATION site, impaired in Refsum disease), C3 = BETA carbon (blue, BETA OXIDATION site, impaired in MCAD), C4 = GAMMA carbon (purple), C5 = CH₃ OMEGA/METHYL carbon (orange). Arrow at C2: impaired in Refsum disease. Arrow at C3: impaired in MCAD.]\n\n📹 [VIDEO LINK]"},{"id":644,"s":"Lipid Chemistry","t":"Fatty Acid Nomenclature","u":"MCAD Beta Carbon Impaired","q":"A 6-month-old infant with MCAD deficiency cannot perform beta-oxidation of medium-chain fatty acids. Oxidation at which of the following carbon atoms is impaired in him, when carbon 1 is the carboxyl carbon and carbon 5 is the methyl carbon?","o":["Carbon 1","Carbon 2","Carbon 3","Carbon 4","Carbon 5"],"a":2,"p":"BETA OXIDATION = oxidation at CARBON 3 (BETA CARBON). The beta carbon (C3) undergoes oxidation → beta-hydroxyacyl-CoA formed → dehydration → cleavage of bond between C2 (alpha) and C3 (beta) → acetyl-CoA released + shortened acyl-CoA. MCAD DEFICIENCY: medium-chain acyl-CoA dehydrogenase absent → cannot perform the first dehydrogenation step of beta-oxidation for C6-C12 fatty acids → hypoketotic hypoglycaemia. Most common fatty acid oxidation defect — detected on newborn screening.\n\n📹 [VIDEO LINK]"},{"id":6441,"s":"Lipid Chemistry","t":"Fatty Acid Nomenclature","u":"Starvation Beta Carbon Survival","q":"A 5-year-old malnourished child rescued from a flood is surviving on stored body fat. Beta oxidation of long-chain fatty acids is keeping him alive. Oxidation at which of the following carbon atoms is making him survive, when carbon 1 is the carboxyl carbon and carbon 5 is the methyl carbon?","o":["Carbon 1","Carbon 2","Carbon 3","Carbon 4","Carbon 5"],"a":2,"p":"BETA OXIDATION at CARBON 3 (beta carbon) → acetyl-CoA → TCA cycle → ATP. In starvation: adipose TAG → free fatty acids → beta-oxidation in LIVER → acetyl-CoA → KETONE BODIES (acetoacetate, beta-hydroxybutyrate) → fuel for brain, heart, and muscle. Insulin falls → glucagon rises → hormone-sensitive lipase activated → fatty acid mobilisation → beta-oxidation → ketogenesis. Children enter starvation ketosis faster than adults due to limited glycogen stores.\n\n📹 [VIDEO LINK]"},{"id":645,"s":"Lipid Chemistry","t":"Fatty Acid Nomenclature","u":"Omega Carbon Methyl End","q":"Which of the following best describes the omega carbon in a fatty acid, when carbon 1 is the carboxyl carbon and carbon 5 is the methyl carbon?","o":["Carbon 1","Carbon 2","Carbon 3","Carbon 4","Carbon 5"],"a":4,"p":"OMEGA CARBON = the TERMINAL METHYL CARBON (last carbon, furthest from carboxyl end). OMEGA FATTY ACID CLASSIFICATION counts double bonds from the methyl end: omega-3 = first double bond at C3 from methyl end (e.g. linolenic acid 18:3, EPA 20:5, DHA 22:6). Omega-6 = first double bond at C6 from methyl end (e.g. linoleic acid 18:2, arachidonic acid 20:4). Omega-9 = first double bond at C9 (e.g. oleic acid 18:1). OMEGA OXIDATION = minor microsomal pathway oxidising fatty acid at omega carbon → dicarboxylic acid → beta-oxidised from both ends.\n\n📹 [VIDEO LINK]"},{"id":646,"s":"Lipid Chemistry","t":"Fatty Acid Nomenclature","u":"Human Desaturases Cannot Introduce Beyond C9","q":"The biochemical basis for nutritional essentiality of omega-6 linoleic and omega-3 linolenic acid is the lack of ability of human desaturases to introduce double bonds between which of the following?","o":["C3 and methyl end","C6 and methyl end","C10 and methyl end","C12 and methyl end","C9 and methyl end"],"a":1,"p":"Human desaturases (Δ4, Δ5, Δ6, Δ9) can only introduce double bonds BETWEEN C1 AND C9 from the carboxyl end. They CANNOT introduce double bonds BETWEEN C6 AND THE METHYL END (i.e. beyond C9 toward the methyl/omega end). Linoleic acid (18:2, omega-6) has double bonds at C9 and C12 (between C6 and methyl end) — cannot be made by humans. Linolenic acid (18:3, omega-3) has double bonds at C9, C12, C15 — cannot be made. Both are therefore ESSENTIAL FATTY ACIDS.\n\n📹 [VIDEO LINK]"},{"id":647,"s":"Lipid Chemistry","t":"Essential Fatty Acids","u":"Linoleic Acid Omega-6","q":"Linoleic acid is which of the following?","o":["Omega-3 fatty acid","Omega-6 fatty acid","Omega-9 fatty acid","Omega-12 fatty acid","Omega-3 and omega-6"],"a":1,"p":"LINOLEIC ACID (18:2, omega-6) = first double bond at C6 from methyl end → OMEGA-6. Essential fatty acid — must be obtained from diet (vegetable oils, nuts, seeds). Precursor for ARACHIDONIC ACID (20:4, omega-6) via elongation and Δ5, Δ6 desaturation. Arachidonic acid → eicosanoids (prostaglandins, thromboxanes, leukotrienes). Omega-6 PUFAs → DECREASE LDL-C AND HDL-C.\n\n📹 [VIDEO LINK]"},{"id":648,"s":"Lipid Chemistry","t":"Essential Fatty Acids","u":"Linolenic Acid Omega-3","q":"Linolenic acid is which of the following?","o":["Omega-3 fatty acid","Omega-6 fatty acid","Omega-9 fatty acid","Omega-12 fatty acid","Omega-3 and omega-9"],"a":0,"p":"LINOLENIC ACID (18:3, omega-3) = first double bond at C3 from methyl end → OMEGA-3. Essential fatty acid. Precursor for EPA (20:5, omega-3) and DHA (22:6, omega-3) via elongation and desaturation. EPA and DHA: suppress cardiac arrhythmias, decrease platelet aggregation, reduce triglycerides (↓ VLDL synthesis), anti-inflammatory (compete with arachidonic acid for COX/LOX). Rich sources: flaxseed (linolenic), fatty fish (EPA/DHA).\n\n📹 [VIDEO LINK]"},{"id":649,"s":"Lipid Chemistry","t":"Essential Fatty Acids","u":"Arachidonic Acid Conditionally Essential","q":"Which of the following is a conditionally essential fatty acid?","o":["Arachidonic acid","Butyric acid","Linolenic acid","Oleic acid","Palmitic acid"],"a":0,"p":"ARACHIDONIC ACID (20:4, omega-6) = CONDITIONALLY ESSENTIAL — normally synthesised from linoleic acid (via elongation + Δ6, Δ5 desaturation) but in states of linoleic acid deficiency or metabolic stress, exogenous arachidonic acid is required. Essential fatty acid deficiency → cannot synthesise arachidonic acid → deficient eicosanoids → ACYLGLUCOSYLCERAMIDE synthesis impaired in skin → RED SCALY DERMATITIS. Arachidonic acid is the primary precursor for 2-series prostaglandins and thromboxanes, and 4-series leukotrienes.\n\n📹 [VIDEO LINK]"},{"id":650,"s":"Lipid Chemistry","t":"Essential Fatty Acids","u":"EFA Deficiency Red Scaly Dermatitis","q":"Deficiency of which of the following is the biochemical basis of red scaly dermatitis in essential fatty acid deficiency?","o":["Acylglucosylceramide","Galactosylceramide","Glucosylceramide","Lactosylceramide","Sphingomyelin"],"a":0,"p":"EFA deficiency → insufficient linoleic acid → cannot synthesise ACYLGLUCOSYLCERAMIDE (a complex ceramide ester with a very long-chain fatty acid in the skin lamellar bodies) → impaired skin barrier formation → water loss through skin → RED SCALY DERMATITIS (similar to fish-scale appearance). Acylglucosylceramide is unique to the skin (stratum corneum) → essential for impermeability. This is the specific biochemical basis of the dermatitis seen in EFA deficiency states.\n\n📹 [VIDEO LINK]"},{"id":651,"s":"Lipid Chemistry","t":"Essential Fatty Acids","u":"Linoleic to Arachidonic Conversion TPN","q":"A 45-year-old man has been on total parenteral nutrition without lipid emulsion for 8 weeks following bowel surgery. He develops red scaly dermatitis, poor wound healing, and alopecia. His body cannot synthesise arachidonic acid because which of the following fatty acids is absent from his TPN?","o":["Docosahexaenoic acid","Eicosapentaenoic acid","Linoleic acid","Linolenic acid","Oleic acid"],"a":2,"p":"LINOLEIC ACID (18:2, omega-6) → Δ6 desaturase → gamma-linolenic acid (18:3) → elongase → DGLA (20:3) → Δ5 desaturase → ARACHIDONIC ACID (20:4, omega-6). TPN without lipid emulsion = CLASSIC CLINICAL SCENARIO for EFA deficiency. Without linoleic acid → no arachidonic acid → no 2-series prostaglandins/thromboxanes or 4-series leukotrienes → impaired skin barrier → red scaly dermatitis. Linolenic acid (omega-3) follows a parallel pathway → EPA/DHA. Both pathways share the same Δ6 and Δ5 desaturases and COMPETE for these enzymes.\n\n📹 [VIDEO LINK]"},{"id":652,"s":"Lipid Chemistry","t":"Essential Fatty Acids","u":"EFA Deficiency Skin TPN","q":"A 35-year-old woman with Crohn's disease has been on total parenteral nutrition without lipid supplementation for 10 weeks. She develops a red scaly rash on her trunk. Deficiency of which of the following is the biochemical basis of her skin condition?","o":["Acylglucosylceramide","Galactosylceramide","Glucosylceramide","Lactosylceramide","Sphingomyelin"],"a":0,"p":"EFA deficiency → insufficient linoleic acid → cannot synthesise ACYLGLUCOSYLCERAMIDE (requires very long-chain fatty acids derived from EFA elongation) → defective skin lamellar bodies → impaired skin barrier → water loss → RED SCALY DERMATITIS. Acylglucosylceramide is unique to stratum corneum. TPN without lipid emulsion is the classic clinical setting for EFA deficiency in adults.\n\n📹 [VIDEO LINK]"},{"id":653,"s":"Lipid Chemistry","t":"Essential Fatty Acids","u":"Lorenzo Oil Erucic Oleic ALD","q":"Lorenzo Odone had adrenoleukodystrophy. His parents discovered that adding two specific fatty acids to olive oil slowed his neurological deterioration. Which of the following fatty acids did they add?","o":["Arachidonic 20:4 and EPA 20:5","DHA 22:6 and EPA 20:5","Erucic acid 22:1 and oleic acid 18:1","Linoleic acid 18:2 and linolenic acid 18:3","Palmitic acid 16:0 and stearic acid 18:0"],"a":2,"p":"ERUCIC ACID (22:1, omega-9) + OLEIC ACID (18:1, omega-9) in 4:1 ratio. Both are monounsaturated omega-9 fatty acids. They compete with VLCFA for the elongase enzyme → reduce VLCFA synthesis → slow neurodegeneration. Do not reverse existing damage. ALD = X-linked, ABCD1 mutation → defective VLCFA peroxisomal transporter.\n\n📹 [VIDEO LINK]"},{"id":654,"s":"Lipid Chemistry","t":"Essential Fatty Acids","u":"VLCFA Accumulate ALD","q":"A 7-year-old boy with progressive behavioural changes, deteriorating school performance, and adrenal insufficiency has markedly elevated plasma C26:0. Accumulation of which of the following is causing him problems?","o":["Long-chain fatty acids","Medium-chain fatty acids","Monounsaturated fatty acids","Short-chain fatty acids","Very long-chain fatty acids"],"a":4,"p":"ADRENOLEUKODYSTROPHY: X-linked, ABCD1 mutation → defective VLCFA transporter on peroxisomal membrane → VERY LONG CHAIN FATTY ACIDS (≥C22) accumulate in him. C26:0 = most elevated diagnostic marker. VLCFA incorporated into myelin → demyelination → neurological deterioration. VLCFA in adrenal cortex → adrenal insufficiency. Treatment: Lorenzo's oil, HSCT (early stages).\n\n📹 [VIDEO LINK]"},{"id":655,"s":"Lipid Chemistry","t":"Essential Fatty Acids","u":"ALD VLCFA Cannot Enter Peroxisomes","q":"A 6-year-old boy with adrenoleukodystrophy cannot oxidise very long-chain fatty acids. Which of the following organelles in him are VLCFA unable to enter?","o":["Endoplasmic reticulum","Lysosomes","Microsomes","Mitochondria","Peroxisomes"],"a":4,"p":"VLCFA are oxidised exclusively in PEROXISOMES. ABCD1 = peroxisomal membrane transporter → mutated in ALD → VLCFA cannot enter peroxisomes in him → accumulate. Mitochondria handle C8-C18 fatty acids. Peroxisomal beta-oxidation also handles branched-chain FA. MCAD deficiency = mitochondrial beta-oxidation defect.\n\n📹 [VIDEO LINK]"},{"id":656,"s":"Lipid Chemistry","t":"Essential Fatty Acids","u":"ALD Myelin Sheath Destruction","q":"A 9-year-old boy with adrenoleukodystrophy has MRI showing widespread cerebral demyelination. Which of the following structures in him are accumulated VLCFA destroying?","o":["Adrenal cortex","Ganglion cells","Leukocytes","Lysosomal membranes","Myelin sheath"],"a":4,"p":"Accumulated VLCFA in him incorporate into myelin phospholipids → destabilise myelin structure → MYELIN SHEATH DESTRUCTION → progressive neurological deterioration. MRI shows posterior cerebral white matter demyelination. Adrenal cortex VLCFA accumulation also occurs → adrenal insufficiency. Cerebral ALD = severe inflammatory demyelination in childhood.\n\n📹 [VIDEO LINK]"},{"id":657,"s":"Lipid Chemistry","t":"Nutritional Significance of Lipids","u":"Saturated Fats LDL Total Cholesterol","q":"A 45-year-old man consumes beef nihari and mutton karahi cooked in desi ghee daily. He is at risk for elevation of which of the following?","o":["HDL and total cholesterol","HDL only","LDL and total cholesterol","VLDL and total cholesterol","VLDL only"],"a":2,"p":"SATURATED FATS (desi ghee, animal fat) → INCREASE LDL-C AND TOTAL CHOLESTEROL by downregulating hepatic LDL receptors. Myristic (14:0) and palmitic (16:0) most potent at raising LDL-C. EXCEPTION: stearic acid (18:0) → converted to oleic acid in liver → does NOT raise LDL-C. AMDR for saturated fat = <10% total calories.\n\n📹 [VIDEO LINK]"},{"id":658,"s":"Lipid Chemistry","t":"Nutritional Significance of Lipids","u":"Saturated Fats Myristic Palmitic 14C 16C","q":"A 30-year-old man enjoys roast beef cooked in beef tallow at a family dinner. In the absorptive state following his meal, which of the following fatty acids are most abundant in his chylomicrons?","o":["2C and 3C","4C and 10C","14C and 16C","18C and 24C","20C and 22C"],"a":2,"p":"BEEF TALLOW is rich in PALMITIC ACID (16C) and MYRISTIC ACID (14C). Post-meal: dietary fats → chylomicrons assembled in enterocytes → carried via lymphatics → bloodstream → ADIPOSE TISSUE in absorptive state. Insulin → activates lipoprotein lipase → releases FA from chylomicrons → FA enter adipocytes → re-esterified to TAG for storage. Both palmitic and myristic acid raise LDL-C.\n\n📹 [VIDEO LINK]"},{"id":659,"s":"Lipid Chemistry","t":"Nutritional Significance of Lipids","u":"MUFA Mediterranean Diet Lipid Profile","q":"A 55-year-old man follows a strict Mediterranean diet with olive oil as his salad dressing and cooking oil. His annual lipid panel consistently shows low LDL-C and elevated HDL-C. Which property of his diet is responsible for this favourable lipid profile?","o":["High monounsaturated fat","High omega-3 fat","High polyunsaturated fat","High saturated fat","High trans fat"],"a":0,"p":"OLIVE OIL is rich in OLEIC ACID (18:1, omega-9 = MONOUNSATURATED). HIGH MONOUNSATURATED FAT → DECREASE LDL-C + INCREASE HDL-C. Additional benefit: prevents free radical propagation, anti-inflammatory. Mediterranean diet → reduced all-cause mortality and cardiovascular events (PREDIMED trial). Compare: high omega-6 (corn oil) → decreases BOTH LDL-C AND HDL-C.\n\n📹 [VIDEO LINK]"},{"id":660,"s":"Lipid Chemistry","t":"Nutritional Significance of Lipids","u":"Omega-6 Decreases LDL and HDL","q":"Which of the following decreases LDL-C but also decreases HDL-C?","o":["Omega-3 fatty acids","Omega-6 fatty acids","Omega-9 fatty acids","Saturated fatty acids","Trans fatty acids"],"a":1,"p":"OMEGA-6 PUFA (linoleic acid, arachidonic acid) → DECREASE LDL-C (beneficial) BUT ALSO DECREASE HDL-C (potentially unfavourable). Compare: OMEGA-9 MUFA (oleic acid) → decreases LDL-C but INCREASES HDL-C (most favourable lipid profile). OMEGA-3 (EPA/DHA) → primarily reduces TRIGLYCERIDES + suppresses cardiac arrhythmias + anti-thrombotic (anti-platelet via competing with arachidonic acid for COX). TRANS FATS → INCREASE LDL-C and DECREASE HDL-C (most unfavourable).\n\n📹 [VIDEO LINK]"},{"id":661,"s":"Lipid Chemistry","t":"Nutritional Significance of Lipids","u":"Omega-3 Cardiac Arrhythmia Thrombosis","q":"A 50-year-old fisherman from coastal Japan consumes fatty fish daily. His triglycerides are consistently low and he has never had a cardiac arrhythmia. Which of the following best explains the benefits of his diet?","o":["Decreased HDL-C","Increased LDL-C","Increased triglycerides","Reduced arrhythmias and triglycerides","Raised total cholesterol"],"a":3,"p":"FISH OIL rich in OMEGA-3 FA (EPA 20:5, DHA 22:6). Primary benefits: (1) REDUCES TRIGLYCERIDES (↓ VLDL synthesis). (2) SUPPRESSES CARDIAC ARRHYTHMIAS (stabilise cardiomyocyte membranes). (3) DECREASES PLATELET AGGREGATION (compete with arachidonic acid for COX → less potent TXA2). Anti-inflammatory. Japanese coastal populations consuming high fish diets have low rates of coronary artery disease and sudden cardiac death.\n\n📹 [VIDEO LINK]"},{"id":662,"s":"Lipid Chemistry","t":"Nutritional Significance of Lipids","u":"Trans Fats Increase LDL Decrease HDL","q":"A medical student consumes trans fat-rich fast food regularly. His biochemistry lecturer says the fat you eat is the fat you wear. These trans fats incorporate into his cell membranes. Which of the following best describes the behaviour of trans fats in his cell membranes?","o":["Decrease fluidity like saturated fats","Have no effect on fluidity","Increase fluidity like cis fats","Increase HDL-C","Prevent lipid peroxidation"],"a":0,"p":"TRANS FATS → raise LDL-C AND lower HDL-C (MOST UNFAVOURABLE lipid effect — worse than saturated fats which only raise LDL). Sources: partially hydrogenated vegetable oils, margarine, fast food, commercially baked goods. Effects: increase LDL-C, decrease HDL-C, promote inflammation, impair membrane fluidity, stop elongation of omega-3 FA (deprive brain of DHA → cognitive impairment, memory problems). WHO target: eliminate industrial trans fats globally by 2023.\n\n📹 [VIDEO LINK]"},{"id":663,"s":"Lipid Chemistry","t":"Nutritional Significance of Lipids","u":"Trans Fats Memory Cognition Omega-3 Elongation","q":"A second-year medical student who relies on fast food encounters difficulty memorising biochemical pathways. Which of the following lipids in his diet are likely contributing to impairment of his memorisation skills by stopping the elongation of omega-3 fatty acids needed by the brain?","o":["Monounsaturated fats","Neutral fats","Saturated fats","Trans fats","Unsaturated fats"],"a":3,"p":"TRANS FATS → inhibit ELONGASE and DESATURASE enzymes → block conversion of dietary alpha-linolenic acid (18:3, omega-3) → DHA (22:6, omega-3). DHA is essential for brain structure (40% of brain PUFA) and neuronal membrane function. Without DHA → impaired membrane fluidity in neurons → impaired synaptic signalling → COGNITIVE IMPAIRMENT and memory problems. Trans fats also promote neuroinflammation and act as cellular stressors affecting memory function.\n\n📹 [VIDEO LINK]"},{"id":664,"s":"Lipid Chemistry","t":"Nutritional Significance of Lipids","u":"MUFA Prevents Free Radical Propagation LDL HDL","q":"A patient with elevated LDL-C and family history of coronary artery disease asks which cooking oil would reduce his cardiovascular risk, lower LDL-C, and increase HDL-C. Cooking oil rich in which of the following would you advise?","o":["Long-chain fatty acids","Monounsaturated fatty acids","Omega-3 fatty acids","Omega-6 fatty acids","Very long-chain fatty acids"],"a":1,"p":"MONOUNSATURATED FATTY ACIDS (MUFA, especially oleic acid 18:1) have a unique triple benefit: (1) PREVENT FREE RADICAL PROPAGATION — one double bond means less susceptibility to lipid peroxidation than PUFA (which have multiple double bonds and generate more peroxidation chain reactions). (2) LOWER LDL-C — upregulate LDL receptors. (3) INCREASE HDL-C — maintain HDL levels better than PUFA. This makes MUFA-rich diets (Mediterranean diet) cardioprotective.\n\n📹 [VIDEO LINK]"},{"id":665,"s":"Lipid Chemistry","t":"Nutritional Significance of Lipids","u":"EPA DHA Lower LDL Cardiovascular Risk","q":"A patient seeks advice on which cooking oil to reduce cardiovascular risk and lower LDL-C. Oil rich in which of the following fatty acids would you recommend?","o":["Butyric and capric acid","Eicosapentaenoic and docosahexaenoic acid","Myristic and palmitic acid","Palmitoleic and stearic acid","Stearic and lignoceric acid"],"a":1,"p":"EPA (20:5, omega-3) and DHA (22:6, omega-3) → lower LDL-C, reduce triglycerides, suppress arrhythmias, anti-thrombotic. MYRISTIC and PALMITIC = raise LDL-C. BUTYRIC and CAPRIC = short/medium-chain saturated, minimal LDL effect. Prescription omega-3 (icosapentaenoic acid) reduces cardiovascular events in statin-treated patients.\n\n📹 [VIDEO LINK]"},{"id":666,"s":"Lipid Chemistry","t":"Nutritional Significance of Lipids","u":"Stearic Acid in Chocolate","q":"A student has consumed a large bar of dark chocolate. Which of the following fatty acids from cocoa butter will be most abundant in his chylomicrons?","o":["Butyric acid","Lauric acid","Myristic acid","Palmitic acid","Stearic acid"],"a":4,"p":"Cocoa butter (in chocolate) is uniquely rich in STEARIC ACID (18:0) — approximately 33% of its fat content. Stearic acid is UNIQUE among saturated fatty acids: it does NOT raise LDL-C because it is rapidly converted to OLEIC ACID (18:1) by hepatic Δ9 desaturase → behaves more like a MUFA in the body. This is why dark chocolate consumption has a neutral or even favourable effect on cardiovascular risk, unlike other saturated fat sources. Other fats in cocoa butter: oleic acid (33%), palmitic acid (25%).\n\n📹 [VIDEO LINK]"},{"id":667,"s":"Lipid Chemistry","t":"Nutritional Significance of Lipids","u":"Linoleic Acid Eicosanoid Precursor","q":"Which of the following is a dietary precursor for eicosanoid synthesis?","o":["Butyric acid","Lignoceric acid","Linoleic acid","Myristic acid","Palmitic acid"],"a":2,"p":"LINOLEIC ACID (18:2, omega-6) → elongation and desaturation → ARACHIDONIC ACID (20:4) → COX and LOX → 2-series prostaglandins, thromboxanes, 4-series leukotrienes → EICOSANOIDS. Linoleic is the dietary INDIRECT eicosanoid precursor (must first become arachidonic acid). Direct eicosanoid precursors: arachidonic acid (20:4), EPA (20:5), DGLA (20:3).\n\n📹 [VIDEO LINK]"},{"id":668,"s":"Lipid Chemistry","t":"Nutritional Significance of Lipids","u":"EPA Direct Eicosanoid Precursor","q":"Which of the following is a direct eicosanoid precursor producing 3-series prostaglandins?","o":["Butyric acid","Eicosapentaenoic acid","Lignoceric acid","Myristic acid","Palmitic acid"],"a":1,"p":"EICOSAPENTAENOIC ACID (EPA, 20:5, omega-3) is a DIRECT eicosanoid precursor → acts as substrate for COX → produces 3-SERIES PROSTAGLANDINS (PGE3, PGI3) and 5-SERIES LEUKOTRIENES — these are LESS POTENT platelet activators and less potent inflammatory mediators than the 2-series from arachidonic acid → EPA has anti-inflammatory and anti-thrombotic effects. Compare: arachidonic acid → 2-series PGs (potent platelet aggregation, inflammation). EPA competes with arachidonic acid for COX and LOX enzymes.\n\n📹 [VIDEO LINK]"},{"id":669,"s":"Lipid Chemistry","t":"Nutritional Significance of Lipids","u":"AMDR Saturated Fat 10 Percent","q":"The acceptable macronutrient distribution range (AMDR) for saturated fats is which of the following?","o":["Less than 1% of total calories","Less than 5% of total calories","Less than 10% of total calories","Less than 20% of total calories","Less than 30% of total calories"],"a":2,"p":"AMDR for SATURATED FATS = LESS THAN 10% of total calories (WHO/AHA recommendation). This is because saturated fats raise LDL-C → increased atherosclerosis risk. AMDR for TOTAL DIETARY FAT = 20-35% of total calories (most guidelines). AMDR for TRANS FATS = as low as possible (ideally <1%). Replacing saturated fats with unsaturated fats (MUFA or PUFA) reduces cardiovascular risk. Countries with the highest intake of saturated fat (mainly from dairy and meat) have higher rates of coronary heart disease.\n\n📹 [VIDEO LINK]"},{"id":670,"s":"Lipid Chemistry","t":"Cholesterol","u":"Cholesterol Ring A Oriented Outward Membrane","q":"When cholesterol is present in a cell membrane, which of its rings is oriented outward along with hydrophilic phosphate groups?","o":["Ring A","Ring B","Ring C","Ring D","None — it is fully buried"],"a":0,"p":"Cholesterol in cell membrane: RING A (with the 3-OH hydroxyl group — the only hydrophilic part of cholesterol) is oriented OUTWARD toward the aqueous environment, alongside the hydrophilic phosphate head groups of phospholipids. The rest of the cholesterol molecule (rings B, C, D and the hydrophobic side chain) is buried in the hydrophobic core of the bilayer. This amphipathic orientation allows cholesterol to integrate into and modulate membrane properties.\n\n[Diagram: Cell membrane bilayer cross-section. Phospholipids shown with polar head groups outward and fatty acid tails inward. Cholesterol molecule inserted between phospholipid tails — Ring A with 3-OH pointing upward toward phosphate head groups (labelled: hydrophilic, oriented outward). Rings B, C, D and isooctyl tail pointing downward (labelled: hydrophobic, buried in core).]\n\n📹 [VIDEO LINK]"},{"id":671,"s":"Lipid Chemistry","t":"Cholesterol","u":"Cholesterol Increases Fluidity Between Saturated FA","q":"In a cell membrane, cholesterol increases its fluidity by inserting in the space between which of the following?","o":["Branched-chain fatty acids","Cis fatty acids","Long-chain fatty acids","Monounsaturated fatty acids","Saturated fatty acids"],"a":4,"p":"CHOLESTEROL inserts between SATURATED FATTY ACIDS → prevents their tight, regular packing → disrupts the gel phase at low temperatures → INCREASES FLUIDITY. At high temperatures: cholesterol stabilises the membrane by restraining excessive motion → DECREASES FLUIDITY (cholesterol is a 'fluidity buffer'). Saturated FA pack tightly without cholesterol (rigid gel phase below Tm). Cis unsaturated FA already have kinks → cholesterol fits between them and REDUCES excessive fluidity.\n\n📹 [VIDEO LINK]"},{"id":672,"s":"Lipid Chemistry","t":"Cholesterol","u":"Cholesterol Decreases Fluidity Between Cis FA","q":"In a cell membrane, cholesterol decreases its fluidity by inserting in the space between which of the following?","o":["Branched-chain fatty acids","Cis unsaturated fatty acids","Long-chain fatty acids","Saturated fatty acids","Trans fatty acids"],"a":1,"p":"CHOLESTEROL inserts between CIS UNSATURATED FATTY ACIDS → occupies the kinked spaces → restrains excessive phospholipid motion → DECREASES FLUIDITY (stabilises the membrane against excessive disorder). Cis unsaturated FA have bent tails → create a loosely packed, fluid bilayer → cholesterol fills in these gaps → increases packing → decreases fluidity. This is why the plasma membrane (rich in both cholesterol and unsaturated FA) has optimal fluidity — not too rigid, not too fluid.\n\n📹 [VIDEO LINK]"},{"id":673,"s":"Lipid Chemistry","t":"Cholesterol","u":"Cholesterol Precursor Vitamin D","q":"Cholesterol is a precursor for which of the following?","o":["Vitamin A","Vitamin B12","Vitamin D","Vitamin E","Vitamin K"],"a":2,"p":"CHOLESTEROL → (skin, UV-B light) → 7-dehydrocholesterol → VITAMIN D3 (cholecalciferol) → 25-hydroxylase (liver) → calcidiol → 1-alpha-hydroxylase (kidney) → CALCITRIOL (active vitamin D). Cholesterol is the precursor for ALL STEROID HORMONES and BILE ACIDS as well. The steroid nucleus (four fused rings) of cholesterol is retained in all steroid hormones. Cholesterol is NOT a precursor for vitamins A, E, or K (these are derived from isoprene units via a separate mevalonate pathway branch or dietary carotenoids).\n\n📹 [VIDEO LINK]"},{"id":674,"s":"Lipid Chemistry","t":"Cholesterol","u":"Cholesterol Precursor Bile Acids","q":"Cholesterol is a precursor for which of the following?","o":["Amino acids","Bile acids","Saturated fatty acids","Triglycerides","Unsaturated fatty acids"],"a":1,"p":"CHOLESTEROL → (liver, CYP7A1 = cholesterol 7-alpha-hydroxylase, rate-limiting step) → PRIMARY BILE ACIDS (cholic acid, chenodeoxycholic acid) → conjugated with glycine or taurine → BILE SALTS → secreted into bile → emulsify dietary fat in small intestine → fat digestion. Secondary bile acids (deoxycholic, lithocholic) = formed by intestinal bacteria. 95% of bile acids are reabsorbed (enterohepatic circulation). STATINS and BILE ACID SEQUESTRANTS (cholestyramine) both used to lower plasma cholesterol.\n\n📹 [VIDEO LINK]"},{"id":675,"s":"Lipid Chemistry","t":"Cholesterol","u":"Cholesterol Precursor Steroid Hormones","q":"Cholesterol acts as a precursor for which of the following?","o":["Growth hormone","Peptide hormone","Steroid hormone","Thyroid hormone","All hormones"],"a":2,"p":"CHOLESTEROL → (mitochondria, StAR protein transports cholesterol in) → side-chain cleavage enzyme (P450scc) → PREGNENOLONE → various STEROID HORMONES: mineralocorticoids (aldosterone — zona glomerulosa), glucocorticoids (cortisol — zona fasciculata), androgens (testosterone — zona reticularis, testes), oestrogens (oestradiol — aromatisation in ovary, adipose), progestogens (progesterone — corpus luteum). THYROID HORMONES = derived from tyrosine (not cholesterol). PEPTIDE HORMONES = derived from amino acids.\n\n📹 [VIDEO LINK]"},{"id":676,"s":"Lipid Chemistry","t":"Cholesterol","u":"Cholesterol Ester Esterified at C3 Ring A","q":"In cholesteryl esters, the fatty acid is esterified to which carbon atom of cholesterol?","o":["Carbon 3","Carbon 9","Carbon 15","Carbon 21","Carbon 27"],"a":0,"p":"CHOLESTERYL ESTERS = cholesterol + fatty acid esterified to the HYDROXYL GROUP AT CARBON 3 (C3) on RING A. The 3-OH group is the only free hydroxyl on cholesterol. Esterification at C3 → makes cholesterol fully hydrophobic → transported in LIPOPROTEIN CORE (as cholesteryl esters). ACAT (acyl-CoA:cholesterol acyltransferase) = intracellular esterification. LCAT (lecithin-cholesterol acyltransferase) = plasma esterification (in HDL). LDL particle = ~70% of plasma cholesteryl esters.\n\n📹 [VIDEO LINK]"},{"id":677,"s":"Lipid Chemistry","t":"Phospholipids","u":"Phosphatidylcholine Glycerophospholipid","q":"Phosphatidyl choline is classified as which of the following?","o":["Glycerophospholipid","Glycolipid","Sphingophospholipid","Simple lipid","Wax"],"a":0,"p":"PHOSPHATIDYLCHOLINE (lecithin) = GLYCEROPHOSPHOLIPID: glycerol backbone + 2 fatty acids (positions 1 and 2) + phosphocholine head group (position 3). Most abundant phospholipid in most membranes. Role in SURFACTANT: dipalmitoylphosphatidylcholine (DPPC) = major surface tension-reducing component of pulmonary surfactant. LECITHIN:SPHINGOMYELIN RATIO in amniotic fluid ≥2 = fetal lung maturity. Compare: SPHINGOMYELIN = sphingophospholipid (sphingosine backbone, not glycerol).\n\n📹 [VIDEO LINK]"},{"id":678,"s":"Lipid Chemistry","t":"Phospholipids","u":"Cardiolipin Two Phosphatidic Acids Glycerol","q":"Which of the following contains two molecules of phosphatidic acid esterified through their phosphate groups to an additional molecule of glycerol?","o":["Cardiolipin","Plasmalogen","Platelet activating factor","Phosphatidylserine","Sphingomyelin"],"a":0,"p":"CARDIOLIPIN (diphosphatidylglycerol) = TWO PHOSPHATIDIC ACID molecules + ONE GLYCEROL. Unique structure: two phosphate groups + three glycerol molecules = 4 fatty acid chains. Found EXCLUSIVELY in the INNER MITOCHONDRIAL MEMBRANE. Essential for: ELECTRON TRANSPORT CHAIN function (stabilises Complex I, III, IV), mitochondrial membrane integrity, apoptosis (released from mitochondria → activates caspases). CARDIOLIPIN is the antigen in WASSERMANN TEST (syphilis serology) and is the target of anticardiolipin antibodies in ANTIPHOSPHOLIPID SYNDROME (SLE).\n\n📹 [VIDEO LINK]"},{"id":679,"s":"Lipid Chemistry","t":"Phospholipids","u":"Phosphatidal Choline Plasmalogen","q":"Phosphatidal choline is classified as which of the following?","o":["Cardiolipin","Lysolecithin","Phosphatidylcholine","Plasmalogen","Sphingomyelin"],"a":3,"p":"PHOSPHATIDAL CHOLINE = PLASMALOGEN (plasmenyl choline): a glycerophospholipid where the C1 fatty acid is replaced by an UNSATURATED LONG-CHAIN ALCOHOL in VINYL ETHER (enol ether) linkage. 'Phosphatidal' = plasmalogen form (vinyl ether at C1). 'Phosphatidyl' = diacyl form (ester linkage at C1). Plasmalogens are abundant in brain, heart, and skeletal muscle. Deficient in Zellweger syndrome (peroxisomal synthesis disorder → impaired plasmalogen production → hypomyelination).\n\n📹 [VIDEO LINK]"},{"id":680,"s":"Lipid Chemistry","t":"Phospholipids","u":"Phosphatidylinositol Signal Transduction","q":"Which of the following phospholipids is involved in signal transduction?","o":["Cardiolipin","Phosphatidyl choline","Phosphatidyl ethanolamine","Phosphatidyl inositol","Phosphatidyl serine"],"a":3,"p":"PHOSPHATIDYL INOSITOL → phosphorylated to PIP2 (phosphatidylinositol 4,5-bisphosphate) → phospholipase C (activated by Gαq) cleaves PIP2 → DAG (diacylglycerol) + IP3 (inositol trisphosphate). IP3 → releases Ca²⁺ from ER. DAG → activates protein kinase C (PKC). This is the phosphoinositide signalling cascade. KEY: PIP2 is also a precursor for PIP3 (via PI3K) → AKT/PKB signalling (insulin signalling, cell survival). Phosphatidylserine = inner leaflet marker + apoptosis signal.\n\n📹 [VIDEO LINK]"},{"id":681,"s":"Lipid Chemistry","t":"Phospholipids","u":"SLE Cardiolipin Antiphospholipid Antibody","q":"A 13-year-old girl presents with butterfly rash and severe weakness, diagnosed with SLE. Autoantibodies to a lipid in the membrane of platelets are seen in this disease. Which of the following membrane lipids is most likely to be involved?","o":["Cardiolipin","Ceramide","Dipalmitoylphosphatidylcholine","Platelet activating factor","Sphingomyelin"],"a":0,"p":"SLE and ANTIPHOSPHOLIPID SYNDROME: ANTICARDIOLIPIN ANTIBODIES are the most common antiphospholipid antibodies in SLE → bind CARDIOLIPIN on platelet membranes → platelet activation + platelet destruction → thrombocytopaenia + paradoxical thrombosis. Cardiolipin is a diphosphatidylglycerol found in inner mitochondrial membrane but also exposed on activated platelets. Anticardiolipin antibodies → venous and arterial thrombosis, recurrent miscarriage (antiphospholipid syndrome). DPPC is the surfactant phospholipid — not an autoantibody target in SLE.\n\n📹 [VIDEO LINK]"},{"id":682,"s":"Lipid Chemistry","t":"Phospholipids","u":"Snake Bite Phospholipase A2 Lysolecithin Haemolysis","q":"Haemolysis caused by poisonous snake bite may be due to formation of which of the following?","o":["Cardiolipin","Cephalin","Lysolecithin","Phosphatidic acid","Sphingomyelin"],"a":2,"p":"SNAKE VENOM contains PHOSPHOLIPASE A2 → cleaves the FATTY ACID at C2 of PHOSPHATIDYLCHOLINE (lecithin) → LYSOLECITHIN (lyso-phosphatidylcholine) + free fatty acid. LYSOLECITHIN is a DETERGENT-LIKE molecule → inserts into RBC membrane → disrupts membrane integrity → HAEMOLYSIS. Also: phospholipase A2 cleaves phospholipids in tissue cell membranes → releases arachidonic acid → eicosanoid synthesis → inflammation. Phospholipase A2 also releases the substrate for platelet activating factor synthesis.\n\n📹 [VIDEO LINK]"},{"id":683,"s":"Lipid Chemistry","t":"Sphingolipids","u":"Galactocerebroside in Myelin","q":"Which of the following is present in myelin?","o":["Fructocerebroside","Galactocerebroside","Glucocerebroside","Lactosylceramide","Sphingomyelin only"],"a":1,"p":"GALACTOCEREBROSIDE (galactosylceramide) = the HALLMARK LIPID of MYELIN. Ceramide + galactose (in beta linkage). Galactose distinguishes it from glucocerebroside (Gaucher disease substrate) which has glucose. Myelin composition: galactocerebroside (22%), sphingomyelin (16%), cholesterol (27%), phospholipids (4%), proteolipid protein (30%). Galactocerebroside deficiency → KRABBE DISEASE (galactosylceramidase deficiency → globoid cell leukodystrophy). SULPHATIDE = sulfated galactocerebroside → also in myelin.\n\n📹 [VIDEO LINK]"},{"id":684,"s":"Lipid Chemistry","t":"Sphingolipids","u":"Sphingolipid Insulates Nerve Cells","q":"Which of the following serves to insulate nerve cells?","o":["Glycerolipid","Glycolipid","Phospholipid","Sphingolipid","Wax"],"a":3,"p":"SPHINGOLIPIDS (specifically GALACTOCEREBROSIDE and SPHINGOMYELIN in myelin) INSULATE NERVE CELLS → prevents ion leakage across axon membrane → allows SALTATORY CONDUCTION (action potential jumps between Nodes of Ranvier) → faster nerve conduction with less energy expenditure. Loss of myelin sphingolipids → demyelinating diseases (multiple sclerosis, Krabbe disease, metachromatic leukodystrophy). Cholesterol also essential for myelin structure (rigidity and stability).\n\n📹 [VIDEO LINK]"},{"id":685,"s":"Lipid Chemistry","t":"Sphingolipids","u":"Cholesterol Required for Myelin Formation","q":"Myelin may fail to form in the absence of which of the following?","o":["Glycolipid","Glycoprotein","Cholesterol","Lipoprotein","Phospholipid"],"a":2,"p":"CHOLESTEROL is ESSENTIAL for MYELIN formation and maintenance. Cholesterol comprises ~27% of myelin lipid content. Functions in myelin: (1) Provides RIGIDITY and STABILITY to the tightly packed myelin membrane. (2) Essential for membrane curvature during myelin wrapping. (3) Condensed with sphingolipids in lipid RAFTS → proper signalling. Genetic block in cholesterol synthesis (e.g. Smith-Lemli-Opitz syndrome — deficient DHCR7) → impaired myelin formation → neurological dysfunction. Oligodendrocytes are the most cholesterol-rich cells in the CNS.\n\n📹 [VIDEO LINK]"},{"id":686,"s":"Lipid Chemistry","t":"Sphingolipids","u":"Grey Matter Brain Sphingomyelin Stearic Acid","q":"Grey matter of the brain comprises mainly of sphingomyelin attached to which of the following?","o":["Lignoceric acid","Linoleic acid","Oleic acid","Palmitic acid","Stearic acid"],"a":4,"p":"GREY MATTER of brain = SPHINGOMYELIN with STEARIC ACID (18:0) as the fatty acid component — the shorter-chain saturated fatty acid. WHITE MATTER = sphingomyelin with LIGNOCERIC ACID (24:0) — longer-chain saturated fatty acid giving greater rigidity needed for the myelinated axon tracts. Sphingomyelin hydrolysis: sphingosine + fatty acid + phosphoric acid + choline (→ ceramide + phosphocholine). Niemann-Pick disease: sphingomyelinase deficiency → sphingomyelin accumulates in grey matter.\n\n📹 [VIDEO LINK]"},{"id":687,"s":"Lipid Chemistry","t":"Sphingolipids","u":"Ceramide Skin Barrier Water Impermeability","q":"Humans are able to swim in water without diffusion of water into the skin. Which of the following is responsible for this?","o":["Cardiolipin","Ceramide","Cerebroside","Ganglioside","Globoside"],"a":1,"p":"CERAMIDE in the STRATUM CORNEUM (outermost layer of epidermis) forms a WATER-IMPERMEABLE BARRIER → prevents water penetration into or out of skin. Ceramide + cholesterol + free fatty acids → arranged in LAMELLAR BODIES → secreted into intercellular space → form multilayered lipid lamellae between keratinocytes → watertight seal. Ceramide deficiency (e.g. essential fatty acid deficiency) → impaired lamellar body formation → permeable skin → water loss → red scaly dermatitis. Atopic eczema is associated with ceramide deficiency in skin.\n\n📹 [VIDEO LINK]"},{"id":688,"s":"Lipid Chemistry","t":"Sphingolipids","u":"Sphingomyelin Hydrolysis Products","q":"Sphingomyelins on hydrolysis yield which of the following?","o":["Glycerol, fatty acid, phosphoric acid, choline","Glycerol, sphingosine, fatty acid, choline","Sphingosine, fatty acid, phosphoric acid, choline","Sphingosine, phosphoric acid, glycerol, inositol","Sphingosine, phosphoric acid, glycerol, serine"],"a":2,"p":"SPHINGOMYELIN = ceramide + phosphocholine. HYDROLYSIS by sphingomyelinase → SPHINGOSINE + FATTY ACID + PHOSPHORIC ACID + CHOLINE. Key: sphingomyelin has NO glycerol (unlike glycerophospholipids). The backbone is SPHINGOSINE (amino alcohol). Compare: lecithin (phosphatidylcholine) hydrolysis → glycerol + 2 fatty acids + phosphoric acid + choline. The presence of SPHINGOSINE (not glycerol) distinguishes sphingomyelin hydrolysis products from glycerophospholipid hydrolysis.\n\n📹 [VIDEO LINK]"},{"id":689,"s":"Lipid Chemistry","t":"Sphingolipids","u":"MS Sphingomyelin Under Immune Attack","q":"A 25-year-old woman presents with muscle weakness, diagnosed with multiple sclerosis. Which of the following is under attack by the immune system?","o":["Glycosphingolipid","Sphinganine","Sphingomyelin","Sphingosine","Ceramide"],"a":2,"p":"MULTIPLE SCLEROSIS (MS) = autoimmune demyelination → immune cells (T cells, macrophages) attack the MYELIN SHEATH. The primary lipid target in myelin is SPHINGOMYELIN (the major phospholipid of myelin, together with galactocerebroside). The protein targets include MYELIN BASIC PROTEIN (MBP) and PROTEOLIPID PROTEIN (PLP). Progressive demyelination → impaired saltatory conduction → motor and sensory deficits. High-dose biotin (vitamin B7) used to promote remyelination in MS by activating acetyl-CoA carboxylase (fatty acid/myelin synthesis).\n\n📹 [VIDEO LINK]"},{"id":690,"s":"Lipid Chemistry","t":"Glycolipids and Gangliosides","u":"Glycolipid Synthesis Palmitoyl CoA Serine Sphinganine","q":"Palmitoyl CoA and serine combine in the presence of pyridoxal phosphate, and the product is reduced in an NADPH-requiring reaction to form which of the following?","o":["Ceramide","Sphinganine","Sulphatide","Sphingomyelin","Sphingosine"],"a":1,"p":"Sphingolipid synthesis: Palmitoyl CoA + serine → (serine palmitoyltransferase, requires PLP) → 3-KETOSPHINGOSINE → (NADPH-dependent reduction) → SPHINGANINE (dihydrosphingosine). Then: sphinganine + fatty acyl-CoA → (ceramide synthase, N-acylation) → DIHYDROCERAMIDE → (desaturase, adds C4-C5 double bond) → CERAMIDE. Then: ceramide is the backbone for all complex sphingolipids. PLP (pyridoxal phosphate = active B6) is required for the initial serine palmitoyltransferase step.\n\n📹 [VIDEO LINK]"},{"id":691,"s":"Lipid Chemistry","t":"Glycolipids and Gangliosides","u":"Sphinganine Desaturated to Sphingosine","q":"Sphinganine is desaturated to form which of the following?","o":["Ceramide","Cerebroside","Globoside","Sphingosine","Sphingomyelin"],"a":3,"p":"SPHINGANINE (dihydrosphingosine) → DESATURASE introduces double bond at C4-C5 → SPHINGOSINE (trans-4-sphingenine). Sphingosine = the backbone long-chain amino alcohol of all sphingolipids. Then: sphingosine + fatty acyl-CoA → (ceramide synthase) → CERAMIDE. From ceramide: + phosphocholine → sphingomyelin. + glucose → glucocerebroside. + galactose → galactocerebroside. + oligosaccharide + sialic acid → ganglioside. Sphingosine-1-phosphate (S1P) = bioactive lipid mediator (produced by sphingosine kinase) → regulates lymphocyte trafficking.\n\n📹 [VIDEO LINK]"},{"id":692,"s":"Lipid Chemistry","t":"Glycolipids and Gangliosides","u":"Glycolipids Built on Ceramide","q":"Glycolipids are constructed on which of the following?","o":["Ceramide","Globoside","Sphinganine","Sphingosine","Glycerol"],"a":0,"p":"GLYCOLIPIDS are built on CERAMIDE (sphingosine + fatty acid). Ceramide = the hydrophobic backbone. Sugar residues are added to the C1-OH of ceramide by glycosyltransferases in the Golgi. Types of glycolipids based on ceramide: CEREBROSIDES = ceramide + one sugar (glucose or galactose). GLOBOSIDES = ceramide + 2 or more neutral sugars. GANGLIOSIDES = ceramide + oligosaccharide + one or more sialic acid (N-acetylneuraminic acid). SULPHATIDES = sulfated ceramide + galactose. ALL glycolipids share ceramide as backbone.\n\n📹 [VIDEO LINK]"},{"id":693,"s":"Lipid Chemistry","t":"Glycolipids and Gangliosides","u":"Glycolipid Synthesis Occurs in Golgi","q":"Synthesis of glycolipids occurs in which of the following?","o":["Cytosol","Endoplasmic reticulum","Golgi apparatus","Lysosomes","Peroxisomes"],"a":2,"p":"GLYCOLIPID SYNTHESIS occurs in the GOLGI APPARATUS. Glycosyltransferases in Golgi membranes sequentially add sugar residues to ceramide → glycolipids formed on the LUMINAL (outer) face of Golgi → packaged into vesicles → transported to plasma membrane → glycolipids expressed on OUTER LEAFLET of plasma membrane (glycocalyx). Compare: ceramide is synthesised in the ER. Degradation of glycolipids occurs in LYSOSOMES (lysosomal glycosidases).\n\n📹 [VIDEO LINK]"},{"id":694,"s":"Lipid Chemistry","t":"Glycolipids and Gangliosides","u":"Ganglioside Contains Neuraminic Acid","q":"A ganglioside contains which of the following distinguishing components?","o":["Glucose only","Galactose only","Lactose only","Neuraminic acid","Phosphate group"],"a":3,"p":"GANGLIOSIDES = ceramide + oligosaccharide + one or more NEURAMINIC ACID (N-acetylneuraminic acid, NANA = sialic acid) residues. The presence of sialic acid gives gangliosides their highly NEGATIVE CHARGE. Found predominantly in NEURONAL CELL MEMBRANES (outer leaflet). Functions: cell recognition, signal transduction, synaptic transmission. GM1 is the receptor for cholera toxin. Ganglioside nomenclature: G = ganglioside. M = monosialo (one NANA). D = disialo (two NANA). T = trisialo. A = asialo (no NANA). Subscript 1, 2, 3 = carbohydrate chain length.\n\n📹 [VIDEO LINK]"},{"id":695,"s":"Lipid Chemistry","t":"Glycolipids and Gangliosides","u":"Ganglioside Subscript 1 Four Sugars","q":"In ganglioside nomenclature, the subscript 1 refers to which carbohydrate sequence attached to the ceramide?","o":["Gal-Glc-ceramide","GalNAc-Gal-Glc-ceramide","Gal-GalNAc-Gal-Glc-ceramide","Gal-Glc-Glc-ceramide","NANA-Gal-ceramide"],"a":2,"p":"GANGLIOSIDE SUBSCRIPT NUMBERING (carbohydrate chain length from ceramide): Subscript 1 = GalGalNAcGalGlc-ceramide (4 sugars: Gal-GalNAc-Gal-Glc, reading from outermost to ceramide). Subscript 2 = GalNAcGalGlc-ceramide (3 sugars). Subscript 3 = GalGlc-ceramide (2 sugars, lactosylceramide). GM1 = mono-sialosyl-4-sugar chain ganglioside. GM2 (Tay-Sachs substrate) = mono-sialosyl-3-sugar chain. GM3 = mono-sialosyl-2-sugar chain (simplest ganglioside).\n\n📹 [VIDEO LINK]"},{"id":696,"s":"Lipid Chemistry","t":"Glycolipids and Gangliosides","u":"Ganglioside Subscript 2 Three Sugars","q":"In ganglioside nomenclature, the subscript 2 refers to which carbohydrate sequence attached to the ceramide?","o":["Gal-Glc-ceramide","GalNAc-Gal-Glc-ceramide","Gal-GalNAc-Gal-Glc-ceramide","Gal-Glc-Glc-ceramide","NANA-Gal-GalNAc-ceramide"],"a":1,"p":"Subscript 2 = GalNAcGalGlc-ceramide (3 sugars: GalNAc-Gal-Glc reading from outer to ceramide). GM2 = the ganglioside that ACCUMULATES in TAY-SACHS DISEASE (beta-hexosaminidase A deficiency → cannot cleave GalNAc from GM2 → GM2 accumulates in neurons → progressive neurodegeneration). GM2 accumulates because beta-hexosaminidase A cannot cleave the terminal GalNAc residue.\n\n📹 [VIDEO LINK]"},{"id":697,"s":"Lipid Chemistry","t":"Glycolipids and Gangliosides","u":"Ganglioside Subscript 3 Two Sugars","q":"In ganglioside nomenclature, the subscript 3 refers to which carbohydrate sequence attached to the ceramide?","o":["Gal-Glc-ceramide","GalNAc-Gal-Glc-ceramide","Gal-GalNAc-Gal-Glc-ceramide","Gal-Glc-Glc-ceramide","NANA-Gal-ceramide"],"a":0,"p":"Subscript 3 = GalGlc-ceramide (2 sugars: Gal-Glc = lactosyl-ceramide). GM3 = mono-sialosyl-lactosylceramide = the SIMPLEST ganglioside. Ganglioside degradation pathway: GM1 → (beta-galactosidase-1) → GM2 → (beta-hexosaminidase A) → GM3 → (neuraminidase) → lactosylceramide → further degradation. Defects at each step cause different sphingolipidoses. GM3 is the starting ganglioside for synthesis of more complex gangliosides.\n\n📹 [VIDEO LINK]"},{"id":698,"s":"Lipid Chemistry","t":"Sphingolipidoses","u":"Tay-Sachs Beta Hexosaminidase A GM2","q":"A 6-month-old infant presents with progressive loss of developmental milestones, exaggerated startle response, and a cherry-red spot on the macula. Tay-Sachs disease is suspected. Which enzyme is deficient?","o":["Alpha-galactosidase A","Beta-galactosidase","Beta-glucocerebrosidase","Beta-hexosaminidase A","Sphingomyelinase"],"a":3,"p":"TAY-SACHS DISEASE: BETA-HEXOSAMINIDASE A deficiency → cannot cleave N-acetylgalactosamine (GalNAc) from GM2 ganglioside → GM2 ACCUMULATES in NEURONS → neuronal swelling → progressive neurodegeneration. Features: progressive loss of milestones (motor, then cognitive), exaggerated startle (hyperacusis), CHERRY-RED SPOT on macula (ganglion cells around fovea swell with GM2 → opaque white, exposing the red fovea below). Death by age 2-4 years. Autosomal recessive. Prevalent in Ashkenazi Jewish population.\n\n[Note to editor: Insert cherry-red spot fundus photograph from net. Insert ganglioside degradation pathway with beta-hexosaminidase A step circled in red showing GM2 → GM3 block.]\n\n📹 [VIDEO LINK]"},{"id":699,"s":"Lipid Chemistry","t":"Sphingolipidoses","u":"Tay-Sachs GM2 Accumulates Neurons","q":"A 6-month-old infant with Tay-Sachs disease presents with progressive neuronal degeneration and a cherry-red spot. Which lipid is causing the neuronal degeneration?","o":["Galactocerebroside","Glucocerebroside","GM1 ganglioside","GM2 ganglioside","Sphingomyelin"],"a":3,"p":"TAY-SACHS: GM2 GANGLIOSIDE accumulates in NEURONS because beta-hexosaminidase A (lysosomal enzyme) cannot cleave terminal N-acetylgalactosamine from GM2. GM2 accumulates in lysosomes of neurons → neurons swell → cell death → progressive neurodegeneration. MACULA (cherry-red spot): ganglion cells of retina swell with GM2 → appear white/opaque → the underlying foveal choroid (red) is exposed by contrast against white swollen surrounding cells. Compare: GM1 accumulation = GM1 gangliosidosis (beta-galactosidase-1 deficiency). GLUCOCEREBROSIDE = Gaucher disease.\n\n[Note to editor: Insert diagram of ganglioside degradation pathway circling GM2 → GM3 step (beta-hexosaminidase A deficiency in red). Insert clinical photograph of cherry-red spot from net.]\n\n📹 [VIDEO LINK]"},{"id":700,"s":"Lipid Chemistry","t":"Sphingolipidoses","u":"Niemann Pick Sphingomyelinase Sphingomyelin","q":"A 6-month-old infant presents with hepatosplenomegaly, progressive neurodegeneration, and a cherry-red spot. Niemann-Pick disease is suspected. Which lipid is causing the hepatosplenomegaly and neurodegeneration in him?","o":["Ceramide","Galactocerebroside","Glucocerebroside","Sphingomyelin","Sulphatide"],"a":3,"p":"NIEMANN-PICK DISEASE Type A: SPHINGOMYELINASE deficiency → SPHINGOMYELIN accumulates in macrophages in liver and spleen (→ hepatosplenomegaly, foam cells) and in neurons (→ neurodegeneration, cherry-red spot in 50%). Type B = visceral only, no neurological involvement. Compare: Tay-Sachs = GM2 accumulates in neurons only, no hepatosplenomegaly in early stages.\n\n[Note to editor: Insert sphingomyelin degradation pathway with sphingomyelinase circled in red. Insert Niemann-Pick foam cell photograph from net.]\n\n📹 [VIDEO LINK]"},{"id":701,"s":"Lipid Chemistry","t":"Sphingolipidoses","u":"Ganglioside Degradation GM1 to GM2","q":"For ganglioside degradation, which of the following enzymes helps in removal of galactose, converting GM1 to GM2?","o":["Alpha-galactosidase","Beta-galactosidase","Beta-galactosidase-1","Beta-hexosaminidase A","Neuraminidase"],"a":2,"p":"GANGLIOSIDE DEGRADATION PATHWAY: GM1 → (BETA-GALACTOSIDASE-1 removes terminal galactose) → GM2. DEFICIENCY of beta-galactosidase-1 = GM1 GANGLIOSIDOSIS → GM1 accumulates → neurodegeneration + skeletal involvement + beta-galactosidase-1 also degrades KERATAN SULFATE → keratan sulfate also accumulates. GM2 → (BETA-HEXOSAMINIDASE A removes GalNAc) → GM3 [TAY-SACHS step]. GM3 → (NEURAMINIDASE removes sialic acid) → LACTOSYLCERAMIDE.\n\n📹 [VIDEO LINK]"},{"id":702,"s":"Lipid Chemistry","t":"Sphingolipidoses","u":"GM2 to GM3 Beta Hexosaminidase A Tay-Sachs","q":"Which of the following enzyme removes N-acetylgalactosamine, converting GM2 to GM3?","o":["Alpha-galactosidase","Beta-galactosidase","Beta-galactosidase-1","Beta-hexosaminidase A","Neuraminidase"],"a":3,"p":"GM2 → GM3: BETA-HEXOSAMINIDASE A removes N-acetylgalactosamine (GalNAc) from GM2. This is the DEFICIENT enzyme in TAY-SACHS DISEASE → GM2 accumulates. Beta-hexosaminidase A = alpha subunit (HEXA gene, mutated in Tay-Sachs) + beta subunit (HEXB gene). SANDHOFF DISEASE = both alpha and beta subunit deficiency (HEXB mutation) → accumulation of BOTH GM2 ganglioside AND globoside. Enzyme replacement therapy not effective for neurological disease (blood-brain barrier).\n\n📹 [VIDEO LINK]"},{"id":703,"s":"Lipid Chemistry","t":"Sphingolipidoses","u":"GM3 to Lactosylceramide Neuraminidase","q":"Which of the following converts GM3 to lactosylceramide?","o":["Alpha-galactosidase","Beta-galactosidase","Beta-galactosidase-1","Beta-hexosaminidase A","Neuraminidase"],"a":4,"p":"GM3 → LACTOSYLCERAMIDE: NEURAMINIDASE (sialidase) removes the sialic acid (N-acetylneuraminic acid/NANA) from GM3 → LACTOSYLCERAMIDE (ceramide + Gal-Glc). NEURAMINIDASE DEFICIENCY = SIALIDOSIS → GM3 and other sialylated glycolipids accumulate → neurological manifestations. Lactosylceramide → further degradation by beta-galactosidase → glucocerebroside → (beta-glucocerebrosidase) → ceramide. Defects at each step cause different lysosomal storage diseases.\n\n📹 [VIDEO LINK]"},{"id":704,"s":"Lipid Chemistry","t":"Sphingolipidoses","u":"GM1 Gangliosidosis Substrate Beta Galactosidase Keratan Sulfate","q":"Which of the following is a substrate for beta-galactosidase (beta-galactosidase-1)?","o":["Chondroitin sulfate","Dermatan sulfate","Heparan sulfate","Keratan sulfate","Heparin sulfate"],"a":3,"p":"BETA-GALACTOSIDASE-1 (lysosomal) has TWO substrates: (1) GM1 ganglioside (removes terminal galactose → GM2). (2) KERATAN SULFATE (cleaves galactose residues in keratan sulfate degradation). Therefore GM1 GANGLIOSIDOSIS (beta-galactosidase-1 deficiency) causes accumulation of BOTH GM1 GANGLIOSIDE (neurological features) AND KERATAN SULFATE (skeletal features — similar to Morquio syndrome). This is why GM1 gangliosidosis presents with neurodegeneration + skeletal dysplasia + coarse facial features.\n\n📹 [VIDEO LINK]"},{"id":705,"s":"Lipid Chemistry","t":"Sphingolipidoses","u":"I-Cell Disease Mannose-6-Phosphate Tagging","q":"A newborn with gargoyle features is found to have lysosomal enzymes present in serum. This newborn likely has a defect in tagging of lysosomal enzymes with which of the following?","o":["Fructose-6-phosphate","Galactose-6-phosphate","Glucose-6-phosphate","Mannose-6-phosphate","Ribose-6-phosphate"],"a":3,"p":"I-CELL DISEASE (inclusion cell disease, Mucolipidosis II): deficiency of GlcNAc-1-phosphotransferase → lysosomal enzymes CANNOT BE TAGGED WITH MANNOSE-6-PHOSPHATE (M6P) → M6P receptor in Golgi cannot direct enzymes to lysosomes → enzymes SECRETED INTO EXTRACELLULAR SPACE (serum) instead → lysosomes lack degradative enzymes → substrates accumulate → inclusion bodies. Features: gargoyle-like facies (similar to Hurler but faster progression), hepatosplenomegaly, skeletal abnormalities. Lysosomal enzymes elevated in SERUM = pathognomonic.\n\n📹 [VIDEO LINK]"},{"id":706,"s":"Lipid Chemistry","t":"Eicosanoids","u":"Steroids Inhibit Phospholipase A2 via Lipocortin","q":"Steroids inhibit prostaglandin synthesis by inhibition of which of the following?","o":["Cyclooxygenase-1","Cyclooxygenase-2","5-Lipoxygenase","Phospholipase A1","Phospholipase A2"],"a":4,"p":"STEROIDS (corticosteroids) → induce LIPOCORTIN (annexin A1) synthesis → lipocortin INHIBITS PHOSPHOLIPASE A2 → prevents release of ARACHIDONIC ACID from membrane phospholipids → blocks BOTH COX pathway (prostaglandins, thromboxanes) AND LOX pathway (leukotrienes). Therefore steroids are the BROADEST anti-inflammatory agents — they inhibit the common substrate supply. Compare: ASPIRIN (NSAID) inhibits only COX (not LOX). Steroids are used for: severe asthma, rheumatoid arthritis, anaphylaxis, autoimmune disease.\n\n📹 [VIDEO LINK]"},{"id":707,"s":"Lipid Chemistry","t":"Eicosanoids","u":"Aspirin Toxicity COX-1 Inhibition","q":"Systemic inhibition of which of the following is the cause of aspirin's gastric toxicity?","o":["Cyclooxygenase-1","Cyclooxygenase-2","Phospholipase A1","Phospholipase A2","5-Lipoxygenase"],"a":0,"p":"ASPIRIN TOXICITY (gastric damage) = inhibition of COX-1 (constitutive enzyme). COX-1 → PGE2 and PGI2 in gastric mucosa → cytoprotective prostaglandins: ↑ mucus secretion, ↑ bicarbonate secretion, ↓ acid secretion, ↑ mucosal blood flow. Aspirin inhibits COX-1 → ↓ protective prostaglandins → exposed gastric mucosa → peptic ulceration + GI bleeding. COX-2 selective inhibitors (rofecoxib) were developed to reduce GI toxicity — but increased cardiovascular risk (↓ PGI2 → unopposed TXA2 → thrombosis). Misoprostol (PGE1 analogue) used to protect gastric mucosa during NSAID therapy.\n\n📹 [VIDEO LINK]"},{"id":708,"s":"Lipid Chemistry","t":"Eicosanoids","u":"COX-1 Physiological Reactions","q":"COX-1 inhibitors inhibit which of the following?","o":["Allergic reactions","Hypersensitivity reactions","Inflammatory reactions","Physiological reactions","Pyrexia"],"a":3,"p":"COX-1 = CONSTITUTIVE (always expressed) → produces PHYSIOLOGICAL prostaglandins required for normal homeostasis: gastric mucosa protection (PGE2, PGI2), platelet aggregation (TXA2 in platelets), renal blood flow maintenance (PGE2, PGI2), vascular tone. COX-1 inhibitors (NSAIDs) → inhibit PHYSIOLOGICAL reactions → gastric damage, impaired platelet aggregation, renal impairment. COX-2 = INDUCIBLE (upregulated by inflammatory stimuli) → produces PATHOLOGICAL prostaglandins → inflammation, fever, pain. COX-2 inhibitors = anti-inflammatory without gastric damage.\n\n📹 [VIDEO LINK]"},{"id":709,"s":"Lipid Chemistry","t":"Eicosanoids","u":"COX-2 Inhibitors Heart Attack PGI2 Decreased","q":"Use of COX-2 inhibitors has been associated with increased risk of heart attacks due to decreased synthesis of which of the following?","o":["PGD2","PGE2","PGF2","PGG2","PGI2"],"a":4,"p":"COX-2 selectively expressed in VASCULAR ENDOTHELIUM → produces PROSTACYCLIN (PGI2) → INHIBITS PLATELET AGGREGATION and causes VASODILATION. COX-2 inhibitors (rofecoxib, celecoxib) → ↓ PGI2 in endothelium → UNOPPOSED TXA2 (from platelets, COX-1) → platelet aggregation + vasoconstriction → THROMBOSIS → HEART ATTACKS. This is why rofecoxib (Vioxx) was withdrawn from market in 2004 after increased cardiovascular events. Selective COX-2 inhibition creates a prothrombotic imbalance.\n\n📹 [VIDEO LINK]"},{"id":710,"s":"Lipid Chemistry","t":"Eicosanoids","u":"Aspirin Irreversible Acetylation COX","q":"Aspirin inhibits COX-1 and COX-2 through irreversible which of the following?","o":["Acetylation","Dehydrogenation","Oxidation","Phosphorylation","Reduction"],"a":0,"p":"ASPIRIN (acetylsalicylic acid) → irreversible ACETYLATION of SERINE RESIDUE (Ser530) in the active site of both COX-1 and COX-2 → permanently blocks access of arachidonic acid to the catalytic site → permanent enzyme inactivation. Irreversible = aspirin effect lasts for the LIFETIME OF THE ENZYME (COX-2, 6-8 hours) or the LIFETIME OF THE CELL (platelets, 8-10 days — no nucleus, cannot synthesise new COX). Compare: ibuprofen, naproxen = REVERSIBLE COX inhibition (compete for active site). The irreversibility of aspirin's platelet COX-1 inhibition is the basis of its antiplatelet effect.\n\n📹 [VIDEO LINK]"},{"id":711,"s":"Lipid Chemistry","t":"Eicosanoids","u":"Platelets No Nuclei COX Cannot Be Replaced","q":"Aspirin irreversibly inhibits COX-1 in platelets. Which of the following explains why platelets cannot synthesise new COX-1 protein to overcome this inhibition?","o":["Absence of endoplasmic reticulum","Absence of Golgi apparatus","Absence of mitochondria","Absence of nuclei","Absence of ribosomes"],"a":3,"p":"Platelets are ANUCLEATE — they have no nuclei → cannot transcribe new COX-1 mRNA → cannot synthesise new COX-1 protein → aspirin's irreversible acetylation of platelet COX-1 is PERMANENT for the lifespan of each platelet (8-10 days). This is the basis of ASPIRIN'S ANTIPLATELET EFFECT: one dose of aspirin inhibits all circulating platelet TXA2 production for 8-10 days. New platelets replacing old ones restore platelet function gradually. Aspirin 75-100 mg daily = antiplatelet therapy for cardiovascular disease prevention.\n\n📹 [VIDEO LINK]"},{"id":712,"s":"Lipid Chemistry","t":"Eicosanoids","u":"Asthma 5-Lipoxygenase Leukotrienes","q":"Asthma may be treated by inhibitors of which of the following?","o":["Cyclooxygenase-1","Cyclooxygenase-2","Phospholipase A2","5-Lipoxygenase","Phospholipase C"],"a":3,"p":"ASTHMA = leukotrienes (LTC4, LTD4, LTE4 — cysteinyl leukotrienes) cause: bronchoconstriction, increased mucus secretion, bronchial oedema. 5-LIPOXYGENASE (5-LOX) = enzyme that converts arachidonic acid → LTA4 → LTB4 (chemotaxis) and cysteinyl leukotrienes (LTC4, LTD4, LTE4). 5-LOX INHIBITOR = ZILEUTON → reduces leukotriene synthesis → treats asthma. LEUKOTRIENE RECEPTOR ANTAGONISTS = MONTELUKAST, ZAFIRLUKAST → block CysLT1 receptor → treat asthma. Compare: aspirin/NSAIDs → inhibit COX → INCREASE leukotriene synthesis (shunting) → WORSEN asthma in susceptible patients.\n\n📹 [VIDEO LINK]"},{"id":713,"s":"Lipid Chemistry","t":"Eicosanoids","u":"Aspirin Induced Asthma Leukotriene Overproduction","q":"In asthmatic individuals, aspirin may cause overproduction of which of the following?","o":["Eicosanoids","Leukotrienes","Prostaglandins","Thromboxanes","All eicosanoids"],"a":1,"p":"ASPIRIN-EXACERBATED RESPIRATORY DISEASE (AERD, Samter's Triad = asthma + nasal polyps + aspirin sensitivity): Aspirin inhibits COX → ↓ PGE2 (which normally INHIBITS 5-LOX) → 5-LOX is DISINHIBITED → OVERPRODUCTION OF LEUKOTRIENES (LTC4, LTD4, LTE4) → severe bronchoconstriction → asthma exacerbation. The key mechanism: PGE2 normally brakes the LOX pathway — remove PGE2 (via COX inhibition) → LOX runs unchecked → leukotriene surge. Treatment: leukotriene receptor antagonists (montelukast), aspirin desensitisation.\n\n📹 [VIDEO LINK]"},{"id":714,"s":"Lipid Chemistry","t":"Eicosanoids","u":"Steroids Emergency Asthma Treatment","q":"Which of the following may serve as emergency treatment of asthma?","o":["Aspirin","Ibuprofen","Montelukast","Phenylbutazone","Steroids"],"a":4,"p":"STEROIDS (systemic corticosteroids — hydrocortisone IV, prednisolone oral) = EMERGENCY TREATMENT of severe acute asthma. Mechanism: induce lipocortin → inhibit phospholipase A2 → BLOCK BOTH COX AND LOX pathways → reduce ALL inflammatory mediators (prostaglandins AND leukotrienes). Most comprehensive anti-inflammatory effect. Also: reduce airway mucosal oedema, reduce mucus secretion, upregulate beta-2 adrenoceptors (synergises with beta-2 agonists). NSAIDs (aspirin, ibuprofen, phenylbutazone) → inhibit COX only → may WORSEN asthma by increasing leukotrienes.\n\n📹 [VIDEO LINK]"},{"id":715,"s":"Lipid Chemistry","t":"Eicosanoids","u":"Asthma Leukotriene Receptor Antagonists","q":"Asthma may be treated by using receptor antagonists for which of the following?","o":["Eicosanoids","Leukotrienes","Prostaglandins","Thromboxanes","Prostacyclins"],"a":1,"p":"LEUKOTRIENE RECEPTOR ANTAGONISTS (MONTELUKAST, ZAFIRLUKAST) block the CysLT1 receptor → prevent LTC4, LTD4, LTE4 from causing bronchoconstriction, mucus hypersecretion, bronchial oedema → treat ASTHMA and ALLERGIC RHINITIS. Used as: maintenance therapy for mild-moderate asthma, especially in aspirin-exacerbated asthma, exercise-induced asthma, and allergic rhinitis. ZILEUTON = 5-LOX inhibitor (inhibits leukotriene synthesis rather than receptor). Both approaches reduce leukotriene activity.\n\n📹 [VIDEO LINK]"},{"id":716,"s":"Lipid Chemistry","t":"Eicosanoids","u":"Aspirin Gastric Mucosa Prostaglandins","q":"Aspirin damages gastric mucosa by decreasing production of which of the following?","o":["Eicosanoids","Leukotrienes","Prostaglandins","Thromboxanes","All of the above"],"a":2,"p":"ASPIRIN → inhibits COX-1 in gastric mucosa → ↓ PGE2 and PGI2 → LOSS OF CYTOPROTECTIVE PROSTAGLANDINS: mucus secretion ↓, bicarbonate secretion ↓, acid secretion ↑, mucosal blood flow ↓ → exposed gastric mucosa → peptic ulceration + GI bleeding. Misoprostol (synthetic PGE1 analogue) is given alongside NSAIDs to protect gastric mucosa (replaces the prostaglandins that aspirin depletes). PROTON PUMP INHIBITORS also used for gastroprotection during NSAID therapy.\n\n📹 [VIDEO LINK]"},{"id":717,"s":"Lipid Chemistry","t":"Eicosanoids","u":"LTB4 Chemotaxis Neutrophils","q":"At a site of bacterial infection, neutrophils are migrating from capillaries toward the infected tissue by chemotaxis. Which of the following leukotrienes is directing this migration?","o":["LTA4","LTB4","LTC4","LTD4","LTE4"],"a":1,"p":"LTB4 = the primary NEUTROPHIL CHEMOTACTIC leukotriene. Produced from LTA4 by LTA4 hydrolase. Binds BLT1 and BLT2 receptors on neutrophils → directed migration toward infection site + enhanced adhesion to endothelium + enhanced phagocytosis. Key mediator in: inflammatory bowel disease, psoriasis, COPD, gout. Compare: LTC4, LTD4, LTE4 = bronchoconstriction. LTA4 = unstable intermediate only.\n\n📹 [VIDEO LINK]"},{"id":7171,"s":"Lipid Chemistry","t":"Eicosanoids","u":"Cysteinyl Leukotrienes Bronchoconstriction Asthma","q":"A 14-year-old asthmatic boy develops severe bronchoconstriction after exposure to dust mites. Which of the following groups of leukotrienes is responsible for his bronchoconstriction?","o":["LTA4 and LTB4","LTA4 and LTC4","LTB4 and LTC4","LTC4 LTD4 and LTE4","LTD4 and LTE4 only"],"a":3,"p":"CYSTEINYL LEUKOTRIENES = LTC4, LTD4, and LTE4 — all three cause bronchoconstriction, mucus hypersecretion, and bronchial oedema by binding CysLT1 receptors in bronchial smooth muscle. LTD4 is the most potent bronchoconstrictor. Formerly called slow-reacting substance of anaphylaxis (SRS-A). Montelukast and zafirlukast block CysLT1 receptor → treat asthma. LTB4 = chemotaxis only. LTA4 = unstable intermediate.\n\n📹 [VIDEO LINK]"},{"id":718,"s":"Lipid Chemistry","t":"Eicosanoids","u":"LTB4 Chemotaxis Neutrophils","q":"Which of the following leukotriene is involved in chemotaxis of neutrophils?","o":["LTA4","LTB4","LTC4","LTD4","LTE4"],"a":1,"p":"LTB4 = NEUTROPHIL CHEMOTACTIC AGENT. Produced from LTA4 (unstable epoxide intermediate) by LTA4 hydrolase. LTB4 binds BLT1 and BLT2 receptors on neutrophils → chemotaxis to sites of infection/inflammation + enhanced neutrophil adhesion to endothelium + enhanced phagocytosis. LTB4 is a key mediator in: inflammatory bowel disease, psoriasis, COPD, gout. Compare: LTC4, LTD4, LTE4 = BRONCHOCONSTRICTION (asthma). LTB4 = CHEMOTAXIS.\n\n📹 [VIDEO LINK]"},{"id":719,"s":"Lipid Chemistry","t":"Eicosanoids","u":"TXA2 COX-1 Platelets Aggregation Calcium","q":"Which of the following is produced by COX-1 in platelets and promotes platelet aggregation?","o":["LTE4","PGD2","PGE2","PGI2","TXA2"],"a":4,"p":"THROMBOXANE A2 (TXA2): produced by COX-1 in PLATELETS via thromboxane synthase. Actions: (1) PROMOTES PLATELET AGGREGATION (positive feedback — recruits more platelets). (2) VASOCONSTRICTION. (3) MOBILISES INTRACELLULAR CALCIUM (via IP3 → Ca²⁺ release from dense granules → degranulation → more platelet activation). Half-life = 30 seconds (unstable → spontaneously hydrolysed to inactive TXB2). Physiological antagonist: PGI2 (prostacyclin, produced by vascular endothelium). Aspirin's antiplatelet effect = irreversible COX-1 inhibition → ↓ TXA2 in platelets.\n\n📹 [VIDEO LINK]"},{"id":720,"s":"Lipid Chemistry","t":"Eicosanoids","u":"PGI2 COX-2 Endothelium Inhibits Platelet Aggregation","q":"Which of the following is produced by COX-2 primarily in vascular endothelium and inhibits platelet aggregation?","o":["LTB4","PGD2","PGE2","PGF2","PGI2"],"a":4,"p":"PROSTACYCLIN (PGI2): produced by COX-2 in VASCULAR ENDOTHELIUM via prostacyclin synthase. Actions: (1) INHIBITS PLATELET AGGREGATION (opposes TXA2). (2) VASODILATION. (3) Increases cAMP in platelets → inhibits platelet activation. TXA2:PGI2 BALANCE = physiological haemostasis. COX-2 inhibitors → ↓ PGI2 → upset balance toward TXA2 → thrombosis → cardiovascular events. Exogenous prostacyclin analogues (epoprostenol, treprostinil) used to treat PULMONARY ARTERIAL HYPERTENSION.\n\n📹 [VIDEO LINK]"},{"id":721,"s":"Lipid Chemistry","t":"Eicosanoids","u":"PGE2 Gastric Mucosa Renal Pain Fever Uterus","q":"Which of the following prostaglandin protects the gastric mucosa by increasing mucus and bicarbonate secretion?","o":["PGD2","PGE2","PGF2","PGG2","PGI2"],"a":1,"p":"PGE2 (PROSTAGLANDIN E2) has MULTIPLE ACTIONS via different EP receptors (EP1-EP4): (1) GASTRIC PROTECTION — ↑ mucus + ↑ bicarbonate + ↓ acid (EP1, EP3). (2) RENAL FUNCTION — regulates GFR and Na⁺ excretion (EP1, EP3, EP4). (3) PAIN POTENTIATION — sensitises nociceptors to bradykinin (EP1, EP2). (4) FEVER — acts on hypothalamus (EP3). (5) UTERINE CONTRACTIONS — used to induce labour (PGE2 gel). (6) BRONCHODILATION at low concentrations. PGE2 is the most versatile prostaglandin with the widest range of actions.\n\n📹 [VIDEO LINK]"},{"id":722,"s":"Lipid Chemistry","t":"Eicosanoids","u":"PGE2 Fever Hypothalamus","q":"Which of the following is required for the fever response?","o":["LTB4","PGD2","PGE2","PGF2","TXA2"],"a":2,"p":"FEVER: PYROGENS (bacteria, endotoxin, cytokines IL-1, IL-6, TNF-alpha) → stimulate COX-2 in hypothalamic blood vessels → ↑ PGE2 synthesis → PGE2 acts on EP3 receptors in HYPOTHALAMUS (preoptic area) → raises thermoregulatory SET POINT → fever. ANTIPYRETICS (aspirin, paracetamol, ibuprofen) inhibit COX → ↓ PGE2 → lower set point → antipyresis. Paracetamol (acetaminophen) works primarily in CNS (inhibits central COX) without peripheral anti-inflammatory effect.\n\n📹 [VIDEO LINK]"},{"id":723,"s":"Lipid Chemistry","t":"Eicosanoids","u":"PGE2 Uterine Contractions Labour","q":"Which of the following stimulates uterine contractions and is used to induce labour?","o":["LTB4","PGD2","PGE2","PGF2","TXA2"],"a":2,"p":"PGE2 stimulates uterine contractions via EP1 and EP3 receptors → USED CLINICALLY TO INDUCE LABOUR (PGE2 gel or pessary applied to cervix → cervical ripening + uterine contractions). Also used for: termination of pregnancy, management of postpartum haemorrhage. PGF2-alpha (dinoprost) also stimulates uterine contractions. NSAIDs (COX inhibitors) given in PREMATURE LABOUR to inhibit PG-driven contractions → tocolysis (e.g. indomethacin).\n\n📹 [VIDEO LINK]"},{"id":724,"s":"Lipid Chemistry","t":"Eicosanoids","u":"PGD2 Sleep Inducing","q":"Which of the following is a potent sleep-inducing factor that accumulates in the brain during prolonged wakefulness?","o":["LTB4","PGD2","PGE2","PGF2","TXA2"],"a":1,"p":"PGD2 (PROSTAGLANDIN D2) = potent SLEEP-INDUCING FACTOR produced by mast cells in the arachnoid membrane (subarachnoid space). PGD2 accumulates during prolonged wakefulness → acts on DP1 receptors → promotes adenosine release → adenosine acts on A2A receptors in basal forebrain → SLEEP induction. PGD2 is the most abundant prostaglandin in the brain. Caffeine works by blocking adenosine receptors (including A2A) → counteracts the sleep-promoting effect downstream of PGD2.\n\n📹 [VIDEO LINK]"},{"id":725,"s":"Lipid Chemistry","t":"Eicosanoids","u":"Aspirin Cardioprotection Inhibits COX TXA2","q":"A 72-year-old grandmother who had coronary stents placed 2 years ago takes a low-dose aspirin tablet every morning to prevent platelet clots from forming in her stents. Aspirin achieves this by which of the following mechanisms?","o":["Inhibits 5-lipoxygenase","Inhibits cyclooxygenase","Inhibits glutathione peroxidase","Inhibits phospholipase A2","Inhibits thromboxane receptor"],"a":1,"p":"ASPIRIN → irreversible ACETYLATION of COX-1 in PLATELETS → ↓ TXA2 synthesis → ↓ PLATELET AGGREGATION → ↓ thrombosis → CARDIOPROTECTIVE. Key: platelets have no nucleus → cannot replace inhibited COX-1 → aspirin effect lasts 8-10 days (platelet lifespan). Dose: 75-100 mg/day (low-dose aspirin). At low doses: preferentially inhibits platelet COX-1 (TXA2) with less effect on endothelial COX-2 (PGI2) → net antithrombotic balance. Higher doses inhibit both TXA2 and PGI2 → less net antiplatelet benefit.\n\n📹 [VIDEO LINK]"},{"id":726,"s":"Lipid Chemistry","t":"Lipid Peroxidation","u":"AGEs Cataracts Lens Elderly","q":"A 70-year-old professor of biochemistry is diagnosed with cataracts. Accumulation of which of the following compounds is likely to be present in his lens?","o":["Advanced glycation end products","Hydrogen peroxide","Oxygen radicals","Reactive oxygen species","Superoxide"],"a":0,"p":"CATARACTS in elderly = accumulation of ADVANCED GLYCATION END PRODUCTS (AGEs) in lens proteins (crystallins). Mechanism: chronic glucose exposure → Maillard reaction → glucose non-enzymatically bonds to amino groups of crystallins → Amadori products → cross-linked AGEs → protein aggregation → loss of lens transparency → CATARACTS. Also: AGEs are formed in diabetic patients (accelerated cataract). Compare: DIABETIC CATARACTS (younger age) = sorbitol accumulation via aldose reductase pathway (different mechanism). AGE formation also damages blood vessels, nerves, kidneys in diabetes.\n\n📹 [VIDEO LINK]"},{"id":727,"s":"Lipid Chemistry","t":"Lipid Peroxidation","u":"G6PD RBC Cannot Fight ROS","q":"A 120-day-old red blood cell is dying as it is incapable of fighting reactive oxygen species because of its inability to synthesise which of the following?","o":["G6P","G6PD","6PGD","GSSG","NADPH"],"a":1,"p":"RBC LIFESPAN = 120 days. As RBCs age → ENZYME ACTIVITY DECLINES including G6PD (glucose-6-phosphate dehydrogenase). G6PD produces NADPH via the pentose phosphate pathway. NADPH → maintains GLUTATHIONE in its reduced form (GSH) via glutathione reductase. GSH → glutathione peroxidase → detoxifies H₂O₂ and lipid peroxides. Without G6PD → ↓ NADPH → ↓ GSH → RBC cannot detoxify ROS → oxidative haemolysis. This is also the mechanism of G6PD DEFICIENCY (X-linked): trigger (malaria drugs, fava beans) → oxidative stress → haemolysis.\n\n📹 [VIDEO LINK]"},{"id":728,"s":"Lipid Chemistry","t":"Lipid Peroxidation","u":"X-Ray Hydroxyl Radical DNA Breaks","q":"An X-ray technician is diagnosed with cancer because X-rays excited cells, generating a radical that caused DNA breaks and mutations. Which of the following radical is likely to have been generated?","o":["NO•","O2•⁻","OH•","RS•","Superoxide"],"a":2,"p":"IONISING RADIATION (X-rays, gamma rays) → RADIOLYSIS OF WATER: H₂O → OH• (HYDROXYL RADICAL) + H•. HYDROXYL RADICAL (OH•) = most reactive ROS known → causes: (1) DNA STRAND BREAKS (single and double-strand). (2) DNA base modifications → mutations. (3) Lipid peroxidation. (4) Protein oxidation. Cannot be scavenged by an enzyme directly — must be prevented by minimising H₂O₂ and free iron (Fenton precursors). Radiation therapy in cancer EXPLOITS this mechanism (directed at tumour cells). Shielding protects X-ray technicians.\n\n📹 [VIDEO LINK]"},{"id":729,"s":"Lipid Chemistry","t":"Lipid Peroxidation","u":"Fenton Reaction H2O2 Iron Hydroxyl Radical","q":"A medical student consumes roasted chicken. As purines are degraded to uric acid by xanthine oxidase, H₂O₂ is produced. Lysosomal iron from ferritin autophagy may react with H₂O₂. Which radical is formed?","o":["NO•","O2•⁻","OH•","RS•","Superoxide"],"a":2,"p":"FENTON REACTION: Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + OH• (HYDROXYL RADICAL). The lysosomal iron pool derived from ferritin autophagy provides Fe²⁺ → reacts with H₂O₂ (produced by xanthine oxidase) → HYDROXYL RADICAL → lysosomal membrane peroxidation → lysosomal membrane rupture → cell death (ferroptosis/lysosome-dependent cell death). Xanthine oxidase produces: H₂O₂ AND O₂•⁻ (superoxide) simultaneously. Superoxide → (dismutase) → H₂O₂ → (Fenton) → OH•. Iron chelation (desferrioxamine) reduces Fenton reaction.\n\n📹 [VIDEO LINK]"},{"id":730,"s":"Lipid Chemistry","t":"Lipid Peroxidation","u":"Catalase H2O2 to Water and Oxygen","q":"Hydrogen peroxide is hydrolysed into water and oxygen by which of the following?","o":["Catalase","Glutathione peroxidase","Glutathione reductase","Peroxidase","Superoxide dismutase"],"a":0,"p":"CATALASE: 2H₂O₂ → 2H₂O + O₂. Located in PEROXISOMES (also cytosol of RBCs). Converts H₂O₂ directly to water and oxygen — no cofactor required. Most efficient enzyme known (turnover number 40 million H₂O₂/second/molecule). Compare: GLUTATHIONE PEROXIDASE: H₂O₂ + 2GSH → 2H₂O + GSSG (uses REDUCED GLUTATHIONE as cofactor — selenium-dependent). SUPEROXIDE DISMUTASE: 2O₂•⁻ + 2H⁺ → H₂O₂ + O₂ (converts superoxide to H₂O₂ — needs to work with catalase or GPx). The specific reaction 'water + oxygen' = CATALASE.\n\n📹 [VIDEO LINK]"},{"id":731,"s":"Lipid Chemistry","t":"Lipid Peroxidation","u":"Hydroxyl Radical 3rd Electron Reduction","q":"Addition of the 3rd electron and subsequent protonation converts oxygen reduction intermediates to which of the following?","o":["O2•⁻","H2O2","OH•","H2O","Peroxide"],"a":2,"p":"Sequential reduction of O₂ (each step + 1 electron): O₂ → +1e⁻ → O₂•⁻ (SUPEROXIDE, radical). O₂•⁻ → +1e⁻ + 2H⁺ → H₂O₂ (HYDROGEN PEROXIDE, not a radical). H₂O₂ → +1e⁻ + H⁺ → OH• + H₂O (HYDROXYL RADICAL — 3rd electron reduction). OH• → +1e⁻ + H⁺ → H₂O (WATER — 4th electron, COMPLETE REDUCTION). HYDROXYL RADICAL (OH•) produced at the 3rd electron step = most reactive ROS → DNA breaks, lipid peroxidation, protein oxidation. Complex IV (cytochrome c oxidase) performs the FULL 4-electron reduction safely.\n\n📹 [VIDEO LINK]"},{"id":732,"s":"Lipid Chemistry","t":"Lipid Peroxidation","u":"Radiation Generates ROS Cancer Treatment","q":"A 53-year-old woman with early breast cancer undergoes radiation therapy. Radiation works, in part, by which of the following?","o":["Generating hypochlorous acid","Generating reactive oxygen species","Inhibiting cytochrome bc1 complex","Inhibiting NADH oxidase","Stimulating glutathione production"],"a":1,"p":"RADIATION THERAPY → GENERATES REACTIVE OXYGEN SPECIES (primarily hydroxyl radical via water radiolysis: H₂O → OH• + H•) → DNA strand breaks → tumour cell death. Cancer cells are more sensitive to radiation because: (1) Faster cell division → less time for DNA repair. (2) Often deficient in DNA repair mechanisms. (3) Hypoxia paradoxically increases ROS sensitivity. Normal tissue protection: antioxidants, DNA repair. Side effects of radiotherapy = ROS damage to surrounding normal tissues.\n\n📹 [VIDEO LINK]"},{"id":733,"s":"Lipid Chemistry","t":"Zellweger Syndrome","u":"Zellweger Dysfunctional Peroxisomes","q":"Zellweger syndrome is caused by which of the following?","o":["Dysfunctional Golgi apparatus","Dysfunctional lysosomes","Dysfunctional mitochondria","Dysfunctional peroxisomes","Dysfunctional ribosomes"],"a":3,"p":"ZELLWEGER SYNDROME (cerebrohepatorenal syndrome) = caused by DYSFUNCTIONAL PEROXISOMES (PEX gene mutations → defective peroxisome biogenesis → absent or non-functional peroxisomes). All peroxisomal functions are lost: (1) VLCFA beta-oxidation → VLCFA accumulate. (2) Plasmalogen synthesis → plasmalogens deficient. (3) Bile acid synthesis → abnormal bile acids. (4) Phytanic acid alpha-oxidation → phytanic acid accumulates. Features: hypotonia, seizures, hepatomegaly, dysmorphic facies, early death (usually <1 year).\n\n📹 [VIDEO LINK]"},{"id":734,"s":"Lipid Chemistry","t":"Zellweger Syndrome","u":"Zellweger VLCFA Accumulate","q":"Which of the following accumulate in Zellweger syndrome?","o":["Long-chain fatty acids","Medium-chain fatty acids","Short-chain fatty acids","Very long-chain fatty acids","Volatile fatty acids"],"a":3,"p":"ZELLWEGER SYNDROME → absent peroxisomes → VLCFA (≥C22) CANNOT be beta-oxidised → VERY LONG CHAIN FATTY ACIDS ACCUMULATE. Specific VLCFAs elevated: C26:0 (hexacosanoic acid) is the most elevated marker. VLCFA incorporation into myelin → myelin destabilisation → hypomyelination → NEUROLOGICAL DYSFUNCTION. Diagnosis: plasma VLCFA assay (C26:0 elevated, C26:0/C22:0 ratio elevated). Also elevated: C26:0/C22:0 ratio in fibroblast culture → prenatal diagnosis possible.\n\n📹 [VIDEO LINK]"},{"id":735,"s":"Lipid Chemistry","t":"Zellweger Syndrome","u":"Zellweger Branched Chain FA Phytanic Acid","q":"Which of the following also accumulate in Zellweger syndrome due to loss of peroxisomal alpha-oxidation?","o":["Essential fatty acids","Long-chain fatty acids","Branched-chain fatty acids","Medium-chain fatty acids","Short-chain fatty acids"],"a":2,"p":"ZELLWEGER SYNDROME → absent peroxisomes → BOTH VLCFA (beta-oxidation) AND BRANCHED-CHAIN FATTY ACIDS (alpha-oxidation) accumulate. PHYTANIC ACID (branched-chain, from dietary chlorophyll) normally undergoes PEROXISOMAL ALPHA-OXIDATION (removes alpha carbon → pristanic acid → then beta-oxidised). In Zellweger: alpha-oxidation absent → PHYTANIC ACID ACCUMULATES. Compare: REFSUM DISEASE = isolated phytanic acid accumulation (only alpha-oxidation defective, not all peroxisomal functions). Zellweger is more severe because ALL peroxisomal functions are lost.\n\n📹 [VIDEO LINK]"},{"id":736,"s":"Lipid Chemistry","t":"Zellweger Syndrome","u":"Zellweger Plasmalogen Deficiency Hypomyelination","q":"Deficiency of which of the following is the biochemical basis of hypomyelination in Zellweger syndrome?","o":["Phosphatidylcholine","Phosphatidylserine","Phosphatidal choline","Phosphatidal ethanolamine","Sphingomyelin"],"a":3,"p":"PLASMALOGEN SYNTHESIS is PEROXISOME-DEPENDENT. In Zellweger: absent peroxisomes → PLASMENYL ETHANOLAMINE (phosphatidal ethanolamine = plasmalogen form of PE) CANNOT BE SYNTHESISED. Plasmalogens (especially plasmenyl ethanolamine) are essential for MYELIN membrane stability → their absence → HYPOMYELINATION → severe neurological deficit. Plasmalogens are also abundant in heart and skeletal muscle. Plasmalogen synthesis: glycerol-3-phosphate + fatty alcohol → DHAP-ether → then vinyl ether bond formed IN PEROXISOMES → exported to ER for completion.\n\n📹 [VIDEO LINK]"},{"id":737,"s":"Lipid Chemistry","t":"Zellweger Syndrome","u":"Zellweger Elevated Fatty Acids C26","q":"Which of the following fatty acids are elevated in Zellweger syndrome?","o":["20:0 and 20:1","22:0 and 22:1","24:0 and 24:1","26:0 and 26:1","28:0 and 28:1"],"a":3,"p":"In Zellweger syndrome: HEXACOSANOIC ACID (C26:0) and its mono-unsaturated form (C26:1) are the most elevated VLCFA markers. Plasma C26:0 level and C26:0/C22:0 ratio are the primary diagnostic tests. These VLCFA (26 carbons) cannot be degraded in the absence of peroxisomes. Saturated (26:0) and monounsaturated (26:1) very long chain fatty acids are pathological markers of Zellweger syndrome and adrenoleukodystrophy (but in ALD, specific ABCD1 gene mutation — not all peroxisomal functions lost).\n\n📹 [VIDEO LINK]"},{"id":738,"s":"Lipid Chemistry","t":"Milky Plasma","u":"Fasting Triglycerides Measurement","q":"Serum triglycerides should be measured during which of the following states?","o":["Absorptive state","Fasting","Post-prandially","Pre-prandially","Any time"],"a":1,"p":"SERUM TRIGLYCERIDES must be measured during FASTING (typically 9-12 hours fast) because: (1) Post-meal chylomicrons raise triglycerides transiently (physiological lipaemia). (2) Fasting sample reflects endogenous TG synthesis (VLDL from liver). Normal fasting TG: <1.7 mmol/L (<150 mg/dL). Borderline: 1.7-5.6 mmol/L. High: 5.6-11.3 mmol/L. Very high: >11.3 mmol/L → risk of PANCREATITIS. Note: unlike cholesterol (which can be measured non-fasting for cardiovascular risk assessment), TRIGLYCERIDES require fasting.\n\n📹 [VIDEO LINK]"},{"id":739,"s":"Lipid Chemistry","t":"Milky Plasma","u":"Post Meal Milky Plasma Chylomicrons","q":"An individual consumed a beef cheeseburger, soon afterwards met an accident and landed in the emergency department. Laboratory results reveal milky plasma. Which of the following is the biochemical cause of milky plasma?","o":["High chylomicrons","High HDL cholesterol","High IDL","High LDL cholesterol","High VLDL"],"a":0,"p":"MILKY PLASMA (lipaemia/lactescent plasma) immediately after a fatty meal = HIGH CHYLOMICRONS. Chylomicrons are synthesised in intestinal enterocytes → transport dietary triglycerides via lymphatics → bloodstream. Chylomicrons are so large they SCATTER LIGHT → plasma appears MILKY (turbid, lipaemic). Peak chylomicronaemia = 3-6 hours after a fatty meal. VLDL also contains TG but is smaller than chylomicrons. Chylomicron TG is cleared by LIPOPROTEIN LIPASE (on capillary walls) → plasma clears after 8-10 hours.\n\n📹 [VIDEO LINK]"},{"id":740,"s":"Lipid Chemistry","t":"Milky Plasma","u":"Milky Plasma Metabolic Syndrome","q":"In which of the following disorders is milky plasma most likely to be present?","o":["Coronary heart disease","Metabolic syndrome","Type 1 diabetes","Type 2 diabetes only","Iron deficiency anaemia"],"a":1,"p":"METABOLIC SYNDROME = cluster of: central obesity + hypertriglyceridaemia (TG >1.7 mmol/L) + low HDL + hypertension + impaired fasting glucose. Hypertriglyceridaemia → elevated VLDL → MILKY PLASMA. Pathogenesis: insulin resistance → ↑ VLDL synthesis (liver receives excess fatty acids from adipose) + ↓ LPL activity → TG accumulation. Severe hypertriglyceridaemia (TG >11.3 mmol/L) → MILKY PLASMA visible + risk of ACUTE PANCREATITIS. Metabolic syndrome prevalence rising with obesity epidemic.\n\n📹 [VIDEO LINK]"},{"id":741,"s":"Lipid Chemistry","t":"Milky Plasma","u":"Milky Plasma High Triglycerides","q":"Milky plasma may be caused by which of the following?","o":["High HDL","High LDL","High cholesterol","High phospholipids","High triglycerides"],"a":4,"p":"MILKY PLASMA (lipaemia) = caused by HIGH TRIGLYCERIDES, specifically very high concentrations of TG-rich particles (CHYLOMICRONS post-meal, VLDL in fasting hypertriglyceridaemia). Mechanism: large TG-rich particles (especially chylomicrons, VLDL) scatter light → plasma appears milky/turbid. Neither HDL nor LDL (relatively TG-poor) cause milky plasma even at very high concentrations. CHOLESTEROL alone (even very high LDL-C) does NOT cause milky plasma. Milky plasma = ALWAYS hypertriglyceridaemia until proven otherwise.\n\n📹 [VIDEO LINK]"},{"id":742,"s":"Lipid Chemistry","t":"Surfactant and ARDS","u":"Surfactant 90 Percent Lipids","q":"The lipids in pulmonary surfactant comprise which of the following?","o":["10 weight percent","30 weight percent","60 weight percent","80 weight percent","90 weight percent"],"a":4,"p":"PULMONARY SURFACTANT COMPOSITION: LIPIDS = ~90 weight percent (wt%). PROTEINS = ~10 wt% (SP-A, SP-B, SP-C, SP-D). LIPID breakdown: DPPC (dipalmitoylphosphatidylcholine) = ~45-50% of total surfactant (most abundant). Other phospholipids (unsaturated PC, PG, PE) = ~35-40%. Neutral lipids (cholesterol) = ~10%. TYPE II PNEUMOCYTES synthesise and secrete surfactant → stored in LAMELLAR BODIES → exocytosed into alveolar space → forms monolayer at air-liquid interface.\n\n📹 [VIDEO LINK]"},{"id":743,"s":"Lipid Chemistry","t":"Surfactant and ARDS","u":"Surfactant Proteins 10 Percent","q":"The surfactant-associated proteins in pulmonary surfactant comprise which of the following?","o":["10 weight percent","30 weight percent","60 weight percent","80 weight percent","90 weight percent"],"a":0,"p":"SURFACTANT PROTEINS = ~10 wt% of surfactant. Four surfactant proteins: SP-A and SP-D = HYDROPHILIC (large) → innate immunity (opsonins, collectins) → defend against pathogens in alveoli. SP-B and SP-C = HYDROPHOBIC (small) → essential for surface tension reduction (facilitate DPPC film spreading and stability). SP-B DEFICIENCY = fatal respiratory failure in neonates. SP-A is the most abundant surfactant protein and regulates tubular myelin formation (surfactant reservoir).\n\n📹 [VIDEO LINK]"},{"id":744,"s":"Lipid Chemistry","t":"Surfactant and ARDS","u":"DPPC Most Abundant Surface Tension Lowering Lipid","q":"The most abundant lipid with surface tension-lowering properties in pulmonary surfactant is which of the following?","o":["Cholesterol","Dipalmitoylphosphatidylcholine","Phosphatidal choline","Phosphatidylethanolamine","Sphingomyelin"],"a":1,"p":"DIPALMITOYLPHOSPHATIDYLCHOLINE (DPPC) = the MAJOR surface tension-reducing component of pulmonary surfactant (~45-50% of total surfactant). Why DPPC is unique: BOTH fatty acids are SATURATED PALMITIC ACID (16:0) → DPPC forms an extremely stable, tightly packed monolayer at the air-liquid interface → reduces surface tension to near zero during expiration → prevents alveolar collapse (atelectasis). Cholesterol and other phospholipids modulate fluidity but DPPC is the primary surface tension reducer. Deficiency → neonatal RDS (respiratory distress syndrome).\n\n📹 [VIDEO LINK]"},{"id":745,"s":"Lipid Chemistry","t":"Surfactant and ARDS","u":"Lecithin Sphingomyelin Ratio Lung Maturity","q":"Which of the following lecithin:sphingomyelin ratio is evidence of fetal lung maturity?","o":["0.25 or more","0.5 or more","1 or more","2 or more","10 or more"],"a":3,"p":"LECITHIN:SPHINGOMYELIN (L/S) RATIO in AMNIOTIC FLUID: ≥2 = FETAL LUNG MATURITY. Before 32 weeks: lecithin (DPPC) and sphingomyelin are roughly equal → L/S ratio ~1. At 32-34 weeks: type II pneumocytes switch from sphingomyelin to lecithin (DPPC) synthesis → L/S ratio rises sharply. ≥2 indicates sufficient surfactant production → lung maturity. <2 = immature lungs → risk of RDS if delivered. Glucocorticoids given to mother before preterm delivery (>24 hours before) → accelerate lung maturity → ↑ DPPC synthesis → ↑ L/S ratio.\n\n📹 [VIDEO LINK]"},{"id":746,"s":"Lipid Chemistry","t":"Surfactant and ARDS","u":"Surfactant Reduces Surface Tension Alveolar Interface","q":"A preterm neonate with respiratory distress syndrome was administered surfactant. The surfactant reduced which of the following at the alveolar air-liquid interface?","o":["Hydrostatic pressure","Osmotic pressure","Oxygen partial pressure","Surface tension","Viscosity"],"a":3,"p":"SURFACTANT reduces SURFACE TENSION at the AIR-LIQUID INTERFACE in alveoli. LaPlace's Law: pressure required to keep alveolus open = 2T/r (T = surface tension, r = radius). Without surfactant: high surface tension → small alveoli require very high pressure to stay open → small alveoli collapse (atelectasis) and large alveoli overdistend. SURFACTANT reduces T → reduces pressure needed → all alveoli can stay open at end of expiration → PREVENTS ATELECTASIS → improves gas exchange. RDS in premature infants = insufficient surfactant → respiratory failure.\n\n📹 [VIDEO LINK]"},{"id":747,"s":"Lipid Chemistry","t":"Lipoproteins","u":"Chylomicron After Fatty Meal","q":"A first-year medical student celebrated his outstanding performance by consuming a double-melt crust pizza with butter topping. Shortly afterwards, which of the following lipoproteins increased in his plasma?","o":["Chylomicrons","HDL","IDL","LDL","VLDL"],"a":0,"p":"After a fatty meal → dietary triglycerides + cholesterol absorbed → CHYLOMICRONS assembled in intestinal enterocytes (apoB-48 + TG + cholesterol + phospholipids) → enter lymphatics via lacteals → thoracic duct → bloodstream → immediate rise in plasma CHYLOMICRONS within 1-2 hours. Peak chylomicronaemia at 3-6 hours post-meal → plasma appears milky (lipaemia). VLDL is made by the liver from endogenous lipids (not the immediate post-meal response). LDL = VLDL remnant after TG removal. HDL = reverse cholesterol transport.\n\n📹 [VIDEO LINK]"},{"id":748,"s":"Lipid Chemistry","t":"Lipoproteins","u":"LDL Highest Cholesterol Content","q":"Which of the following has the highest cholesterol content?","o":["Chylomicron","Chylomicron remnants","HDL","IDL","LDL"],"a":4,"p":"LIPOPROTEIN COMPOSITION (approximate): LDL = ~50% CHOLESTEROL (highest cholesterol content) + 25% protein + 20% phospholipids + 10% TG. VLDL = ~65% TG + 20% cholesterol. HDL = ~20% cholesterol + 50% protein + 25% phospholipids. CHYLOMICRONS = ~85% TG + 5% cholesterol. IDL = intermediate composition. LDL delivers CHOLESTEROL to peripheral tissues via LDL receptor (apoB-100 ligand). LDL = PRIMARY TARGET for cardiovascular risk reduction (statins). Familial hypercholesterolaemia = LDL receptor deficiency → extreme LDL elevation.\n\n📹 [VIDEO LINK]"},{"id":749,"s":"Lipid Chemistry","t":"Lipoproteins","u":"VLDL Highest Triglyceride Content","q":"The highest content of triglycerides is found in which of the following lipoproteins?","o":["Chylomicron remnants","HDL","IDL","LDL","VLDL"],"a":4,"p":"LIPOPROTEIN TRIGLYCERIDE CONTENT (approximate): CHYLOMICRONS = ~85% TG (highest absolute TG but VLDL is the main fasting TG carrier). VLDL = ~65% TG (highest TG content among FASTING lipoproteins). IDL = ~35% TG. LDL = ~10% TG. HDL = ~5% TG. VLDL is synthesised by the LIVER → transports endogenous TG to peripheral tissues → LPL cleaves TG → VLDL remnant = IDL → further TG removal → LDL. Hypertriglyceridaemia (elevated VLDL) → milky plasma at high concentrations.\n\n📹 [VIDEO LINK]"},{"id":750,"s":"Lipid Chemistry","t":"Lipoproteins","u":"Energy from Fat 9 kcal Per Gram","q":"Energy liberated on metabolism of 1 gram of fat is which of the following?","o":["4 kcal","5 kcal","7 kcal","9 kcal","15 kcal"],"a":3,"p":"ENERGY YIELD: FAT (triacylglycerol) = 9 kcal/g (38 kJ/g) — highest energy density of all macronutrients. CARBOHYDRATE = 4 kcal/g. PROTEIN = 4 kcal/g. ALCOHOL = 7 kcal/g. Fat yields more energy because fatty acids are more reduced (more C-H bonds) than carbohydrates → more electrons transferred to ETC → more ATP generated. 1 molecule of palmitic acid (16:0) → beta-oxidation → 8 acetyl-CoA → TCA cycle + ETC → ~106 ATP. Compare: 1 glucose → ~30-32 ATP. This energy density makes TAG the most efficient long-term energy storage.\n\n📹 [VIDEO LINK]"},{"id":751,"s":"Lipid Chemistry","t":"Lipoproteins","u":"Membrane Fluidity Increased by Unsaturated FA","q":"Fluidity of the cell membrane may be increased if which of the following is increased?","o":["Branched-chain fatty acids","Cyclic fatty acids","Saturated fatty acids","Trans fatty acids","Unsaturated fatty acids"],"a":4,"p":"UNSATURATED FATTY ACIDS (cis configuration) → CIS DOUBLE BONDS create a KINK in the acyl chain → chains cannot pack tightly → INCREASE MEMBRANE FLUIDITY. Saturated FA pack tightly → reduce fluidity. Trans FA are more linear (like saturated) → pack tightly → DECREASE fluidity (similar to saturated FA). Cholesterol = fluidity buffer (increases fluidity among saturated FA, decreases fluidity among cis unsaturated FA). Temperature: high temperature → more fluidity. Cells regulate fluidity by adjusting FA composition (homeoviscous adaptation).\n\n📹 [VIDEO LINK]"},{"id":752,"s":"Lipid Chemistry","t":"Lipoproteins","u":"Essential FA Fat Free Diet Most Deficient","q":"A teenager maintains a fat-free diet for several weeks. He would be most deficient in his ability to synthesise which of the following?","o":["Cholesterol","Phospholipids","Saturated fatty acids","Triacylglycerol","Unsaturated fatty acids"],"a":3,"p":"Fat-free diet → no dietary fatty acids → body relies entirely on DE NOVO LIPOGENESIS from carbohydrates/proteins to make fatty acids. TRIACYLGLYCEROL synthesis is most directly impaired because: (1) TAGs require fatty acids as primary substrate. (2) De novo lipogenesis from carbohydrates can supply some saturated FA (palmitate) but is limited. (3) Cholesterol can be synthesised from acetyl-CoA (independent of dietary fat). (4) ESSENTIAL FATTY ACIDS (linoleic, linolenic) CANNOT be synthesised at all → their absence is the most critical specific deficiency → impaired eicosanoid synthesis → impaired skin barrier. However, TRIACYLGLYCEROL represents the QUANTITATIVELY greatest lipid that cannot be produced without adequate dietary fat.\n\n📹 [VIDEO LINK]"},{"id":756,"s":"Protein Chemistry","t":"Amino Acid Classification","u":"Amino Acids Amphoteric","q":"Amino acids can act either as an acid or a base. They are defined as which of the following?","o":["Acidic","Amphoteric","Basic","Non-polar","Polar"],"a":1,"p":"AMPHOTERIC = capable of acting as both ACID (donating H⁺ from the carboxyl group) and BASE (accepting H⁺ on the amino group). All amino acids are amphoteric because they have both a carboxyl group (–COOH, acidic) and an amino group (–NH₂, basic). In solution they exist as ZWITTERIONS (internal salts) — both groups are ionised simultaneously. This dual nature is the biochemical basis for their role as BUFFERS in blood and tissues.\n\n📹 [VIDEO LINK]"},{"id":757,"s":"Protein Chemistry","t":"Amino Acid Classification","u":"Isoelectric Point Neutral State","q":"Isoelectric point is the point at which the amino acid is present in which of the following states?","o":["Charged","Hydrophobic","Negatively charged","Neutral","Positively charged"],"a":3,"p":"ISOELECTRIC POINT (pI) = the pH at which an amino acid has a NET CHARGE of ZERO (neutral zwitterion — equal positive and negative charges, but net charge = 0). At pI: the amino acid does not migrate in an electric field. Below pI: net positive charge (protonated). Above pI: net negative charge (deprotonated). Clinical use: ISOELECTRIC FOCUSING separates proteins by their unique pI values. At pI, amino acid has minimum solubility → PRECIPITATION occurs (used in protein purification).\n\n📹 [VIDEO LINK]"},{"id":758,"s":"Protein Chemistry","t":"Amino Acid Classification","u":"Proline Beta Bends Turns","q":"Which of the following amino acids is present in beta bends and turns in protein structures?","o":["Asparagine","Isoleucine","Proline","Tryptophan","Tyrosine"],"a":2,"p":"PROLINE = found specifically in BETA BENDS and TURNS. Its rigid pyrrolidine ring introduces a fixed kink → DISRUPTS alpha-helix → naturally accommodated in BETA BENDS (type I, II, II' turns). Asparagine, tryptophan, and tyrosine disrupt alpha-helix due to bulky or polar side chains but are NOT specifically associated with beta bends. Isoleucine = branched-chain, nonpolar, found in hydrophobic core of beta-pleated sheets and helices — NOT in turns.\n\n📹 [VIDEO LINK]"},{"id":7581,"s":"Protein Chemistry","t":"Amino Acid Classification","u":"Non-Standard Amino Acid No Codon Hydroxyproline","q":"Which of the following amino acids has no codon in the genetic code and is therefore classified as a non-standard amino acid?","o":["Alanine","Hydroxyproline","Leucine","Phenylalanine","Valine"],"a":1,"p":"HYDROXYPROLINE = NON-STANDARD AMINO ACID — it has NO CODON in the genetic code. Formed POST-TRANSLATIONALLY by PROLYL HYDROXYLASE (vitamin C-dependent, in RER) acting on PROLINE after translation. Other non-standard amino acids with no codons: HYDROXYLYSINE (from lysine), GAMMA-CARBOXYGLUTAMATE (from glutamate, vitamin K-dependent), DESMOSINE (from lysine in elastin), CITRULLINE and ORNITHINE (urea cycle intermediates, no codons). Standard amino acids (alanine, leucine, phenylalanine, valine) each have one or more codons → incorporated directly during translation.\n\n📹 [VIDEO LINK]"},{"id":759,"s":"Protein Chemistry","t":"Amino Acid Classification","u":"Hepatic Encephalopathy BCAA","q":"An old man known case of hepatitis C presents with confusion, hepatomegaly, and a flapping tremor. He is diagnosed with hepatic encephalopathy. A diet rich in which of the following sets of amino acids is suitable for him?","o":["Aspartate, glutamate, asparagine","Glycine, alanine, serine","Methionine, proline, cysteine","Tryptophan, phenylalanine, tyrosine","Valine, leucine, isoleucine"],"a":4,"p":"HEPATIC ENCEPHALOPATHY: damaged liver cannot clear AROMATIC AMINO ACIDS (tryptophan, phenylalanine, tyrosine) → these cross the blood-brain barrier → converted to FALSE NEUROTRANSMITTERS (octopamine, serotonin) → encephalopathy. BRANCHED-CHAIN AMINO ACIDS (BCAA) = VALINE, LEUCINE, ISOLEUCINE → metabolised in MUSCLE (not liver) → safe in liver disease → BCAA diet competes with aromatic AA for BBB transport (same neutral amino acid carrier, LAT1) → reduces false neurotransmitter formation → improves encephalopathy.\n\n📹 [VIDEO LINK]"},{"id":760,"s":"Protein Chemistry","t":"Amino Acid Classification","u":"Tryptophan Mental Asylum Corn Diet","q":"Inmates of a mental asylum fed on corn as staple diet recovered from mental illnesses after their diet was fortified with neutral amino acids. Which of the following amino acids is likely to have helped in their recovery?","o":["Arginine","Aspartate","Glutamate","Lysine","Tryptophan"],"a":4,"p":"CORN is low in TRYPTOPHAN → insufficient tryptophan → cannot synthesise niacin (60mg tryptophan = 1mg niacin) → PELLAGRA → psychiatric manifestations (dementia, psychosis). Fortification with neutral amino acids including TRYPTOPHAN → restored niacin synthesis → mental illness reversed. HARTNUP DISEASE also involves defective neutral amino acid transport (SLC6A19) → tryptophan lost in urine → pellagra-like symptoms treated by tryptophan/niacin supplementation. Tryptophan is a NEUTRAL, AROMATIC, ESSENTIAL amino acid.\n\n📹 [VIDEO LINK]"},{"id":761,"s":"Protein Chemistry","t":"Amino Acid Classification","u":"Hartnup Disease Tryptophan Neutral Amino Acid","q":"A 7-year-old boy presents with a rash on sun-exposed skin, diarrhoea, and unsteady gait. Urinalysis shows increased neutral amino acids. He is diagnosed with Hartnup disease. Loss of which of the following amino acids in his urine is causing his symptoms?","o":["Alanine","Phenylalanine","Serine","Threonine","Tryptophan"],"a":4,"p":"HARTNUP DISEASE: SLC6A19 mutation → defective neutral amino acid transport → ALL NEUTRAL AMINO ACIDS lost in urine including TRYPTOPHAN. TRYPTOPHAN loss is clinically critical → insufficient niacin synthesis → PELLAGRA-LIKE SYMPTOMS (photosensitive dermatitis, cerebellar ataxia, psychiatric symptoms). Alanine, phenylalanine, serine, and threonine are also neutral amino acids and also lost — but their loss does not produce the characteristic symptoms. Tryptophan → niacin pathway is uniquely critical. Presents in CHILDHOOD (first few years of life).\n\n📹 [VIDEO LINK]"},{"id":762,"s":"Protein Chemistry","t":"Amino Acid Classification","u":"Strychnine Glycine Inhibitory Neurotransmitter","q":"A 46-year-old man is brought to the emergency room with muscle twitching, spasms, and difficulty swallowing after exposure to strychnine rodenticide. Strychnine blocks which of the following inhibitory neurotransmitters?","o":["Acetylcholine","Aspartate","Dopamine","Glutamate","Glycine"],"a":4,"p":"STRYCHNINE = competitive antagonist of GLYCINE RECEPTORS in the spinal cord. GLYCINE = inhibitory neurotransmitter (chloride channels → hyperpolarisation → inhibition of motor neurons). Strychnine blocks glycine receptors → motor neurons disinhibited → uncontrolled muscle contractions → TETANIC SPASMS → opisthotonus, trismus, dysphagia. GABA = the other major inhibitory neurotransmitter. GLUTAMATE and ASPARTATE = EXCITATORY. ACETYLCHOLINE and DOPAMINE = neither inhibitory amino acid neurotransmitters.\n\n📹 [VIDEO LINK]"},{"id":763,"s":"Protein Chemistry","t":"Amino Acid Classification","u":"Alanine Gluconeogenesis Starvation","q":"A mountaineer is trapped without food for 70 hours. Which of the following amino acids is helping him maintain blood glucose levels?","o":["Alanine","Citrulline","Leucine","Lysine","Ornithine"],"a":0,"p":"GLUCOSE-ALANINE CYCLE: starvation → muscle protein catabolism → ALANINE (from transamination of pyruvate) → liver → pyruvate → GLUCONEOGENESIS → glucose. Alanine = primary gluconeogenic amino acid. LEUCINE and LYSINE = purely KETOGENIC. CITRULLINE and ORNITHINE = NON-STANDARD urea cycle intermediates — not glucogenic through the alanine-glucose cycle.\n\n📹 [VIDEO LINK]"},{"id":764,"s":"Protein Chemistry","t":"Amino Acid Classification","u":"Ninhydrin Purple Colour Amino Acid","q":"During a ninhydrin test, a compound in solution reacts with ninhydrin resulting in release of CO₂ and formation of a purple colour. Which of the following is present in the solution?","o":["Glutamate","Malate","Oxaloacetate","Phytate","Pyruvate"],"a":0,"p":"NINHYDRIN reacts with PRIMARY AMINO ACIDS → RUHEMANN'S PURPLE (blue-purple) + CO₂ + aldehyde. GLUTAMATE = amino acid → positive ninhydrin (purple + CO₂). Malate, oxaloacetate, pyruvate = organic acids (no alpha-amino group) → NEGATIVE ninhydrin. Phytate = inositol hexaphosphate (no amino group) → negative. The purple colour + CO₂ evolution specifically indicates a PRIMARY ALPHA-AMINO ACID in solution.\n\n📹 [VIDEO LINK]"},{"id":7641,"s":"Protein Chemistry","t":"Amino Acid Classification","u":"Ninhydrin Yellow Colour Proline Hydroxyproline","q":"During a ninhydrin test, a compound in solution reacts with ninhydrin and a YELLOW colour is formed instead of purple. Which of the following is present in the solution?","o":["Alanine and glycine","Asparagine and glutamine","Aspartate and glutamate","Proline and hydroxyproline","Tyrosine and tryptophan"],"a":3,"p":"PROLINE and HYDROXYPROLINE = SECONDARY AMINO/IMINO ACIDS (nitrogen is part of pyrrolidine ring — not a free primary amine). Ninhydrin reacts differently with imino acids → YELLOW/ORANGE colour (not purple). NO CO₂ evolved. All other options (alanine, glycine, asparagine, glutamine, aspartate, glutamate, tyrosine, tryptophan) = PRIMARY AMINO ACIDS → purple colour + CO₂. Yellow colour with ninhydrin specifically identifies proline and hydroxyproline.\n\n📹 [VIDEO LINK]"},{"id":765,"s":"Protein Chemistry","t":"Amino Acid Classification","u":"GGT Gamma Glutamyl Cycle Amino Acid Absorption","q":"A 57-year-old comatose chronic alcoholic presents with elevated serum gamma-glutamyl transferase (GGT). As a result of elevated GGT activity, absorption of which of the following will be increased in him?","o":["Amino acids","Carbonic acids","Cholic acids","Fatty acids","Keto acids"],"a":0,"p":"GAMMA-GLUTAMYL CYCLE (Meister cycle): GGT (gamma-glutamyl transferase) transfers gamma-glutamyl group from GLUTATHIONE to an AMINO ACID → gamma-glutamyl-amino acid enters cell → released amino acid absorbed. GGT is present on the brush border of intestinal and renal tubular cells → facilitates AMINO ACID ABSORPTION. Elevated GGT in alcoholism = CYP450 enzyme induction by ethanol → increased GGT synthesis. GGT is the most sensitive marker of alcohol abuse and hepatobiliary disease.\n\n📹 [VIDEO LINK]"},{"id":766,"s":"Protein Chemistry","t":"Protein Structure","u":"Peptide Bond Primary Structure","q":"Which bond is present in the primary structure of a protein?","o":["Disulfide bond","Ester bond","Hydrogen bond","Ionic bond","Peptide bond"],"a":4,"p":"PRIMARY STRUCTURE of protein = the LINEAR SEQUENCE of amino acids linked by PEPTIDE BONDS (covalent amide bonds between alpha-carboxyl group of one amino acid and alpha-amino group of the next, with loss of water). Peptide bond characteristics: (1) RIGID and PLANAR (partial double bond character). (2) Trans configuration (R groups on opposite sides). (3) No free rotation around C-N bond. All other bonds (hydrogen bonds = secondary structure, disulfide bonds, ionic bonds, hydrophobic interactions = tertiary structure) stabilise higher levels of protein organisation.\n\n📹 [VIDEO LINK]"},{"id":767,"s":"Protein Chemistry","t":"Protein Structure","u":"Hydrolysis Breaks Peptide Bonds","q":"Hydrolysis of proteins involves breakdown of which of the following?","o":["Hydrogen bonds","Hydrophobic interactions","Ionic bonds","Peptide bonds","Van der Waals interactions"],"a":4,"p":"HYDROLYSIS of proteins = cleavage of PEPTIDE BONDS (covalent amide bonds) by addition of water → individual amino acids released. Catalysed by: PROTEASES (pepsin, trypsin, chymotrypsin, elastase — digestive enzymes). Chemical hydrolysis: acid or base + heat. Peptide bonds are COVALENT bonds within the primary structure. Hydrogen bonds, hydrophobic interactions, ionic bonds = non-covalent bonds of secondary/tertiary structure — these are broken by DENATURATION, not hydrolysis. Hydrolysis specifically cleaves the covalent peptide (amide) bonds.\n\n📹 [VIDEO LINK]"},{"id":768,"s":"Protein Chemistry","t":"Protein Structure","u":"Denaturation Does Not Break Peptide Bonds","q":"Denaturation of proteins does NOT break which of the following bonds?","o":["Disulfide bonds","Hydrogen bonds","Hydrophobic interactions","Ionic bonds","Peptide bonds"],"a":4,"p":"DENATURATION does NOT break PEPTIDE BONDS (covalent amide bonds — primary structure preserved). Denaturation DOES break: hydrogen bonds (secondary structure), hydrophobic interactions, ionic bonds (tertiary structure), and with strong reducing agents — disulfide bonds. PRIMARY STRUCTURE is the only level preserved after denaturation. Prions demonstrate this: PrPC → PrPSc involves secondary structure change with primary structure unchanged.\n\n📹 [VIDEO LINK]"},{"id":769,"s":"Protein Chemistry","t":"Protein Structure","u":"Denaturation Primary Structure Preserved","q":"When proteins become denatured, which of the following is preserved?","o":["Primary structure","Quaternary structure","Secondary structure","Supersecondary structure","Tertiary structure"],"a":0,"p":"DENATURATION preserves PRIMARY STRUCTURE (the sequence of amino acids held together by COVALENT PEPTIDE BONDS). Secondary, tertiary, and quaternary structures are held together by NON-COVALENT interactions (hydrogen bonds, hydrophobic interactions, ionic bonds) → these are disrupted by heat, pH extremes, detergents, urea. Some denaturation is REVERSIBLE (e.g. RNase A — renaturation restores activity — Anfinsen's experiment). Prion diseases involve MISFOLDING without changes to primary structure — demonstrating that primary structure alone does not determine final conformation.\n\n📹 [VIDEO LINK]"},{"id":770,"s":"Protein Chemistry","t":"Protein Structure","u":"Permanent Wave Tertiary Structure Disulfide","q":"A bride's hairdresser uses gel to break apart the S-S bonds in cystine units, reducing them to SH groups, then sets the hair in rollers to create a permanent wave. Which level of protein structure is most greatly affected?","o":["Primary structure","Secondary structure","Supersecondary structure","Tertiary structure","Quaternary structure"],"a":3,"p":"DISULFIDE BONDS (S-S bonds between cysteine residues) are TERTIARY STRUCTURE bonds — they stabilise the three-dimensional folding of polypeptide chains. Breaking disulfide bonds (with reducing agents like ammonium thioglycolate in permanent wave solutions) → unfolds the tertiary structure of KERATIN (hair protein) → hair can be reshaped → re-oxidation (with H₂O₂ or air) in new position → new disulfide bonds form → new 3D shape locked in = PERMANENT WAVE. The amino acid sequence (primary structure) and hydrogen bonds (secondary) are unaffected.\n\n📹 [VIDEO LINK]"},{"id":771,"s":"Protein Chemistry","t":"Protein Structure","u":"Glycation Glucose Structural Proteins Diabetes","q":"A 65-year-old man with type 2 diabetes has blurred vision and numbness in his toes. His HbA1c is 9.0. Non-enzymatic covalent bonds formed between glucose and structural proteins are responsible for his symptoms. This reaction is best classified as which of the following?","o":["Acylation","Carboxylation","Esterification","Glycation","Hydroxylation"],"a":3,"p":"GLYCATION = non-enzymatic, spontaneous covalent bonding between glucose and the free amino groups of proteins (Maillard reaction → AMADORI PRODUCTS → ADVANCED GLYCATION END PRODUCTS, AGEs). HbA1c = glycated haemoglobin → diagnostic marker for chronic hyperglycaemia. AGEs cause: diabetic nephropathy (glomerular basement membrane thickening), retinopathy (basement membrane changes → blurred vision), neuropathy (myelin glycation → numbness). Compare: GLYCOSYLATION = enzymatic addition of sugars to proteins (post-translational modification in Golgi).\n\n📹 [VIDEO LINK]"},{"id":772,"s":"Protein Chemistry","t":"Protein Structure","u":"Alzheimer Secondary Structure Alpha Helix to Beta Sheet","q":"An 85-year-old male presents with impairment of intellectual function and memory loss over 6 months. A provisional diagnosis of Alzheimer disease is made. Alteration at which of the following level of structural organisation occurred in the affected protein in his brain?","o":["Primary structure","Quaternary structure","Secondary structure","Supersecondary structure","Tertiary structure"],"a":2,"p":"ALZHEIMER'S DISEASE: Amyloid precursor protein (APP) → abnormally cleaved by BETA and GAMMA SECRETASES → Aβ42 peptide. Aβ42 undergoes conformational change from ALPHA-HELIX (normal, soluble) → BETA-PLEATED SHEET (abnormal, insoluble) → self-aggregates → AMYLOID PLAQUES (extracellular). This alpha-helix to beta-sheet transition = SECONDARY STRUCTURE alteration. The primary structure (amino acid sequence) of Aβ is unchanged — the problem is MISFOLDING at the secondary/tertiary level.\n\n📹 [VIDEO LINK]"},{"id":773,"s":"Protein Chemistry","t":"Protein Structure","u":"Alzheimer APP Abnormally Cleaved","q":"An 80-year-old male presents with impaired intellectual abilities and memory loss. Abnormal cleavage of which of the following protein is likely to occur in his brain?","o":["Amyloid A","Amyloid beta","Amyloid L","Amyloid precursor protein","Serum amyloid A"],"a":3,"p":"ALZHEIMER'S DISEASE: AMYLOID PRECURSOR PROTEIN (APP) is the protein that undergoes ABNORMAL CLEAVAGE. Normal pathway: ALPHA SECRETASE cleaves APP within the Aβ sequence → non-amyloidogenic → soluble fragments. Abnormal pathway: BETA SECRETASE (BACE1) + GAMMA SECRETASE sequentially cleave APP → releases Aβ42 PEPTIDE → misfolding → amyloid plaques. APP is a normal transmembrane protein — the problem is WHICH secretases cleave it. AMYLOID BETA is the PRODUCT of abnormal cleavage — not the protein being cleaved.\n\n📹 [VIDEO LINK]"},{"id":774,"s":"Protein Chemistry","t":"Protein Structure","u":"Alzheimer Autopsy Amyloid Beta Plaques","q":"An 83-year-old female dies after years of progressive memory loss. Which of the following is the likely autopsy finding in her brain?","o":["Amyloid A","Amyloid alpha","Amyloid beta","Amyloid L","Serum amyloid A"],"a":2,"p":"ALZHEIMER'S DISEASE autopsy: two hallmark findings: (1) EXTRACELLULAR AMYLOID PLAQUES composed of AMYLOID BETA (Aβ42) — core surrounded by dystrophic neurites. (2) INTRACELLULAR NEUROFIBRILLARY TANGLES composed of HYPERPHOSPHORYLATED TAU protein. Amyloid A = secondary amyloidosis (deposits in liver, spleen, kidney). Amyloid L = primary amyloidosis (from immunoglobulin light chains). Serum amyloid A = precursor in secondary amyloidosis. Brain autopsy in Alzheimer's = Aβ42 plaques + tau tangles.\n\n📹 [VIDEO LINK]"},{"id":775,"s":"Protein Chemistry","t":"Protein Structure","u":"Alzheimer Hyperphosphorylated Tau Tangles","q":"An 83-year-old female dies after years of memory loss. Which of the following is the likely biochemical composition of tangled bundles of fibres seen in her brain at autopsy?","o":["Alpha-synuclein aggregates","Dephosphorylated tau protein","Hyperphosphorylated tau protein","Normal tau protein","Serum amyloid A"],"a":2,"p":"NEUROFIBRILLARY TANGLES in Alzheimer's = HYPERPHOSPHORYLATED TAU PROTEIN. Normal tau: associated with microtubules → stabilises microtubule assembly → neuronal transport. In AD: KINASES (CDK5, GSK-3β) HYPERPHOSPHORYLATE tau → tau dissociates from microtubules → self-aggregates into PAIRED HELICAL FILAMENTS → NEUROFIBRILLARY TANGLES (intracellular). Microtubule collapse → disrupted axonal transport → neuronal death. Tau tangles = INTRACELLULAR (in neuron bodies). Aβ plaques = EXTRACELLULAR.\n\n📹 [VIDEO LINK]"},{"id":776,"s":"Protein Chemistry","t":"Protein Structure","u":"Alzheimer Abnormal Secretases Beta Gamma","q":"An 80-year-old male with Alzheimer disease. Which of the following enzymes abnormally cleave the affected protein in him?","o":["Alpha and beta secretases","Alpha and gamma secretases","Alpha secretase only","Beta and gamma secretases","Gamma secretase only"],"a":3,"p":"ABNORMAL (amyloidogenic) APP cleavage: BETA SECRETASE (BACE1) cleaves at N-terminus of Aβ domain → C99 fragment. GAMMA SECRETASE then cleaves C99 at C-terminus → releases Aβ42 (toxic, aggregation-prone) → amyloid plaques. Both BETA AND GAMMA SECRETASES are required for the abnormal pathway. ALPHA SECRETASE is the normal pathway enzyme — it cleaves within the Aβ sequence → CANNOT release intact Aβ42 → non-amyloidogenic. If alpha secretase is overactive, Alzheimer's risk decreases.\n\n📹 [VIDEO LINK]"},{"id":777,"s":"Protein Chemistry","t":"Protein Structure","u":"Alzheimer Normal Alpha Gamma Secretases","q":"An 80-year-old male with Alzheimer disease. Which of the following enzymes normally cleave the affected protein before the development of disease?","o":["Alpha and beta secretases","Alpha and gamma secretases","Alpha secretase only","Beta and gamma secretases","Gamma secretase only"],"a":1,"p":"NORMAL (non-amyloidogenic) APP cleavage: ALPHA SECRETASE cleaves APP WITHIN the Aβ sequence → sAPPα (soluble, neuroprotective) + C83 fragment. GAMMA SECRETASE then cleaves C83 → releases p3 (non-toxic, non-amyloidogenic). So the normal pathway involves ALPHA + GAMMA SECRETASES → produces harmless fragments. The ABNORMAL pathway substitutes BETA SECRETASE for ALPHA SECRETASE → produces Aβ42 instead of p3. The key difference: alpha vs beta secretase determines which pathway dominates.\n\n📹 [VIDEO LINK]"},{"id":778,"s":"Protein Chemistry","t":"Protein Structure","u":"Alzheimer Biochemical Mechanism Abnormal Cleavage","q":"An 80-year-old male presents with impaired intellectual function and memory loss from the last 6 months, tentatively diagnosed with Alzheimer disease. Which of the following is the biochemical mechanism involved in the development of this disease?","o":["Abnormal proteolytic cleavage of a normal protein","Altered gene expression","Altered post-transcriptional modification","Altered post-translational modification","Altered version of a normal protein"],"a":0,"p":"ALZHEIMER'S DISEASE: The primary biochemical mechanism = ABNORMAL PROTEOLYTIC CLEAVAGE of AMYLOID PRECURSOR PROTEIN (APP) — a NORMAL protein. APP (normal, transmembrane) → normally cleaved by alpha secretase (non-amyloidogenic). In AD: beta + gamma secretases cleave APP → Aβ42 → misfolding → plaques. APP gene and protein sequence are NORMAL — the PROCESSING is abnormal. Compare: PRION DISEASE = altered version of a normal protein (PrPC → PrPSc, same sequence). This distinguishes Alzheimer's from prion disease biochemistry.\n\n📹 [VIDEO LINK]"},{"id":779,"s":"Protein Chemistry","t":"Protein Structure","u":"Alzheimer Amyloid Plaque Composition","q":"An 81-year-old female dies in a road accident. Autopsy reveals amyloid plaques in her brain. Which of the following is the composition of these amyloid plaques?","o":["Abnormal tau protein","Amyloid beta peptide","Amyloid precursor protein","Normal tau protein","Serum amyloid A"],"a":1,"p":"AMYLOID PLAQUES in Alzheimer's brain = AMYLOID BETA PEPTIDE (Aβ42) arranged in BETA-PLEATED SHEET configuration → insoluble, self-aggregating fibrils → extracellular deposits. Aβ42 is a 42 amino acid fragment produced by abnormal APP cleavage. Tau protein = NEUROFIBRILLARY TANGLES (intracellular) — not plaques. Amyloid precursor protein (APP) = the PRECURSOR that gets cleaved, not the plaque component itself. Serum amyloid A = secondary amyloidosis precursor. BRAIN amyloid plaques = exclusively Aβ42.\n\n📹 [VIDEO LINK]"},{"id":780,"s":"Protein Chemistry","t":"Prion Disease","u":"CJD Transmissible Protein Agent Cadaveric hGH","q":"A 19-year-old girl who received cadaveric growth hormone develops iatrogenic Creutzfeldt-Jakob disease. Which of the following is the transmissible agent in this disease?","o":["Carbohydrate","DNA","Lipid","Protein","RNA"],"a":3,"p":"PRION DISEASES (CJD, kuru, scrapie, BSE) are caused by PROTEINACEOUS INFECTIOUS PARTICLES (PRIONS) — the transmissible agent is a PROTEIN (PrPSc) with NO associated nucleic acid (no DNA, no RNA). PrPSc = misfolded isoform of normal PrPC (prion protein). Cadaveric growth hormone was extracted from human pituitary glands — if contaminated with PrPSc → IATROGENIC CJD. This ended the use of cadaveric-derived pituitary hormones (replaced by recombinant hGH). The protein-only hypothesis = Nobel Prize, Stanley Prusiner 1997.\n\n📹 [VIDEO LINK]"},{"id":781,"s":"Protein Chemistry","t":"Prion Disease","u":"Kuru Alpha Helices to Beta Sheet","q":"People from New Guinea practised cannibalism eating the brains of dead people. Deaths from Kuru were reported. Which alteration in protein structure caused the death of these people?","o":["Alpha helices replaced by beta bends","Alpha helices replaced by beta sheet","Beta sheet replaced by alpha helices","Beta sheet replaced by amyloid beta","Motifs replaced by beta sheet"],"a":1,"p":"KURU = prion disease spread by cannibalism (eating infected brains). PrPC (normal prion protein) = rich in ALPHA HELICES (soluble, functional). PrPSc (pathological) = rich in BETA SHEETS (insoluble, protease-resistant). Conversion: ALPHA HELICES → BETA SHEET (secondary structure change, same primary sequence). PrPSc acts as a TEMPLATE → converts normal PrPC molecules by inducing the same conformational change → exponential accumulation → neurodegeneration → KURU (ataxia, tremor, death). The primary structure of PrP is UNCHANGED — only the folding changes.\n\n📹 [VIDEO LINK]"},{"id":782,"s":"Protein Chemistry","t":"Prion Disease","u":"CJD Pathologist Prion Protein in Circulation","q":"A pathologist doing an autopsy of a patient who died from Creutzfeldt-Jakob disease accidentally cuts himself while examining the brain. He is primarily concerned about which of the following materials entering his circulation?","o":["Bacteria","Lipid","Nucleotide","Protein","Virus"],"a":3,"p":"The pathologist is concerned about PROTEIN — specifically PrPSc (the misfolded prion protein). Prions are PROTEIN-ONLY infectious agents — no bacteria, no virus, no nucleic acid. If PrPSc enters circulation via the cut → can potentially infect the pathologist's nervous system → CJD. PrPSc is PROTEOLYTICALLY RESISTANT (resistant to proteinase K in standard concentrations) and NOT inactivated by standard sterilisation procedures (autoclaving must be at higher temperatures for prion decontamination). Special precautions required during prion disease autopsies.\n\n📹 [VIDEO LINK]"},{"id":783,"s":"Protein Chemistry","t":"Prion Disease","u":"Prion Acts as Template","q":"An altered version of a normal protein in prion disease acts as which of the following?","o":["Activator","Catalyst","Inhibitor","Template","Vector"],"a":3,"p":"PrPSc acts as a TEMPLATE (mould) — it induces conformational change in normal PrPC molecules without changing their primary structure. Mechanism: PrPSc binds PrPC → forces PrPC to adopt the beta-sheet-rich PrPSc conformation → one PrPSc creates two → exponential propagation. This is analogous to a template in DNA replication but purely at the PROTEIN level (no nucleic acid involved). Prusiner's protein-only hypothesis: misfolded protein can propagate its misfolded conformation — a revolutionary concept in biology.\n\n📹 [VIDEO LINK]"},{"id":784,"s":"Protein Chemistry","t":"Prion Disease","u":"CJD Alpha Helices to Beta Sheet Biochemical Alteration","q":"A 60-year-old female is diagnosed with sporadic Creutzfeldt-Jakob disease after rapid memory deterioration, unsteady gait, and jerking movements. Which of the following biochemical alteration is present in the affected protein?","o":["Alpha helices to beta bends","Alpha helices to beta sheet","Beta bends to beta barrel","Beta sheet to alpha helices","Beta sheet to beta bends"],"a":1,"p":"CJD: PrPC (normal) = predominantly ALPHA HELICES (about 40% alpha-helix, 3% beta-sheet). PrPSc (pathological) = predominantly BETA SHEETS (about 43% beta-sheet, 30% alpha-helix). Conversion: ALPHA HELICES → BETA SHEET. Same primary structure, different folding. PrPSc is: (1) Insoluble. (2) Protease-resistant. (3) Forms amyloid fibrils. (4) Neurotoxic. The beta-sheet-rich conformation is common to ALL amyloidogenic proteins (Alzheimer's Aβ, transthyretin, alpha-synuclein) — they all undergo this transition to form insoluble aggregates.\n\n📹 [VIDEO LINK]"},{"id":785,"s":"Protein Chemistry","t":"Prion Disease","u":"Parkinson Alpha Synuclein Lewy Bodies","q":"Which of the following misfolded protein aggregates are found in Parkinson disease?","o":["Alpha-synuclein","Amyloid beta peptide","Beta-2-microglobulin","Prion protein","Transthyretin"],"a":0,"p":"PARKINSON'S DISEASE: misfolded ALPHA-SYNUCLEIN aggregates → LEWY BODIES (intracytoplasmic inclusions in dopaminergic neurons of substantia nigra) → neuronal death → dopamine depletion → Parkinson's features (bradykinesia, rigidity, resting tremor, postural instability). Alpha-synuclein = small protein normally at presynaptic terminals, role in vesicle trafficking. In PD: alpha-synuclein misfolds → oligomers → fibrils → Lewy bodies. Compare: Alzheimer's = Aβ42 plaques + tau tangles. CJD = PrPSc. Secondary amyloidosis = amyloid A. Dialysis amyloidosis = beta-2-microglobulin.\n\n📹 [VIDEO LINK]"},{"id":786,"s":"Protein Chemistry","t":"Prion Disease","u":"Secondary Amyloidosis Amyloid A Ex Army","q":"An ex-army chief suffered multiple problems during retirement due to secondary amyloidosis. Which of the following misfolded protein aggregates accumulated in him?","o":["Amyloid beta","Amyloid precursor protein","Amyloid A","Islet amyloid polypeptide","Transthyretin"],"a":2,"p":"SECONDARY AMYLOIDOSIS (AA amyloidosis): precursor protein = SERUM AMYLOID A (SAA, a positive acute phase reactant elevated in chronic inflammation) → SAA fragments misfold → deposit as AMYLOID A (AA). Common in: chronic inflammatory diseases (rheumatoid arthritis, TB, osteomyelitis, bronchiectasis — common in military veterans with chronic infections). Deposits primarily in LIVER, SPLEEN, KIDNEYS. An ex-army chief with chronic infections = classic secondary amyloidosis history. Compare: PRIMARY amyloidosis = amyloid L (immunoglobulin light chains).\n\n📹 [VIDEO LINK]"},{"id":787,"s":"Protein Chemistry","t":"Plasma Proteins","u":"Albumin Oncotic Pressure","q":"Which of the following is the major determinant of plasma osmotic pressure?","o":["Albumin","Fibrinogen","Globulin","Haptoglobulin","Hepcidin"],"a":0,"p":"ALBUMIN (MW ~66 kDa) = most abundant plasma protein (~4g/dL, ~60% of total plasma protein) → MAJOR DETERMINANT of COLLOID OSMOTIC PRESSURE (oncotic pressure). Albumin's large size prevents it from crossing capillary walls → retains water in vascular compartment via osmotic effect. HYPOALBUMINAEMIA → ↓ oncotic pressure → water moves from blood into tissues → OEDEMA (pedal oedema, ascites, pulmonary oedema). Causes of hypoalbuminaemia: liver disease (decreased synthesis), nephrotic syndrome (renal loss), malnutrition (decreased synthesis), protein-losing enteropathy.\n\n📹 [VIDEO LINK]"},{"id":788,"s":"Protein Chemistry","t":"Plasma Proteins","u":"Albumin Transports Fatty Acids","q":"Albumin facilitates movement of which of the following through circulation?","o":["Amino acids","Carbohydrates","Carboxylic acids","Fatty acids","Vitamins"],"a":3,"p":"ALBUMIN transports FREE FATTY ACIDS (non-esterified fatty acids) in blood. Free fatty acids are insoluble in aqueous plasma → must be bound to albumin for transport. One albumin molecule can carry up to 6 fatty acids. Albumin also transports: bilirubin (indirect/unconjugated), bile acids, drugs (warfarin, NSAIDs, penicillin), hormones (thyroid hormones, cortisol), metals (Cu²⁺, Ca²⁺). Albumin has no enzymatic activity — it is a TRANSPORT and BUFFER protein. Negative acute phase reactant (decreases in inflammation).\n\n📹 [VIDEO LINK]"},{"id":789,"s":"Protein Chemistry","t":"Plasma Proteins","u":"Digoxin Classified as Glycoside","q":"A 67-year-old man with congestive heart failure takes digoxin. Digoxin is an ester containing a sugar component and a non-sugar aglycone component attached via oxygen. Digoxin would best be classified as which of the following?","o":["Glucosteroid","Glycoprotein","Glycoside","Oligosaccharide","Thioester"],"a":2,"p":"GLYCOSIDE = a compound containing a sugar (glycone) component linked via an OXYGEN (or other atom) to a non-sugar (aglycone) component. DIGOXIN = CARDIAC GLYCOSIDE: steroid aglycone + three digitoxose sugar units linked via O-glycosidic bond. Mechanism: inhibits Na⁺/K⁺-ATPase → ↑ intracellular Na⁺ → Na⁺/Ca²⁺ exchanger brings in more Ca²⁺ → ↑ cardiac contractility (positive inotropic effect). THIOESTER = sulfur instead of oxygen. GLYCOPROTEIN = protein with covalently attached oligosaccharides. GLYCOSIDE = non-protein sugar-aglycone conjugate.\n\n📹 [VIDEO LINK]"},{"id":790,"s":"Protein Chemistry","t":"Plasma Proteins","u":"C-Peptide Insulinoma Hypoglycaemia","q":"A 47-year-old woman passes out at work and is found to have blood glucose of 24 mg/dL. C-peptide is much higher than normal. What is a possible cause of her condition?","o":["Anorexia","Exogenous insulin administration","High glucose diet","High protein diet","Insulin-secreting islet cell tumour"],"a":4,"p":"HIGH C-PEPTIDE + HYPOGLYCAEMIA = ENDOGENOUS INSULIN OVERPRODUCTION = INSULINOMA (insulin-secreting islet cell tumour). Proinsulin → cleaved → insulin + C-PEPTIDE (equimolar). EXOGENOUS INSULIN administration → LOW C-peptide (commercial insulin has no C-peptide, suppresses endogenous insulin secretion) + hypoglycaemia — factitious hypoglycaemia. INSULINOMA → HIGH C-peptide + HIGH insulin + hypoglycaemia. C-peptide measurement is the key diagnostic test to distinguish factitious from endogenous hypoglycaemia. Insulinoma = most common pancreatic endocrine tumour.\n\n📹 [VIDEO LINK]"},{"id":791,"s":"Protein Chemistry","t":"Collagen","u":"OI Collagen I Mutation Blue Sclera Fractures","q":"A neonate sustains multiple fractures during delivery and has bluish sclera. Mutation in which of the following type of collagen results in the development of her disease?","o":["Collagen I","Collagen II","Collagen III","Collagen IV","Collagen V"],"a":0,"p":"OSTEOGENESIS IMPERFECTA (brittle bone disease): mutations in COL1A1 or COL1A2 genes → defective TYPE I COLLAGEN. Type I collagen is the most abundant collagen (present in bone, skin, tendons, sclera). Features: BLUE SCLERA (thin sclera allows underlying choroid to show through), MULTIPLE FRACTURES (on minimal trauma), diminished hearing (ossicular chain collagen defects), dentinogenesis imperfecta (opalescent teeth), hypermobile joints. The most severe form (OI Type II) = blue sclera + fractures at birth (perinatal lethal).\n\n[Note to editor: Insert diagram of collagen synthesis pathway showing COL1A1/COL1A2 mutation sites]\n\n📹 [VIDEO LINK]"},{"id":792,"s":"Protein Chemistry","t":"Collagen","u":"OI Brittle Bone Disease","q":"Which of the following is known as brittle bone disease?","o":["Alpha-1-antitrypsin deficiency","Ehler's Danlos syndrome","Marfan's syndrome","Osteogenesis Imperfecta","Rickets"],"a":3,"p":"OSTEOGENESIS IMPERFECTA (OI) = BRITTLE BONE DISEASE. Type I collagen mutation → structurally defective bone matrix → fragile bones → fractures on minimal trauma. Hallmarks: blue sclera, multiple fractures, diminished hearing, dentinogenesis imperfecta. FOUR TYPES: Type I (mild, blue sclera, fractures, normal stature), Type II (perinatal lethal), Type III (severe, progressive deformity), Type IV (moderate, normal sclera). Treatment: bisphosphonates (reduce osteoclast activity), orthopaedic interventions.\n\n📹 [VIDEO LINK]"},{"id":793,"s":"Protein Chemistry","t":"Collagen","u":"OI Glycine Replaced by Bulky Side Chains","q":"A baby with blue sclerae presents with a history of fracture on minor trauma. He likely has a mutation in genes encoding alpha1 or alpha2 chains in type I collagen, which leads to replacement of glycine with amino acids containing which of the following?","o":["Acidic side chains","Basic side chains","Bulky side chains","Non-polar side chains","Polar side chains"],"a":2,"p":"In OI (and many collagen disorders): GLYCINE (the smallest amino acid, required at every third position in the Gly-X-Y collagen repeat sequence) is replaced by amino acids with BULKY SIDE CHAINS. Glycine's small H side chain is the only group that fits in the centre of the collagen triple helix. Replacing glycine with ANY other amino acid (especially bulky ones) → DISRUPTS TRIPLE HELIX FORMATION → structurally weak collagen → OI. Even conservative replacements (serine, alanine) cause disease. The more bulky the replacement, the more severe the OI phenotype.\n\n📹 [VIDEO LINK]"},{"id":794,"s":"Protein Chemistry","t":"Collagen","u":"EDS Collagen Mutations Types I III V","q":"Mutations in which of the following types of collagen are responsible for Ehlers-Danlos syndrome?","o":["I, III and V","II, III and VI","IV, V and VIII","IX, X and XII","VI, VIII and IX"],"a":0,"p":"EHLERS-DANLOS SYNDROME (EDS) — multiple types with different collagen mutations: CLASSIC EDS = Collagen V mutation (COL5A1/COL5A2). VASCULAR EDS = Collagen III mutation (COL3A1) — most severe, risk of arterial rupture. KYPHOSCOLIOTIC EDS = Lysyl hydroxylase deficiency (PLOD1). ARTHROCHALASIA EDS = Collagen I mutation (COL1A1/COL1A2). EDS features: hyperextensible skin, hypermobile joints, easy bruising. Collagen II mutations → skeletal/cartilage disorders (achondroplasia). Collagen IV mutations → Alport syndrome (glomerular basement membrane).\n\n📹 [VIDEO LINK]"},{"id":795,"s":"Protein Chemistry","t":"Collagen","u":"EDS Collagen Defect Hyperextensible Joints","q":"A 15-year-old female with chronic musculoskeletal pain, fatigue, and hypermobility of joints is confirmed to have mild classic Ehlers-Danlos syndrome. She has a defect in the synthesis of which of the following?","o":["Collagen","Elastin","Fibrillin","Keratin","Myosin"],"a":0,"p":"CLASSIC EHLERS-DANLOS SYNDROME = defect in COLLAGEN (specifically collagen V, from COL5A1/COL5A2 mutations). Collagen V is a fibrillar collagen that regulates the diameter of collagen I fibrils. Without proper collagen V → abnormally thick collagen fibrils → structurally weak connective tissue → hyperextensible skin (stretches easily, velvety texture), HYPERMOBILE JOINTS (Beighton score ≥5), easy bruising, chronic musculoskeletal pain, fatigue. Not to be confused with MARFAN'S (fibrillin defect) or OI (collagen I defect).\n\n📹 [VIDEO LINK]"},{"id":796,"s":"Protein Chemistry","t":"Collagen","u":"Vitamin C Triple Helix Formation Impaired","q":"Formation of which of the following is impaired in vitamin C deficiency?","o":["Alpha helix","Beta barrel","Beta bends","Beta pleated sheet","Triple helix"],"a":4,"p":"VITAMIN C (ascorbate) is required for PROLYL HYDROXYLASE and LYSYL HYDROXYLASE (both require Fe²⁺, O₂, alpha-ketoglutarate, and vitamin C as electron donor to maintain Fe²⁺). Without vitamin C: Fe²⁺ → Fe³⁺ (oxidised) → hydroxylases inactive → PROLINE and LYSINE NOT HYDROXYLATED → HYDROXYPROLINE and HYDROXYLYSINE not formed → COLLAGEN TRIPLE HELIX CANNOT BE STABILISED (hydroxyproline provides hydroxyl groups for hydrogen bonds that stabilise the triple helix) → unstable collagen → SCURVY.\n\n📹 [VIDEO LINK]"},{"id":797,"s":"Protein Chemistry","t":"Collagen","u":"Vitamin C Collagen Processing in RER","q":"Vitamin C is required for processing of collagen in which of the following?","o":["Cytosol","Extracellular matrix","Golgi apparatus","Rough endoplasmic reticulum","Smooth endoplasmic reticulum"],"a":3,"p":"Vitamin C is required for PROLYL HYDROXYLASE and LYSYL HYDROXYLASE which operate in the ROUGH ENDOPLASMIC RETICULUM (RER) — where collagen procollagen chains are hydroxylated after translation. Collagen synthesis sequence: RIBOSOMES → pre-procollagen synthesised → signal sequence cleaved in RER → PROLINE and LYSINE HYDROXYLATED in RER lumen (vitamin C-dependent hydroxylases) → glycosylation in RER and Golgi → triple helix formation in RER → procollagen secreted → Golgi → secretory vesicles → extracellular space.\n\n📹 [VIDEO LINK]"},{"id":798,"s":"Protein Chemistry","t":"Collagen","u":"Vitamin C Lysyl Hydroxylase Ferrous Iron","q":"Vitamin C maintains ferrous iron in the active site of which of the following collagen-processing enzymes?","o":["C-procollagen peptidase","Glycosyl transferase","Lysyl hydroxylase","Metalloproteinase","N-procollagen peptidase"],"a":2,"p":"VITAMIN C maintains Fe²⁺ in the active site of LYSYL HYDROXYLASE (and prolyl hydroxylase). During hydroxylation, Fe²⁺ is oxidised to Fe³⁺ → vitamin C REDUCES Fe³⁺ back to Fe²⁺ → enzyme remains active. IMPORTANT DISTINCTIONS: LYSYL OXIDASE = COPPER-dependent enzyme — NOT vitamin C-dependent — crosslinks collagen and elastin extracellularly. METALLOPROTEINASE = ZINC-dependent enzyme that DEGRADES collagen in ECM remodelling — not involved in biosynthesis. GLYCOSYL TRANSFERASE = adds sugar residues in Golgi. Vitamin C → hydroxylases only.\n\n📹 [VIDEO LINK]"},{"id":799,"s":"Protein Chemistry","t":"Collagen","u":"Hydroxylase Requires O2 and Ferrous Iron","q":"Which of the following is required for the proper functioning of prolyl hydroxylase and lysyl hydroxylase?","o":["Molecular O₂ and copper","Molecular O₂ and ferrous iron","Oxygen radical and copper","Oxygen radical and ferric iron","Vitamin K and calcium"],"a":1,"p":"PROLYL HYDROXYLASE and LYSYL HYDROXYLASE require: (1) MOLECULAR O₂ (not reactive oxygen species). (2) Fe²⁺ (FERROUS IRON — maintained by vitamin C). (3) Alpha-ketoglutarate (2-oxoglutarate) as co-substrate. (4) VITAMIN C (electron donor to regenerate Fe²⁺). Reaction: proline + O₂ + alpha-ketoglutarate + Fe²⁺ → hydroxyproline + succinate + CO₂. Iron must remain as Fe²⁺ (ferrous) — vitamin C prevents oxidation to Fe³⁺ (ferric). Without vitamin C → Fe³⁺ → hydroxylases inactive → scurvy.\n\n📹 [VIDEO LINK]"},{"id":800,"s":"Protein Chemistry","t":"Collagen","u":"Signal Sequence Cleaved in RER","q":"The signal sequence of preprocollagen is cleaved inside which of the following organelles?","o":["Golgi apparatus","Lysosomes","Rough endoplasmic reticulum","Smooth endoplasmic reticulum","Secretory vesicles"],"a":2,"p":"Collagen synthesis: RIBOSOMES synthesise PREPROCOLLAGEN (with N-terminal signal sequence). SIGNAL SEQUENCE directs the polypeptide to the ROUGH ER membrane → co-translational translocation into RER lumen → SIGNAL PEPTIDASE in RER membrane CLEAVES signal sequence → now called PROCOLLAGEN (minus signal sequence) inside RER. Then: hydroxylation (vitamin C-dependent), glycosylation, triple helix formation — all in RER. Further glycosylation and processing in Golgi. Extracellular: propeptide cleavage → tropocollagen → fibril assembly.\n\n📹 [VIDEO LINK]"},{"id":801,"s":"Protein Chemistry","t":"Collagen","u":"Fibrillar Collagen Triple Helix Rope","q":"Which of the following has long rigid structures in which three collagen polypeptide chains are wrapped around one another in a rope-like triple helix?","o":["Fibril-associated collagen","Fibrillar collagen","Network-forming collagen","Short-chain collagen","Tropocollagen"],"a":1,"p":"FIBRILLAR COLLAGENS (Type I, II, III, V, XI) = LONG, CONTINUOUS TRIPLE HELIX structure. Three alpha chains wound around each other in a right-handed triple helix (each chain is a left-handed polyproline II helix). The triple helix requires GLYCINE at every third position (Gly-X-Y repeat, where X is often proline, Y is often hydroxyproline). Network-forming collagens (Type IV) = interrupted triple helix → sheet-like networks (basement membrane). Fibril-associated collagens (FACITs) = Type IX, XII → on surface of fibrils.\n\n📹 [VIDEO LINK]"},{"id":802,"s":"Protein Chemistry","t":"Collagen","u":"Tropocollagen Spontaneously Forms Fibrils","q":"Which of the following spontaneously associates to form collagen fibrils?","o":["Collagen","Preprocollagen","Procollagen","Propeptide","Tropocollagen"],"a":3,"p":"TROPOCOLLAGEN = the basic structural unit of collagen fibrils (300 nm long, 1.5 nm diameter). Formed extracellularly when procollagen peptidases remove N- and C-terminal propeptides from procollagen → TROPOCOLLAGEN. Tropocollagen molecules then SPONTANEOUSLY SELF-ASSEMBLE in a quarter-stagger arrangement → COLLAGEN FIBRILS → COLLAGEN FIBRES. The self-assembly is driven by hydrophobic interactions between tropocollagen molecules. PROCOLLAGEN cannot self-assemble (propeptides prevent premature fibril formation inside the cell).\n\n📹 [VIDEO LINK]"},{"id":803,"s":"Protein Chemistry","t":"Collagen","u":"Procollagen Peptidases Produce Tropocollagen","q":"During collagen synthesis, procollagen peptidases remove terminal propeptides producing which of the following?","o":["Collagen","Mature fibril","Preprocollagen","Procollagen","Tropocollagen"],"a":4,"p":"PROCOLLAGEN (with N- and C-terminal propeptides) → SECRETED INTO EXTRACELLULAR SPACE → PROCOLLAGEN PEPTIDASES (N-proteinase and C-proteinase) cleave BOTH TERMINAL PROPEPTIDES → TROPOCOLLAGEN released. The propeptides prevent premature fibril assembly inside cells. Deficiency of procollagen peptidases → DERMATOSPARAXIS EDS (type VII EDS) → accumulation of pNCollagen (with N-propeptide intact) → abnormal fibril formation → extreme skin fragility. Tropocollagen → spontaneous self-assembly → fibrils.\n\n📹 [VIDEO LINK]"},{"id":804,"s":"Protein Chemistry","t":"Collagen","u":"Lysyl Oxidase Covalent Crosslinks Collagen","q":"Which of the following is essential for covalent crosslinking of collagen?","o":["Lysyl hydroxylase","Lysyl oxidase","Prolyl hydroxylase","Procollagen peptidase","Transglutaminase"],"a":1,"p":"LYSYL OXIDASE (copper-dependent extracellular enzyme) = ESSENTIAL for COVALENT CROSSLINKING of COLLAGEN (and elastin). Mechanism: oxidatively deaminates lysine and hydroxylysine → ALLYSINE and HYDROXYALLYSINE (reactive aldehydes) → spontaneous condensation with adjacent lysine/allysine → PYRIDINOLINE and PYRROLE crosslinks → stabilise collagen fibrils. Deficiency: MENKES DISEASE (copper deficiency → lysyl oxidase inactive → weak connective tissue). LATHYRISM = beta-aminopropionitrile (BAPN, from sweet peas) inhibits lysyl oxidase → reduced collagen crosslinks → joint laxity.\n\n📹 [VIDEO LINK]"},{"id":805,"s":"Protein Chemistry","t":"Collagen","u":"Lysyl Hydroxylase EDS Deficiency","q":"Deficiency of which of the following may cause Ehlers-Danlos syndrome?","o":["Fibrillin-1","Lysyl hydroxylase","Lysyl oxidase","Prolyl hydroxylase","Transglutaminase"],"a":1,"p":"KYPHOSCOLIOTIC EDS (EDS Type VI) = LYSYL HYDROXYLASE deficiency (PLOD1 gene mutation). Lysyl hydroxylase → hydroxylates lysine → HYDROXYLYSINE in RER → hydroxylysine is essential for: (1) O-glycosylation of collagen. (2) Formation of stable crosslinks via lysyl oxidase pathway. Without hydroxylysine → UNSTABLE CROSSLINKS → EDS features: hyperextensible skin, hypermobile joints + SCOLIOSIS and OCULAR FRAGILITY (specific to kyphoscoliotic type). Lysyl OXIDASE deficiency → also causes connective tissue fragility but via different mechanism.\n\n📹 [VIDEO LINK]"},{"id":806,"s":"Protein Chemistry","t":"Collagen","u":"Lysyl Oxidase Forms Allysine Hydroxyallysine","q":"Which of the following forms reactive aldehydes allysine and hydroxyallysine during collagen synthesis?","o":["Lysyl hydroxylase","Lysyl oxidase","Prolyl hydroxylase","Procollagen peptidase","Transglutaminase"],"a":1,"p":"LYSYL OXIDASE (extracellular, copper-dependent): oxidatively deaminates LYSINE → ALLYSINE and HYDROXYLYSINE → HYDROXYALLYSINE. These reactive ALDEHYDES then undergo spontaneous ALDOL CONDENSATION with adjacent allysine/lysine residues → COVALENT CROSSLINKS → stable collagen and elastin fibres. Without crosslinks → weak, fragile connective tissue (as seen in copper deficiency — Menkes disease, scurvy — where vitamin C-dependent hydroxylases are impaired, and lathyrism where lysyl oxidase is inhibited). Allysine formation is the rate-limiting step in crosslink formation.\n\n📹 [VIDEO LINK]"},{"id":807,"s":"Protein Chemistry","t":"Elastin","u":"Elastin Rich in Glycine Alanine Valine","q":"Elastin is rich in which of the following amino acids?","o":["Alanine, proline and lysine","Glutamate, aspartate and glutamine","Glycine, alanine and valine","Histidine, arginine and lysine","Valine, leucine and isoleucine"],"a":2,"p":"ELASTIN = rich in GLYCINE (33%), ALANINE, and VALINE — three small, NONPOLAR amino acids that give elastin its hydrophobic, elastic character. Also contains PROLINE and LYSINE (required for crosslinking). Elastin is DEFICIENT in HYDROXYPROLINE and HYDROXYLYSINE (unlike collagen which is rich in these modified amino acids). The nonpolar hydrophobic amino acids allow elastin to form a RANDOM COIL structure that can stretch and recoil — the basis of its elasticity.\n\n📹 [VIDEO LINK]"},{"id":808,"s":"Protein Chemistry","t":"Elastin","u":"Elastin Scant Hydroxyproline Hydroxylysine","q":"Which of the following amino acids are present in scant amounts in elastin?","o":["Alanine and glycine","Glycine, alanine and valine","Hydroxyproline and hydroxylysine","Proline and lysine","Valine and alanine"],"a":2,"p":"ELASTIN has SCANT AMOUNTS of HYDROXYPROLINE and HYDROXYLYSINE (the modified amino acids that are abundant in collagen). This distinguishes elastin from collagen biochemically. Hydroxyproline is essential for collagen triple helix stability but is largely absent from elastin. Elastin's elastic properties come from its GLYCINE-RICH hydrophobic domains forming random coils (not triple helices). The few crosslinks in elastin are DESMOSINE crosslinks (unique to elastin, formed from 4 lysine residues by lysyl oxidase).\n\n📹 [VIDEO LINK]"},{"id":809,"s":"Protein Chemistry","t":"Elastin","u":"Lysyl Oxidase Desmosine Crosslinks Elastin","q":"Which of the following enzyme is responsible for formation of desmosine crosslinks in elastin?","o":["Lysyl hydroxylase","Lysyl oxidase","Procollagen peptidase","Prolyl hydroxylase","Transglutaminase"],"a":1,"p":"LYSYL OXIDASE (copper-dependent) → oxidises LYSINE residues in TROPOELASTIN → ALLYSINE (reactive aldehydes) → FOUR ALLYSINE RESIDUES (from 4 tropoelastin molecules) spontaneously condense → DESMOSINE (unique to elastin) or ISODESMOSINE crosslinks. Desmosine = UNIQUE CROSSLINK found ONLY in elastin (not collagen). Elastin crosslinks are more extensive than collagen crosslinks → creates an insoluble, stable, three-dimensional rubber-like network. Copper deficiency (Menkes disease) → impaired lysyl oxidase → reduced desmosine crosslinks → fragile elastic fibres.\n\n📹 [VIDEO LINK]"},{"id":810,"s":"Protein Chemistry","t":"Elastin","u":"Fibrillin Scaffold for Tropoelastin","q":"Which of the following acts as a scaffold for deposition of tropoelastin?","o":["Collagen","Elastin","Fibrillin","Keratin","Laminin"],"a":2,"p":"FIBRILLIN MICROFIBRILS (fibrillin-1 and fibrillin-2) serve as a SCAFFOLD on which TROPOELASTIN molecules are deposited during elastic fibre assembly. Sequence of elastic fibre formation: (1) Fibrillin microfibrils assembled first (extracellular). (2) TROPOELASTIN secreted and deposited onto fibrillin scaffold. (3) LYSYL OXIDASE crosslinks tropoelastin → desmosine crosslinks → mature ELASTIN. In MARFAN SYNDROME: fibrillin-1 mutation → abnormal scaffold → impaired elastic fibre assembly → aortic wall weakness (aortic dilatation, dissection) + lens dislocation (zonular fibres = fibrillin).\n\n📹 [VIDEO LINK]"},{"id":811,"s":"Protein Chemistry","t":"Elastin","u":"Marfan Syndrome Fibrillin Mutation","q":"A young female who is much taller than her peers has exceptionally long arms and fingers and a mid-systolic click associated with mitral valve prolapse. Which protein is likely to be defective?","o":["Collagen","Elastin","Fibrillin","Keratin","Myosin"],"a":2,"p":"MARFAN SYNDROME: FBN1 gene mutation → defective FIBRILLIN-1. Features: TALL STATURE (long limbs, arachnodactyly — spider fingers), LENS DISLOCATION (ectopia lentis — upward), CARDIOVASCULAR abnormalities (aortic root dilatation → aortic regurgitation, mitral valve prolapse → mid-systolic click + murmur). Mechanism: abnormal fibrillin-1 → (1) Defective scaffold for elastic fibre assembly. (2) Excess TGF-β signalling (fibrillin normally sequesters TGF-β). The tall stature results from excess bone growth due to unregulated TGF-β signalling in growth plates.\n\n📹 [VIDEO LINK]"},{"id":812,"s":"Protein Chemistry","t":"Elastin","u":"Marfan Syndrome Aortic Dissection Athlete","q":"A 24-year-old male athlete collapses suddenly during a basketball match. He was tall with long arms and fingers. Echocardiogram reveals aortic root dilatation of 5.2 cm. He is likely to be suffering from which of the following?","o":["Alpha-1-antitrypsin deficiency","Ehlers-Danlos syndrome","Marfan's syndrome","Osteogenesis Imperfecta","Rickets"],"a":2,"p":"MARFAN SYNDROME: tall stature + arachnodactyly + AORTIC ROOT DILATATION → risk of AORTIC DISSECTION → sudden cardiac death in young athletes. FBN1 gene mutation → fibrillin-1 defect. Classic history: tall male athlete collapses during sport — Marfan's until proven otherwise. Management: beta-blockers, losartan, elective aortic root repair when diameter ≥5.0 cm, avoid contact sports.\n\n📹 [VIDEO LINK]"},{"id":813,"s":"Protein Chemistry","t":"Amyloidoses","u":"Primary Amyloidosis Amyloid L","q":"Which of the following is the precursor protein and the deposited amyloid protein in primary amyloidosis?","o":["Immunoglobulin heavy chain and amyloid H","Immunoglobulin light chain and amyloid L","Prealbumin and amyloid T","Serum amyloid A and amyloid A","Transthyretin and amyloid T"],"a":1,"p":"PRIMARY AMYLOIDOSIS (AL amyloidosis): PRECURSOR = IMMUNOGLOBULIN LIGHT CHAINS (produced by abnormal plasma cells — multiple myeloma or monoclonal gammopathy). DEPOSITED PROTEIN = AMYLOID L (AL). Deposits in: heart (restrictive cardiomyopathy), kidney (nephrotic syndrome), liver (hepatomegaly), tongue (macroglossia), peripheral nerves. Primary amyloidosis is the MOST COMMON systemic amyloidosis. Treatment: treat the underlying plasma cell dyscrasia.\n\n📹 [VIDEO LINK]"},{"id":814,"s":"Protein Chemistry","t":"Amyloidoses","u":"Secondary Amyloidosis Serum Amyloid A","q":"Which of the following is the precursor protein and the deposited amyloid protein in secondary amyloidosis?","o":["Immunoglobulin light chain and amyloid L","Islet amyloid polypeptide and amyloid I","Prealbumin and amyloid T","Serum amyloid A and amyloid A","Transthyretin and amyloid T"],"a":3,"p":"SECONDARY AMYLOIDOSIS (AA amyloidosis): PRECURSOR = SERUM AMYLOID A (SAA) — a positive acute phase reactant elevated in chronic inflammatory conditions. DEPOSITED PROTEIN = AMYLOID A (AA). Causes: chronic inflammatory diseases (rheumatoid arthritis, TB, bronchiectasis, osteomyelitis, inflammatory bowel disease). Deposits primarily in: KIDNEY (glomeruli → nephrotic syndrome), LIVER (hepatomegaly), SPLEEN (splenomegaly). Secondary amyloidosis reflects years of chronic inflammation with persistently elevated SAA.\n\n📹 [VIDEO LINK]"},{"id":815,"s":"Protein Chemistry","t":"Amyloidoses","u":"Familial Amyloidosis Transthyretin","q":"Which of the following is the precursor protein and the deposited amyloid protein in familial amyloidosis?","o":["Immunoglobulin light chain and amyloid L","Islet amyloid polypeptide and amyloid I","Serum amyloid A and amyloid A","Transthyretin and amyloid T","Beta-2-microglobulin and amyloid B"],"a":3,"p":"FAMILIAL AMYLOIDOSIS (hereditary transthyretin amyloidosis, hATTR): PRECURSOR = TRANSTHYRETIN (TTR, formerly called prealbumin) — a plasma protein that transports thyroxine and retinol-binding protein. Mutant TTR destabilises and misfolds → deposits as AMYLOID T. Features: PERIPHERAL NEUROPATHY, AUTONOMIC NEUROPATHY, CARDIOMYOPATHY. Wild-type TTR amyloidosis (senile cardiac amyloidosis) = non-mutant TTR deposits in elderly men → cardiac amyloidosis. Treatment: TAFAMIDIS (TTR stabiliser), patisiran/inotersen (siRNA/antisense to reduce TTR production).\n\n📹 [VIDEO LINK]"},{"id":816,"s":"Protein Chemistry","t":"Protein Techniques","u":"Polypeptide Sequencing Step 1 Cleaving","q":"Which of the following is the first step for the sequencing of a larger polypeptide?","o":["Characterising","Cleaving","Decoding","Ordering","Sequencing"],"a":1,"p":"POLYPEPTIDE SEQUENCING of larger proteins — three steps: STEP 1: CLEAVING — the large polypeptide is cleaved into smaller, manageable fragments using: (1) PROTEASES (trypsin → cleaves after Arg/Lys; chymotrypsin → after Phe/Trp/Tyr; V8 protease → after Glu/Asp) or (2) CHEMICAL AGENTS (CNBr → after methionine). Multiple different cleavage methods produce OVERLAPPING FRAGMENTS. Step 2: SEQUENCING each fragment (Edman degradation). Step 3: ORDERING fragments into the correct sequence using overlapping data.\n\n📹 [VIDEO LINK]"},{"id":817,"s":"Protein Chemistry","t":"Protein Techniques","u":"Polypeptide Sequencing Step 2 Sequencing","q":"Which of the following is the second step for the sequencing of a larger polypeptide?","o":["Characterising","Cleaving","Decoding","Ordering","Sequencing"],"a":4,"p":"STEP 2 of polypeptide sequencing: SEQUENCING each fragment using EDMAN DEGRADATION. Edman degradation: phenyl isothiocyanate (PTC) + terminal alpha-amino group → PHENYLTHIOHYDANTOIN (PTH)-amino acid → cleaved from N-terminus → identified by HPLC. Repeated sequentially → reads N to C terminal sequence. Modern automated sequencers can read ~50-60 residues per run. SANGER'S method (1-fluoro-2,4-dinitrobenzene, FDNB) was the original method — labels the N-terminal amino acid permanently (one-time use per fragment). Edman degradation is cyclic and allows multiple reads.\n\n📹 [VIDEO LINK]"},{"id":818,"s":"Protein Chemistry","t":"Protein Techniques","u":"Polypeptide Sequencing Step 3 Ordering","q":"Which of the following is the third step for the sequencing of a larger polypeptide?","o":["Characterising","Cleaving","Decoding","Ordering","Sequencing"],"a":3,"p":"STEP 3 of polypeptide sequencing: ORDERING — aligning the sequences of overlapping fragments from different cleavage methods to reconstruct the complete protein sequence. Example: trypsin fragments + chymotrypsin fragments → OVERLAPPING SEQUENCES → unique assembly into full sequence. This is analogous to shotgun sequencing in DNA analysis. Once the complete amino acid sequence is known → can predict structure, design mutations, synthesise the protein. Computerised alignment algorithms now perform this ordering step automatically.\n\n📹 [VIDEO LINK]"},{"id":819,"s":"Protein Chemistry","t":"Protein Techniques","u":"Salting Out Protein Purification","q":"Salting out occurs when hydrated organic ions compete with which of the following?","o":["Amino group hydration","Carboxyl group ionisation","Protein hydration shell","Protein isoelectric point","Protein surface charge"],"a":2,"p":"SALTING OUT: at HIGH SALT CONCENTRATIONS, inorganic ions (Na⁺, SO₄²⁻) compete with proteins for WATER MOLECULES → ions strip away the protein's HYDRATION SHELL → proteins lose their solubilising water layer → exposed hydrophobic patches interact with each other → PROTEIN AGGREGATION → PRECIPITATION. Different proteins salt out at different ammonium sulphate concentrations → used for PARTIAL PURIFICATION. Compare: SALTING IN = LOW salt concentration increases protein solubility (ions reduce electrostatic repulsion between protein molecules → more soluble). Both phenomena used in protein purification.\n\n📹 [VIDEO LINK]"},{"id":820,"s":"Protein Chemistry","t":"Clinical Protein Scenarios","u":"OI Blue Sclera Fractures Child Abuse","q":"A social worker refers a 2-year-old child because of suspected child abuse. The child has blue sclera and diminished hearing, and radiographs show recent and healing long bone fractures. The child is diagnosed with osteogenesis imperfecta. Which protein is defective?","o":["Calcitonin","Collagen","Myosin","Spectrin","Transthyretin"],"a":1,"p":"OSTEOGENESIS IMPERFECTA: defective TYPE I COLLAGEN (COL1A1/COL1A2 mutations) → structurally weak bone matrix. Blue sclera = thin scleral collagen → underlying choroid visible. Multiple fractures = brittle bones → may mimic non-accidental injury (NAI). Diminished hearing = ossicular chain collagen defect. KEY CLINICAL POINT: OI must always be considered when evaluating children with unexplained fractures — a genetic diagnosis, not parental abuse. Family history, skeletal survey, and genetic testing distinguish OI from NAI.\n\n📹 [VIDEO LINK]"},{"id":821,"s":"Protein Chemistry","t":"Clinical Protein Scenarios","u":"Marfan Fibrillin Volleyball Player","q":"A young female volleyball player is much taller than her peers with exceptionally long arms and fingers. Upon auscultation there is a mid-systolic click with mitral valve prolapse murmur. The physician suspects Marfan's syndrome, which is a defect in which of the following proteins?","o":["Ankyrin","Collagen","Fibrillin","Myosin","Spectrin"],"a":2,"p":"MARFAN SYNDROME: FBN1 gene mutation → defective FIBRILLIN-1 → impaired elastic fibre scaffold + excess TGF-β → tall stature (arachnodactyly), cardiovascular abnormalities (aortic dilatation, MITRAL VALVE PROLAPSE → mid-systolic click), lens dislocation. Volleyball players are recruited based on height — marfanoid habitus gives an athletic advantage. Sudden cardiac death risk in athletes with undiagnosed Marfan's from aortic dissection. Screening: echocardiogram for aortic root diameter.\n\n📹 [VIDEO LINK]"},{"id":822,"s":"Protein Chemistry","t":"Clinical Protein Scenarios","u":"Fibrillin Mutation Marfan Syndrome","q":"Mutations in fibrillin-1-protein gene are responsible for which of the following?","o":["Alpha-1-antitrypsin deficiency","Ehlers-Danlos syndrome","Homocystinuria","Marfan's syndrome","Osteogenesis Imperfecta"],"a":3,"p":"FBN1 gene (fibrillin-1) mutations → MARFAN SYNDROME. Fibrillin-1: (1) Structural component of extracellular microfibrils. (2) Sequesters TGF-β in ECM. Mutation → excess TGF-β → overgrowth of long bones, aortic wall weakness, ciliary zonule defects (lens dislocation). HOMOCYSTINURIA = also presents with marfanoid features (tall stature, arachnodactyly) but due to CBS gene mutation → downward lens dislocation, intellectual disability, positive urine homocystine test — distinguishable from Marfan's.\n\n📹 [VIDEO LINK]"},{"id":823,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"FMN Prosthetic Group NADH Dehydrogenase Complex I","q":"Which of the following is the prosthetic group of NADH dehydrogenase (Complex I)?","o":["FAD","FMN","NAD plus","NADH","NADPH"],"a":1,"p":"COMPLEX I (NADH-Q oxidoreductase) contains FMN (FLAVIN MONONUCLEOTIDE) as its PROSTHETIC GROUP — tightly bound, non-covalently. FMN accepts hydride (H⁻) from NADH → FMNH₂ → electrons passed to iron-sulfur clusters → CoQ. Compare: COMPLEX II (succinate dehydrogenase) contains FAD (not FMN) as its prosthetic group. NADPH is the reductive biosynthesis coenzyme — NOT part of the respiratory chain. NADH is the SUBSTRATE of Complex I, not its prosthetic group.\n\n📹 [VIDEO LINK]"},{"id":824,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Cytochrome One Electron Carrier","q":"Which of the following ETC components accepts only one electron at a time?","o":["Coenzyme Q","Cytochrome b","FAD","FMN","Oxygen"],"a":1,"p":"CYTOCHROMES (including cytochrome b) = ONE-ELECTRON CARRIERS. Each cytochrome contains a haem group with IRON that undergoes Fe²⁺ ↔ Fe³⁺ — a single electron change per cycle. CoQ, FAD, FMN = TWO-ELECTRON CARRIERS (accept hydride H⁻, two electrons simultaneously). O₂ = accepts FOUR ELECTRONS (complete reduction to 2H₂O at Complex IV). This is why CoQ acts as the junction point — receiving two-electron packets from Complex I (via FMN) and Complex II (via FAD) and passing single electrons to cytochromes downstream.\n\n📹 [VIDEO LINK]"},{"id":825,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Oxygen Strongest Tendency to Gain Electrons","q":"Which of the following has the strongest tendency to gain electrons?","o":["Coenzyme Q","Cytochrome c","Flavin adenine dinucleotide","NAD plus","Oxygen"],"a":4,"p":"STANDARD REDUCTION POTENTIALS (E⁰'): O₂ = +0.82V (highest — strongest oxidising agent, greatest tendency to gain electrons). Cytochrome c = +0.23V. CoQ = +0.04V. FAD = -0.18V. NAD⁺ = -0.32V (lowest — strongest reducing agent). Electrons flow from MOST NEGATIVE to MOST POSITIVE reduction potential — from NADH (−0.32V) to O₂ (+0.82V) — a total potential difference of 1.14V — releasing ~220 kJ/mol free energy to drive ATP synthesis.\n\n📹 [VIDEO LINK]"},{"id":826,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"ADP Controls Rate of Respiration","q":"The rate of respiration of mitochondria can be controlled by the availability of which of the following?","o":["ADP","ATP","FAD","FMN","NAD"],"a":0,"p":"RESPIRATORY CONTROL (ACCEPTOR CONTROL): mitochondrial respiration rate is primarily controlled by ADP availability. HIGH ADP → ATP synthase runs → proton gradient falls → electron transport accelerates → O₂ consumption increases. LOW ADP → ATP synthase slows → proton gradient builds (back-pressure) → electron transport slows → O₂ consumption falls. This is the normal physiological mechanism matching ATP production to cellular ATP demand. Exercise → ADP rises → respiration increases.\n\n📹 [VIDEO LINK]"},{"id":827,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"CoA Not Component of Respiratory Chain","q":"Which of the following coenzymes is NOT a component of the respiratory chain?","o":["Coenzyme A","Coenzyme Q","FAD","FMN","NAD"],"a":0,"p":"COENZYME A (CoA) = NOT a component of the respiratory chain. CoA functions in: TCA cycle (acetyl-CoA, succinyl-CoA), fatty acid oxidation, fatty acid synthesis. RESPIRATORY CHAIN COMPONENTS: NAD (Complex I substrate), FMN (Complex I prosthetic group), FAD (Complex II prosthetic group), CoQ (mobile electron carrier between Complexes I/II and III), cytochromes b, c1, c, a, a3, iron-sulfur clusters. CoA has no redox function in the electron transport chain.\n\n📹 [VIDEO LINK]"},{"id":828,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Nicotinamide Vitamin Component Respiratory Chain","q":"Which of the following vitamins acts as a component of the mitochondrial respiratory chain?","o":["Biotin","Cyanocobalamin","Folic acid","Nicotinamide","Retinal"],"a":3,"p":"NICOTINAMIDE (vitamin B3/niacin) → NAD⁺ (nicotinamide adenine dinucleotide) = essential respiratory chain coenzyme. NAD⁺ accepts electrons from TCA cycle substrates → NADH → Complex I substrate. RIBOFLAVIN (vitamin B2) → FAD and FMN — also respiratory chain components. BIOTIN = CO₂ fixation (pyruvate carboxylase, ACC). CYANOCOBALAMIN (B12) = methyl transfer reactions. RETINAL = visual cycle. FOLIC ACID = one-carbon transfer.\n\n📹 [VIDEO LINK]"},{"id":829,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"NAD Derived from Niacin Vitamin B3","q":"NAD is derived from which of the following vitamins?","o":["Biotin","Niacin","Pyridoxine","Riboflavin","Thiamine"],"a":1,"p":"NAD (nicotinamide adenine dinucleotide) is derived from NIACIN (NICOTINAMIDE/NICOTINIC ACID, vitamin B3). Can also be synthesised from TRYPTOPHAN (60mg tryptophan = 1mg niacin). RIBOFLAVIN (B2) → FAD and FMN. THIAMINE (B1) → TPP. PYRIDOXINE (B6) → PLP. BIOTIN (B7) → CO₂ fixation. Deficiency of niacin → PELLAGRA (dermatitis, diarrhoea, dementia).\n\n📹 [VIDEO LINK]"},{"id":8291,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"NAD Highest Concentration Mitochondria","q":"NAD is found in the highest concentration in which of the following?","o":["Cytosol","Lysosomes","Mitochondria","Peroxisomes","Ribosomes"],"a":2,"p":"NAD is found in HIGHER CONCENTRATION IN MITOCHONDRIA than cytosol. The mitochondrial and cytosolic NAD pools are SEPARATE — NAD CANNOT cross the inner mitochondrial membrane (hence the need for shuttle systems). Mitochondrial NAD is regenerated by TCA cycle dehydrogenases and used by Complex I. Cytosolic NAD is regenerated by lactate dehydrogenase under anaerobic conditions or transferred via malate-aspartate and glycerol-3-phosphate shuttles.\n\n📹 [VIDEO LINK]"},{"id":830,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Malate Aspartate Shuttle Heart NADH Transferred","q":"NADH derived from glycolysis in the heart is transferred to the mitochondria in which of the following forms?","o":["Alpha-ketoglutarate","Aspartate","Glutamate","Malate","Oxaloacetate"],"a":3,"p":"MALATE-ASPARTATE SHUTTLE (heart, liver, kidney): cytosolic NADH → malate dehydrogenase → OAA reduced to MALATE → MALATE transferred into mitochondria via malate-alpha-KG antiporter → oxidised back to OAA → generates MITOCHONDRIAL NADH → Complex I → 2.5 ATP per cytosolic NADH. This shuttle delivers the full NADH equivalent — highest ATP yield.\n\n📹 [VIDEO LINK]"},{"id":8301,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Glycerol 3 Phosphate Shuttle Brain NADH Transferred","q":"NADH derived from glycolysis in the brain is transferred to the mitochondria in which of the following forms?","o":["Alpha-ketoglutarate","Aspartate","Glutamate","Glycerol-3-phosphate","Oxaloacetate"],"a":3,"p":"GLYCEROL-3-PHOSPHATE SHUTTLE (brain, skeletal muscle, white adipose tissue): cytosolic NADH → DHAP reduced to GLYCEROL-3-PHOSPHATE → transferred to inner mitochondrial membrane → mitochondrial FAD-linked glycerol-3-phosphate dehydrogenase → FADH₂ → CoQ → 1.5 ATP per cytosolic NADH. Lower ATP yield than malate-aspartate shuttle because enters at CoQ (bypasses Complex I).\n\n📹 [VIDEO LINK]"},{"id":8302,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Malate Aspartate Shuttle ATP Yield 2.5","q":"NADH from glycolysis transferred to mitochondria via the malate-aspartate shuttle generates which of the following ATP yield per cytosolic NADH?","o":["0.5 ATP","1.0 ATP","1.5 ATP","2.0 ATP","2.5 ATP"],"a":4,"p":"MALATE-ASPARTATE SHUTTLE → cytosolic NADH → mitochondrial NADH → Complex I → 2.5 ATP per cytosolic NADH. Same yield as directly mitochondrially-generated NADH because the shuttle delivers NADH directly to Complex I. Operates in heart, liver, kidney.\n\n📹 [VIDEO LINK]"},{"id":8303,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Glycerol 3 Phosphate Shuttle ATP Yield 1.5","q":"NADH from glycolysis transferred to mitochondria via the glycerol-3-phosphate shuttle generates which of the following ATP yield per cytosolic NADH?","o":["0.5 ATP","1.0 ATP","1.5 ATP","2.0 ATP","2.5 ATP"],"a":2,"p":"GLYCEROL-3-PHOSPHATE SHUTTLE → cytosolic NADH → FADH₂ (mitochondrial) → CoQ → Complex III → 1.5 ATP per cytosolic NADH. Enters at CoQ (BYPASSES Complex I) → fewer protons pumped → LESS ATP than malate-aspartate shuttle. Operates in brain and muscle where speed matters more than efficiency.\n\n📹 [VIDEO LINK]"},{"id":8304,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Glycerol Phosphate Shuttle Less ATP Because FADH2","q":"The glycerol-3-phosphate shuttle produces less ATP per cytosolic NADH than the malate-aspartate shuttle because it generates which of the following in the mitochondria?","o":["Acetyl-CoA","FADH₂ instead of NADH","Less CoQ","NADH instead of FADH₂","Oxaloacetate"],"a":1,"p":"GLYCEROL-3-PHOSPHATE SHUTTLE produces FADH₂ (enters at CoQ, BYPASSES Complex I → one less proton-pumping complex) → 1.5 ATP. MALATE-ASPARTATE SHUTTLE produces NADH (enters at Complex I → three proton-pumping complexes used) → 2.5 ATP. The difference (1.0 ATP) represents the energy from bypassing Complex I.\n\n📹 [VIDEO LINK]"},{"id":8305,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Malate Aspartate Shuttle Location Heart Liver Kidney","q":"Which of the following tissues primarily uses the malate-aspartate shuttle to transfer cytosolic NADH into mitochondria?","o":["Brain","Heart and liver","Red blood cells","Skeletal white muscle","White adipose tissue"],"a":1,"p":"MALATE-ASPARTATE SHUTTLE = present in HEART, LIVER, KIDNEY (high energy demand, oxidative tissues). These tissues prioritise maximal ATP yield (2.5 ATP per NADH). GLYCEROL-3-PHOSPHATE SHUTTLE = present in brain, skeletal muscle, white adipose tissue. RED BLOOD CELLS = no mitochondria → cannot use either shuttle → rely entirely on anaerobic glycolysis.\n\n📹 [VIDEO LINK]"},{"id":831,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"CO Poisoning Complex IV Charcoal Brazier","q":"A 45-year-old construction worker is found unconscious in a poorly ventilated basement where a charcoal brazier had been burning overnight. He is brought to the emergency department with carbon monoxide poisoning. Inhibition of which of the following complexes of the electron transport chain led to his collapse?","o":["Complex I","Complex II","Complex III","Complex IV","ATP synthase"],"a":3,"p":"CO → binds Fe²⁺ in haem a₃ of CYTOCHROME C OXIDASE (COMPLEX IV) → blocks electron transfer to O₂. Treatment: 100% O₂, hyperbaric O₂.\n\n📹 [VIDEO LINK]"},{"id":832,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"CO Poisoning Complex IV Closed Garage","q":"A thief hides in his car in a closed garage with the engine running to avoid police. He is brought to the emergency department with carbon monoxide poisoning. Inhibition of which of the following complexes of the electron transport chain led to his collapse?","o":["Complex I","Complex II","Complex III","Complex IV","ATP synthase"],"a":3,"p":"CO → binds Fe²⁺ in haem a₃ of CYTOCHROME C OXIDASE (COMPLEX IV) → blocks electron transfer to O₂. CO affinity for Hb = 250× greater than O₂ → carboxyhaemoglobin. Double mechanism of hypoxia: O₂ delivery AND utilisation both impaired. Cherry-red skin. Treatment: 100% O₂.\n\n📹 [VIDEO LINK]"},{"id":833,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"CO Poisoning Complex IV Generator Shop","q":"A businessman runs a generator in his closed shop and is brought to the emergency department with carbon monoxide poisoning. Inhibition of which of the following complexes of the electron transport chain led to his collapse?","o":["Complex I","Complex II","Complex III","Complex IV","ATP synthase"],"a":3,"p":"CO → binds Fe²⁺ in haem a₃ of CYTOCHROME C OXIDASE (COMPLEX IV) → ETC arrested → ATP depletion → neurological collapse + cardiac toxicity. Generators, heaters, cars in enclosed spaces = most common CO poisoning sources. Prevention: CO detectors, ventilation. Treatment: 100% O₂.\n\n📹 [VIDEO LINK]"},{"id":834,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"Rotenone Inhibits Complex I NADH to CoQ","q":"A gardener spraying rotenone in fields develops laboured breathing and convulsions. Inhibition of which of the following steps of the electron transport chain led to his symptoms?","o":["Electron transfer from cytochrome a1 and a3 to oxygen","Electron transfer from cytochrome c to cytochrome a1 and a3","Electron transfer from NADH to coenzyme Q","Oxidation of coenzyme Q","Reduction of cytochrome c"],"a":2,"p":"ROTENONE → inhibits COMPLEX I (NADH-Q oxidoreductase) → blocks electron transfer from NADH to CoQ. Insecticide and fish poison. Associated with Parkinson-like neurodegeneration in exposed individuals. Complex I inhibition → NADH accumulates → ATP depletion.\n\n📹 [VIDEO LINK]"},{"id":835,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"Inhibitors Decrease Oxygen Consumption","q":"A woman is served curd sprinkled with arsenic. Which of the following will happen in her as a result of inhibition of her electron transport chain?","o":["Decreased oxygen consumption","Increased CO₂ production","Increased heat production","Increased NADH oxidation","Stimulation of TCA cycle"],"a":0,"p":"ETC INHIBITORS → block electron flow → O₂ CONSUMPTION DECREASES. NADH accumulates, TCA slows, CO₂ decreases, ATP depleted. Compare: UNCOUPLERS → O₂ consumption INCREASES. Key distinction: inhibitors ↓ O₂; uncouplers ↑ O₂.\n\n📹 [VIDEO LINK]"},{"id":836,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"Inhibitors Inhibit TCA Cycle","q":"A man is poisoned with arsenic in his food. Which of the following will happen in him as a result of inhibition of his electron transport chain?","o":["Increased CO₂ production","Increased heat production","Increased oxygen consumption","Inhibition of TCA cycle","Stimulation of electron transport chain"],"a":3,"p":"ETC INHIBITORS → NADH and FADH₂ accumulate → NAD and FAD not regenerated → TCA CYCLE INHIBITED. CO₂ decreases. Heat decreases. Arsenite (As(III)) also directly inhibits PDH and alpha-KG dehydrogenase by binding lipoate.\n\n📹 [VIDEO LINK]"},{"id":837,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"Cyanide Decreased NADH Oxidation","q":"A child presents with acute cyanide poisoning after eating cassava seeds and is deteriorating rapidly. Which of the following is happening in this child leading to deterioration of his symptoms?","o":["Decreased heat production","Decreased NADH oxidation","Increased CO₂ production","Increased oxygen consumption","Stimulation of TCA cycle"],"a":1,"p":"CYANIDE → Complex IV inhibited → NADH OXIDATION DECREASES (NADH accumulates, cannot be oxidised) → ETC arrested → ATP depletion → lactic acidosis → deterioration. All downstream processes slow: TCA inhibited, CO₂ decreases, O₂ consumption decreases, heat production decreases. DECREASED NADH OXIDATION is the primary biochemical event causing deterioration.\n\n📹 [VIDEO LINK]"},{"id":838,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"Inhibitors Decrease CO2 Production","q":"A sailor is found unconscious after antimycin A-containing pesticide exposure in a ship's hold. Which of the following will happen in him as a result?","o":["Decreased CO₂ production","Increased heat production","Increased NADH oxidation","Increased oxygen consumption","Stimulation of TCA cycle"],"a":0,"p":"ETC INHIBITORS (antimycin A → Complex III) → TCA cycle inhibited → DECARBOXYLATION REACTIONS slow → CO₂ PRODUCTION DECREASES. TCA decarboxylation steps all require NAD which is not regenerated when ETC is blocked.\n\n📹 [VIDEO LINK]"},{"id":839,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"Arsenic Substrate Level Phosphorylation","q":"A 30-year-old man is exposed to arsenic by drinking contaminated groundwater. Pentavalent arsenic acts as a substrate for which of the following enzymes?","o":["Alpha-ketoglutarate dehydrogenase","Branched-chain alpha-ketoacid dehydrogenase","Glyceraldehyde-3-phosphate dehydrogenase","Isocitrate dehydrogenase","Pyruvate dehydrogenase"],"a":2,"p":"ARSENATE (As(V), pentavalent arsenic) = PHOSPHATE ANALOGUE → acts as substrate for GLYCERALDEHYDE-3-PHOSPHATE DEHYDROGENASE (GAPDH) in glycolysis → 1-arseno-3-phosphoglycerate formed instead of 1,3-BPG → spontaneously hydrolyses → 3-phosphoglycerate produced with NO ATP formation (ARSENOLYSIS). This uncouples substrate-level phosphorylation at the GAPDH step. ARSENITE (As(III), trivalent arsenic) binds lipoate → inhibits PDH and alpha-KG dehydrogenase. Both forms deplete ATP but by different mechanisms.\n\n📹 [VIDEO LINK]"},{"id":841,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Uncouplers of ETC","u":"Uncouplers Increase Oxygen Consumption DNP","q":"A woman collapses at a gym with hyperthermia, profuse sweating, and tachycardia after taking an illegal slimming pill that acts by uncoupling the electron transport chain. Her electron transport chain is facing which of the following effects?","o":["Arrest of electron transport","Decreased CO₂ production","Decreased oxygen consumption","Increased oxygen consumption","Inhibition of TCA cycle"],"a":3,"p":"UNCOUPLERS → proton gradient dissipated → ETC runs unchecked → O₂ CONSUMPTION INCREASES markedly. DNP overdose: hyperthermia + high O₂ demand + sweating + tachycardia → can be fatal. THERMOGENIN (UCP-1) in brown adipose tissue = physiological uncoupler.\n\n📹 [VIDEO LINK]"},{"id":842,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Uncouplers of ETC","u":"Uncouplers Increase CO2 Neonate BAT Thermogenin","q":"A premature neonate is being nursed in an incubator. When the electricity breaks down and the incubator temperature drops, her brown adipose tissue activates thermogenin (UCP-1) to uncouple the electron transport chain and generate heat. Which of the following is happening in this neonate at the level of the electron transport chain?","o":["Arrest of electron transport","Decreased CO₂ production","Decreased oxygen consumption","Increased CO₂ production","Inhibition of TCA cycle"],"a":3,"p":"UNCOUPLERS (UCP-1/thermogenin) → electron transport runs faster → NAD regenerated → TCA STIMULATED → more CO₂ produced. Brown adipose tissue in neonates = primary thermogenic organ. Activated by: cold exposure → noradrenaline → lipolysis → free fatty acids activate UCP-1.\n\n📹 [VIDEO LINK]"},{"id":843,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Uncouplers of ETC","u":"Uncouplers Cause Metabolic Acidosis DNP","q":"After being prescribed dinitrophenol by a quack for weight loss, a 30-year-old woman presents with hyperthermia, tachycardia, and profuse sweating. Which of the following acid-base disturbances is expected?","o":["Metabolic acidosis","Metabolic alkalosis","Respiratory acidosis","Respiratory alkalosis","No acid-base change"],"a":0,"p":"UNCOUPLERS → ATP synthesis abolished → anaerobic glycolysis → LACTIC ACID → METABOLIC ACIDOSIS (↓ pH, ↓ HCO₃⁻, high anion gap, normal pCO₂). DNP poisoning = hyperthermia + metabolic acidosis + tachycardia. Respiratory acidosis = CO₂ retention (lung disease) — not the mechanism here.\n\n📹 [VIDEO LINK]"},{"id":844,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Uncouplers of ETC","u":"Uncouplers Increase NADH Oxidation Hibernation","q":"A hibernating bear activates brown adipose tissue by uncoupling the electron transport chain to maintain body temperature. Which of the following is happening in the mitochondria of his brown adipose tissue?","o":["Arrest of electron transport","Decreased NADH oxidation","Decreased oxygen consumption","Increased NADH oxidation","Inhibition of TCA cycle"],"a":3,"p":"UNCOUPLERS → ETC runs without back-pressure → Complex I oxidises NADH faster → NADH OXIDATION INCREASES → NAD regenerated → TCA cycle accelerated. Energy released as HEAT instead of ATP. UCP-1 (thermogenin) in BAT = the physiological uncoupler. Cold → noradrenaline → lipolysis → free fatty acids activate UCP-1.\n\n📹 [VIDEO LINK]"},{"id":845,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Uncouplers of ETC","u":"Uncouplers Dissipate Proton Gradient Neonate","q":"A 1-week-old neonate born in a slum without heating is struggling to maintain body temperature in winter. Her brown adipose tissue activates thermogenin (UCP-1) to uncouple the electron transport chain. Which of the following is happening in the mitochondria of her brown adipose tissue?","o":["Arrest of electron transport","Decreased heat production","Dissipation of proton gradient","Increased gluconeogenesis","Inhibition of TCA cycle"],"a":2,"p":"THERMOGENIN (UCP-1) = proton channel in inner mitochondrial membrane of BAT → allows protons to flow back from intermembrane space to matrix WITHOUT passing through ATP synthase → PROTON GRADIENT IS DISSIPATED as HEAT instead of being used to synthesise ATP. Neonates rely heavily on BAT thermogenesis as they cannot shiver. Cold → noradrenaline → beta-3 adrenoceptors → lipolysis → fatty acids activate UCP-1.\n\n📹 [VIDEO LINK]"},{"id":846,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Uncouplers of ETC","u":"DNP Weight Loss Uncoupler Quack","q":"A quack is selling a weight-loss medicine at a market, claiming it burns fat without dieting. In theory, which of the following compounds could make this claim?","o":["Amytal","Antimycin","Atractyloside","Dinitrophenol","Rotenone"],"a":3,"p":"DINITROPHENOL (DNP) = CLASSIC UNCOUPLER → lipid-soluble weak acid → shuttles protons across inner mitochondrial membrane → dissipates proton gradient → electron transport runs without ATP synthesis → energy released as HEAT → weight loss. ROTENONE, AMYTAL = Complex I inhibitors. ANTIMYCIN A = Complex III inhibitor. ATRACTYLOSIDE = inhibits ANT.\n\n📹 [VIDEO LINK]"},{"id":847,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Uncouplers of ETC","u":"DNP Electron Flow Continues ATP Synthesis Abolished","q":"If 2,4-dinitrophenol is added to mitochondria, which of the following best describes the effect on coupling?","o":["Both electron flow and ATP synthesis will be increased","Both electron flow and ATP synthesis will stop","Electron flow will continue but ATP synthesis will not occur","Electron flow will stop but ATP synthesis will continue","Oligomycin will cause ATP hydrolysis if added"],"a":2,"p":"DNP = UNCOUPLER → dissipates proton gradient → ATP synthase has NO PROTON GRADIENT to drive it → ATP SYNTHESIS STOPS. However electron transport continues (and increases) because the back-pressure of the proton gradient is removed → electrons flow freely to O₂. ELECTRON FLOW CONTINUES — ATP SYNTHESIS DOES NOT OCCUR. This is the definition of uncoupling: OXIDATION (electron transport) is dissociated from PHOSPHORYLATION (ATP synthesis). If oligomycin is THEN added to DNP-uncoupled mitochondria → oligomycin blocks Fo channel — but since proton gradient is already dissipated by DNP → no further effect on electron transport.\n\n📹 [VIDEO LINK]"},{"id":848,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Oxidative Phosphorylation","u":"Oligomycin Inhibits Complex V ATP Synthase","q":"A microbiologist adds oligomycin to isolated mitochondria in a lab experiment. Oligomycin will inhibit which of the following?","o":["Complex I","Complex II","Complex III","Complex IV","Complex V"],"a":4,"p":"OLIGOMYCIN → binds the Fo SUBUNIT of COMPLEX V (ATP SYNTHASE) → blocks proton channel → ATP synthesis stops → proton gradient builds up → back-pressure inhibits electron transport → O₂ consumption decreases. NOT an uncoupler. Does NOT inhibit Complexes I–IV directly. Adding ADP to oligomycin-inhibited mitochondria does NOT stimulate respiration because the Fo channel is blocked.\n\n📹 [VIDEO LINK]"},{"id":849,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Oxidative Phosphorylation","u":"Chemiosmotic Hypothesis Proton Gradient Primary Driving Force","q":"Mitchell's chemiosmotic hypothesis proposes that during oxidative phosphorylation, electron transport through Complexes I, III, and IV pumps protons from the matrix to the intermembrane space, creating a proton gradient that drives ATP synthesis. Which of the following represents the primary driving force for ATP generation?","o":["Electron transfer","Protein phosphorylation","Proton gradient","Substrate phosphorylation","NADH transport"],"a":2,"p":"MITCHELL'S CHEMIOSMOTIC HYPOTHESIS: PROTON GRADIENT (ΔpH) + MEMBRANE POTENTIAL (ΔΨ) across inner mitochondrial membrane = PROTON MOTIVE FORCE → drives protons back through Fo-F1 ATP synthase → ATP synthesised. SUBSTRATE PHOSPHORYLATION = glycolysis/TCA cycle mechanism (separate). ELECTRON TRANSFER = powers proton pumping but is not the direct driving force for ATP synthesis itself. NADH cannot cross the inner mitochondrial membrane.\n\n📹 [VIDEO LINK]"},{"id":850,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Oxidative Phosphorylation","u":"NADH Oxidation Yields 2.5 ATP","q":"In oxidative phosphorylation, the oxidation of one molecule of NADH produces which of the following?","o":["Zero ATP","1 ATP","2 ATP","2.5 ATP","5 ATP"],"a":3,"p":"MODERN P/O RATIO: oxidation of NADH → 2.5 ATP (current biochemistry consensus, Harper's 31st edition). TRADITIONAL TEACHING VALUE: 3 ATP (still in many exam contexts). FADH₂ → 1.5 ATP (enters at CoQ, bypasses Complex I). TOTAL ATP from one glucose (aerobic): ~30-32 ATP (modern values).\n\n📹 [VIDEO LINK]"},{"id":851,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Clinical ETC Disorders","u":"Complex I Mutation Poor NADH Oxidation Normal Succinate","q":"A 6-year-old child has muscle weakness worsening over 6 months. Mitochondria from a muscle biopsy show normal succinate oxidation but very poor oxidation of NADH-generating substrates. Malate dehydrogenase activity is normal. The patient may have a mutation in a gene encoding a subunit of which of the following?","o":["ATP synthase","Complex I","Complex II","Complex III","Complex IV"],"a":1,"p":"SUCCINATE → Complex II → CoQ: NORMAL (Complex II intact). NADH-generating substrates → COMPLEX I → CoQ: POOR (Complex I defective). Malate dehydrogenase normal = TCA intact. Block is specifically at Complex I. Most common mitochondrial respiratory chain disorder — presents with MELAS, Leigh syndrome, exercise intolerance.\n\n📹 [VIDEO LINK]"},{"id":852,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Clinical ETC Disorders","u":"Kearns Sayre Complex II Succinate Impaired","q":"An 8-year-old boy with ophthalmoplegia and pigmentary retinopathy is suspected to have Kearns-Sayre syndrome. Assuming the defect is a mutation in Complex II, electron transfer from which of the following would be impaired?","o":["Alpha-ketoglutarate","Isocitrate","Malate","Pyruvate","Succinate"],"a":4,"p":"COMPLEX II (SUCCINATE-Q REDUCTASE = succinate dehydrogenase): oxidises SUCCINATE → FUMARATE + FADH₂ → electrons passed to CoQ. If Complex II is mutated → SUCCINATE CANNOT be oxidised → succinate accumulates. Other TCA substrates (malate, isocitrate, alpha-KG, pyruvate) → produce NADH → enter via COMPLEX I (not Complex II) → unaffected by Complex II mutation. KEARNS-SAYRE SYNDROME = mitochondrial deletion syndrome → ophthalmoplegia, pigmentary retinopathy, cardiac conduction defects, onset before age 20.\n\n📹 [VIDEO LINK]"},{"id":853,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Clinical ETC Disorders","u":"Luft Disease Uncoupling Hypermetabolism","q":"A muscle biopsy from a patient in whom Luft disease is suspected shows abnormally large mitochondria with packed cristae and ATPase activity seven times normal. Oxidation and phosphorylation are partially uncoupled. Which of the following is correct?","o":["Decreased core temperature","Decreased metabolic rate","Increased ATP production","Increased core temperature","Increased somnolence"],"a":3,"p":"LUFT DISEASE = partial uncoupling → electrons flow faster → proton gradient DISSIPATED as heat → INCREASED CORE TEMPERATURE + HYPERMETABOLISM. ATP production is LOWER than normal (uncoupling abolishes efficient ATP synthesis — option C wrong). Metabolic rate is INCREASED not decreased (option B wrong). Core temperature INCREASED not decreased (option A wrong). Patient is HYPERACTIVE not somnolent (option E wrong).\n\n📹 [VIDEO LINK]"},{"id":854,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Clinical ETC Disorders","u":"Malignant Hyperthermia Fo F1 ATPase Uncoupling","q":"A 23-year-old footballer sustains a fracture requiring surgery. The anaesthesiologist notes a significantly increased body temperature of 39°C. The operation is terminated as malignant hyperthermia is suspected. Which of the following components of the ETC is likely to be responsible for this phenomenon?","o":["Complex I","Complex II","Complex III","Complex IV","The Fo-F1 ATPase"],"a":4,"p":"MALIGNANT HYPERTHERMIA: RYR1 mutation → triggered by halogenated anaesthetics (halothane) or succinylcholine → uncontrolled Ca²⁺ release from SR → sustained muscle contraction → massive ATP consumption → ATPase activity uncoupled → heat generated without useful work → HYPERTHERMIA. The Fo-F1 ATPase is implicated in the uncoupling — mitochondria run at maximum rate to replace ATP but HEAT is generated faster than it can be dissipated. Treatment: DANTROLENE (blocks RYR1 Ca²⁺ channel) + active cooling.\n\n📹 [VIDEO LINK]"},{"id":855,"s":"Heme Metabolism","t":"Heme Synthesis","u":"ALA Synthase Catalyses Rate Limiting Step","q":"Delta-aminolevulinic acid synthase catalyses which of the following steps in haem synthesis?","o":["Condensation of porphobilinogen units to uroporphyrinogen","Conversion of coproporphyrinogen to protoporphyrinogen","Conversion of succinyl-CoA and glycine to delta-aminolevulinic acid","Insertion of iron into protoporphyrin IX","Oxidation of uroporphyrinogen to uroporphyrin"],"a":2,"p":"ALA SYNTHASE (ALAS): SUCCINYL-CoA + GLYCINE → DELTA-AMINOLEVULINIC ACID (ALA) + CO₂. Rate-limiting step. Requires PLP (B6). Located in MITOCHONDRIA. Haem synthesis steps: (1) ALAS: succinyl-CoA + glycine → ALA (mitochondria). (2) ALA dehydratase: 2 ALA → porphobilinogen. (3) PBG deaminase: 4 PBG → uroporphyrinogen III. (4-6) Cytosolic modifications. (7) Ferrochelatase: protoporphyrin IX + Fe²⁺ → HAEM (mitochondria).\n\n📹 [VIDEO LINK]"},{"id":856,"s":"Heme Metabolism","t":"Heme Synthesis","u":"Sideroblastic Anaemia Isoniazid ALA Synthase","q":"A 62-year-old man being treated for tuberculosis develops microcytic hypochromic anaemia with increased ferritin and ring sideroblasts. A decrease in which enzyme activity is most directly responsible?","o":["Cytochrome oxidase","Cytochrome P450 oxidase","Delta-aminolevulinate synthase","Lysyl oxidase","Pyruvate kinase"],"a":2,"p":"ISONIAZID (INH) = PYRIDOXINE (B6) ANTAGONIST → inhibits PLP-dependent enzymes including DELTA-AMINOLEVULINATE SYNTHASE (ALAS, requires PLP) → decreased haem synthesis → iron cannot be incorporated → IRON ACCUMULATES IN MITOCHONDRIA of erythroblasts → RING SIDEROBLASTS (iron deposits around nucleus). Sideroblastic anaemia: microcytic, hypochromic anaemia + elevated serum ferritin + ring sideroblasts on iron stain. Treatment: pyridoxine (B6) supplementation. Compare: lead poisoning also causes sideroblastic anaemia (inhibits ALA dehydratase AND ferrochelatase).\n\n📹 [VIDEO LINK]"},{"id":857,"s":"Heme Metabolism","t":"Porphyrias","u":"PCT Photosensitivity Dark Urine Skin Blisters","q":"A 50-year-old man on a desert safari presents with photosensitivity, skin eruptions, and red-brown discolouration of urine. He is likely to be suffering from which of the following?","o":["Acute intermittent porphyria","Erythropoietic protoporphyria","Hereditary coproporphyria","Porphyria cutanea tarda","Variegate porphyria"],"a":3,"p":"PORPHYRIA CUTANEA TARDA (PCT): UROPORPHYRINOGEN DECARBOXYLASE deficiency (most common porphyria). Features: PHOTOSENSITIVITY (porphyrins absorb UV light → reactive oxygen species → skin blistering), skin fragility, DARK URINE (uroporphyrins excreted), hypertrichosis (increased facial hair), no acute attacks (no neurological symptoms). Triggers: alcohol, oestrogens, hepatitis C, iron overload. Compare: ACUTE INTERMITTENT PORPHYRIA (AIP) = neurological attacks, no photosensitivity. Desert/sun exposure precipitates PCT symptoms.\n\n📹 [VIDEO LINK]"},{"id":858,"s":"Heme Metabolism","t":"Porphyrias","u":"Lead Poisoning Zinc Protoporphyrin ALA Dehydratase","q":"A 48-year-old man with fatigue and muscle weakness has high urinary delta-aminolevulinic acid. Analysis of his red blood cells reveals 70% contain elevated zinc protoporphyrin. The correct diagnosis is which of the following?","o":["Acute intermittent porphyria","Barbiturate addiction","Congenital erythropoietic porphyria","Iron deficiency","Lead poisoning"],"a":4,"p":"LEAD POISONING: inhibits ALA DEHYDRATASE → ALA elevated + inhibits ferrochelatase → zinc replaces iron → ZINC PROTOPORPHYRIN elevated in RBCs. Peripheral motor neuropathy, basophilic stippling, microcytic anaemia. Iron deficiency = also elevated zinc protoporphyrin but NO ALA elevation. AIP = elevated ALA AND PBG (different pattern).\n\n📹 [VIDEO LINK]"},{"id":859,"s":"Heme Metabolism","t":"Porphyrias","u":"CEP Uroporphyrinogen III Synthase Deficiency","q":"A 10-year-old boy has skin that blisters easily, urine that darkens on standing, and stained teeth. High levels of uroporphyrin I and coproporphyrin I are found in his plasma. The most likely biochemical pathology is which of the following?","o":["Deficiency of ALA synthase","Deficiency of ALA dehydratase","Deficiency of bilirubin glucuronyltransferase","Deficiency of uroporphyrinogen III synthase","Down-regulation of tyrosinase"],"a":3,"p":"CONGENITAL ERYTHROPOIETIC PORPHYRIA (CEP, Günther disease): UROPORPHYRINOGEN III SYNTHASE deficiency → Type I isomers (uroporphyrinogen I) accumulate instead of the functional Type III isomers → deposited in teeth (ERYTHRODONTIA — red-brown stained teeth, pink fluorescence under UV), skin (severe photosensitivity, blistering), and excreted in urine (dark urine). Uroporphyrin I is the non-functional, dead-end isomer. Most severe photosensitive porphyria — requires sun protection.\n\n📹 [VIDEO LINK]"},{"id":860,"s":"Heme Metabolism","t":"Heme Catabolism","u":"Bilirubin UDP Glucuronosyl Transferase Conjugation","q":"The enzyme responsible for conjugation of bilirubin is which of the following?","o":["Bilirubin carboxylase","Bilirubin glucuronyl transferase","Bilirubin oxidase","Bilirubin UDP-glucuronosyl transferase","None of the above"],"a":3,"p":"BILIRUBIN UDP-GLUCURONOSYL TRANSFERASE (UGT1A1): in HEPATOCYTES, conjugates BILIRUBIN with GLUCURONIC ACID → BILIRUBIN DIGLUCURONIDE (conjugated bilirubin, water-soluble, direct bilirubin). Deficiency/absence: CRIGLER-NAJJAR SYNDROME Type I (absent UGT1A1 → fatal neonatal jaundice without liver transplant) and Type II (reduced UGT1A1 → manageable with phototherapy). GILBERT SYNDROME = mildly reduced UGT1A1 (promoter variant) → benign mild unconjugated hyperbilirubinaemia. Inducible by phenobarbital (used in Crigler-Najjar Type II).\n\n📹 [VIDEO LINK]"},{"id":861,"s":"Heme Metabolism","t":"Heme Catabolism","u":"First Bile Pigment Biliverdin Bruise Green Colour","q":"A 30-year-old woman notices her bruise changes colour over several days — initially dark purple-red, then green, then yellow-brown, before fading. In haem catabolism, the first bile pigment formed, responsible for the green colour, is which of the following?","o":["Bilirubin","Biliverdin","Cholic acid","Deoxycholic acid","Urobilinogen"],"a":1,"p":"HAEM CATABOLISM in bruise: Hb released from ruptured RBCs → HAEM OXYGENASE → BILIVERDIN (GREEN colour) + CO + Fe²⁺ → BILIVERDIN REDUCTASE → BILIRUBIN (YELLOW-BROWN). Dark red/purple = deoxyhaemoglobin. GREEN = biliverdin (first bile pigment). YELLOW = bilirubin. Fade = bilirubin cleared by macrophages. Perfect clinical illustration of sequential haem catabolism.\n\n📹 [VIDEO LINK]"},{"id":862,"s":"Heme Metabolism","t":"Heme Catabolism","u":"Fate Conjugated Bilirubin Urobilinogen Intestine","q":"Which of the following best describes the fate of conjugated bilirubin in the intestine?","o":["Biliverdin","Coproporphyrinogen","Haem","Protoporphyrin","Urobilinogen"],"a":4,"p":"CONJUGATED BILIRUBIN (from liver via bile) → intestine → INTESTINAL BACTERIA reduce it → UROBILINOGEN (colourless). Then: urobilinogen → (1) Most oxidised → STERCOBILIN (brown, gives faeces colour) → excreted in faeces. (2) Small amount reabsorbed → portal vein → re-excreted in bile (enterohepatic circulation) OR → systemic circulation → kidney → urine → UROBILIN (yellow). BILIVERDIN = upstream (before bilirubin). PROTOPORPHYRIN = haem biosynthetic intermediate, not catabolism product.\n\n📹 [VIDEO LINK]"},{"id":863,"s":"Heme Metabolism","t":"Heme Catabolism","u":"Heme Oxygenase Cleaves Protoporphyrin IX Ring","q":"In haem catabolism, the catabolism of haemoglobin involves oxidative cleavage of which of the following?","o":["Coproporphyrinogen III ring","Protoporphyrin IX ring","Protoporphyrinogen IX ring","Uroporphyrinogen I ring","Uroporphyrinogen III ring"],"a":1,"p":"HAEM OXYGENASE cleaves the ALPHA-METHENE BRIDGE of the PROTOPORPHYRIN IX RING (the porphyrin ring of haem, after iron is already in the ferrous state). This ring-opening reaction: (1) Releases CO (one carbon from the alpha-methene bridge — the only endogenous source of CO). (2) Releases Fe²⁺. (3) Forms BILIVERDIN (linear tetrapyrrole). The ring opened is PROTOPORPHYRIN IX (not uroporphyrin or coproporphyrin — those are biosynthetic intermediates).\n\n📹 [VIDEO LINK]"},{"id":864,"s":"Heme Metabolism","t":"Heme Catabolism","u":"Full Term Infant UGT Protects Against Icterus Gravis","q":"Full-term infants are protected against icterus gravis neonatorum due to the activity of which of the following enzymes?","o":["ALA synthase","Bilirubin UDP-glucuronosyl transferase","Biliverdin reductase","Haem oxygenase","Porphobilinogen deaminase"],"a":1,"p":"FULL-TERM INFANTS have sufficient BILIRUBIN UDP-GLUCURONOSYL TRANSFERASE (UGT1A1) activity → can conjugate bilirubin → water-soluble conjugated bilirubin excreted in bile → protected from severe jaundice (ICTERUS GRAVIS NEONATORUM). PRETERM INFANTS: immature UGT1A1 → cannot conjugate bilirubin adequately → unconjugated bilirubin accumulates → kernicterus risk. PHOTOTHERAPY: converts bilirubin to water-soluble isomers (lumirubin) → can be excreted WITHOUT conjugation → decreases the REQUIREMENT for UGT1A1.\n\n📹 [VIDEO LINK]"},{"id":865,"s":"Heme Metabolism","t":"Heme Catabolism","u":"Phototherapy Neonate Decreased Bilirubin UGT Requirement","q":"A neonate with physiological jaundice receiving phototherapy is likely to have decreased activity of which of the following enzymes required for bilirubin clearance?","o":["ALA synthase","Bilirubin UDP-glucuronosyl transferase","Biliverdin reductase","Haem oxygenase","Porphobilinogen deaminase"],"a":1,"p":"PHOTOTHERAPY converts bilirubin → LUMIRUBIN (water-soluble structural isomer) and PHOTOBILIRUBIN → these can be excreted in bile and urine WITHOUT hepatic conjugation → BYPASSES the requirement for BILIRUBIN UDP-GLUCURONOSYL TRANSFERASE (UGT1A1). Therefore phototherapy effectively DECREASES THE NEED for UGT1A1 activity. This is why phototherapy works in Crigler-Najjar Type II (reduced but not absent UGT1A1) — phototherapy + residual UGT1A1 maintains acceptable bilirubin levels.\n\n📹 [VIDEO LINK]"},{"id":866,"s":"Heme Metabolism","t":"Heme Catabolism","u":"Haemolytic Anaemia Bilirubin Greater 300mg Day","q":"A 12-year-old boy presents with jaundice and bone pains after a hiking trip. His peripheral blood smear shows anaemia consistent with haemolysis. Bilirubin production in this boy is likely to be which of the following?","o":["Equal to normal at 250mg per day","Greater than 100mg per day","Greater than 300mg per day","Less than 100mg per day","Less than 300mg per day"],"a":2,"p":"NORMAL bilirubin production = ~250-300 mg/day. HAEMOLYTIC ANAEMIA → increased RBC destruction → bilirubin production GREATER THAN 300 mg/day → unconjugated hyperbilirubinaemia. Elevated urinary urobilinogen. No bilirubin in urine (unconjugated not water-soluble). Hiking trip = possible sickle cell crisis or G6PD-induced haemolysis.\n\n📹 [VIDEO LINK]"},{"id":867,"s":"Heme Metabolism","t":"Jaundice","u":"Unconjugated Bilirubin Gilbert Syndrome","q":"Unconjugated bilirubin is a characteristic finding in which of the following?","o":["Chronic active hepatitis","Dubin-Johnson syndrome","Gilbert's syndrome","Primary biliary cirrhosis","Rotor syndrome"],"a":2,"p":"GILBERT SYNDROME = mildly reduced UGT1A1 activity (promoter variant TA7 repeat) → MILD UNCONJUGATED HYPERBILIRUBINAEMIA. Features: benign, asymptomatic, often noticed incidentally or during stress/fasting/illness. Normal liver function. Confirmed by genetic testing or rifampicin provocation test. ROTOR and DUBIN-JOHNSON = CONJUGATED (direct) bilirubin elevation (defective biliary excretion). CHRONIC ACTIVE HEPATITIS = mixed hyperbilirubinaemia. Gilbert's = unconjugated = the classic examination answer.\n\n📹 [VIDEO LINK]"},{"id":868,"s":"Heme Metabolism","t":"Jaundice","u":"Gilbert Syndrome Young Man Mild Unconjugated","q":"A healthy 19-year-old man recovering from a respiratory infection has mild yellow discolouration of skin and eyes. Lab tests reveal mild increased unconjugated bilirubin with no other abnormalities. The most likely diagnosis is which of the following?","o":["Crigler-Najjar syndrome Type I","Crigler-Najjar syndrome Type II","Erythropoietin deficiency","Gilbert syndrome","Lead poisoning"],"a":3,"p":"GILBERT SYNDROME: mild unconjugated hyperbilirubinaemia + no other abnormalities + precipitated by STRESS (infections, fasting, fatigue). Most common inherited disorder of bilirubin metabolism (~8% of population). Benign — no treatment needed. UGT1A1 activity reduced to ~30% of normal. Distinguish from: CRIGLER-NAJJAR Type I = severely elevated unconjugated bilirubin + neurological damage (kernicterus). Type II = moderate elevation, responds to phenobarbital. ERYTHROPOIETIN deficiency = anaemia, no jaundice.\n\n📹 [VIDEO LINK]"},{"id":869,"s":"Heme Metabolism","t":"Jaundice","u":"Obstructive Jaundice Stone CBD Conjugated Bilirubin","q":"A 45-year-old febrile woman presents with jaundice, abdominal pain, and right upper quadrant tenderness. Ultrasound shows a stone in the common bile duct. She is likely to have which of the following?","o":["Decreased total bilirubin","Increased conjugated bilirubin","Increased unconjugated bilirubin","Normal serum bilirubin","Increased indirect bilirubin"],"a":1,"p":"OBSTRUCTIVE JAUNDICE (choledocholithiasis): stone in common bile duct → bile cannot drain → CONJUGATED BILIRUBIN (direct, water-soluble) backs up from bile into blood → ELEVATED CONJUGATED BILIRUBIN. Features: DARK URINE (conjugated bilirubin excreted in urine — bilirubin is water-soluble), PALE STOOLS (no bilirubin reaching intestine → no stercobilin), JAUNDICE, PRURITUS (bile salts in skin). Fever = Charcot's triad (jaundice + pain + fever) = cholangitis. Raised ALP and GGT are key liver enzyme markers.\n\n📹 [VIDEO LINK]"},{"id":870,"s":"Heme Metabolism","t":"Jaundice","u":"Hepatitis A ALT Raised Medical Students","q":"A batch of medical students visiting a fort consumed drinks from a local vendor. Three to four weeks later they all developed jaundice. Which of the following enzymes is likely to be raised in their serum?","o":["Alanine transaminase","Amylase","Aspartate transaminase","Creatine kinase","Lipase"],"a":0,"p":"HEPATITIS A: faecal-oral transmission, incubation 2-4 weeks, epidemic spread (shared food/water source). Hepatocellular damage → ALANINE TRANSAMINASE (ALT/SGPT) is the most SENSITIVE and SPECIFIC marker of hepatocellular injury (located primarily in hepatocytes, rises earlier and higher than AST). ALT:AST ratio >1 = viral hepatitis. AMYLASE = pancreatitis. CREATINE KINASE = myocardial or skeletal muscle damage. AST (option C) also rises but is less specific (found in heart, muscle, RBCs too).\n\n📹 [VIDEO LINK]"},{"id":871,"s":"Heme Metabolism","t":"2,3-BPG and Oxygen Transport","u":"Stored Blood Depleted 2,3-BPG Tissue Hypoxia","q":"A patient injured by bomb blast is transfused stored blood but dies. His tissues did not receive adequate oxygen because the blood transfused had depleted stores of which of the following?","o":["2,3-bisphosphoglycerate","Bicarbonate","Carbonic acid","Glycerol-3-phosphate","Glyceraldehyde-3-phosphate"],"a":0,"p":"STORED BLOOD rapidly depletes 2,3-BPG (2,3-bisphosphoglycerate) within 1-2 weeks of storage. 2,3-BPG binds to beta chains of deoxyHb → DECREASES O₂ AFFINITY of Hb (shifts O₂-Hb dissociation curve RIGHT) → facilitates O₂ RELEASE TO TISSUES. Depleted 2,3-BPG → HIGH O₂ AFFINITY → O₂ BOUND TIGHTLY → not released to tissues → TISSUE HYPOXIA despite adequate circulating haemoglobin. Modern blood banks use additive solutions (AS-1, AS-3) that extend 2,3-BPG preservation. For massively transfused patients, fresher blood improves O₂ delivery.\n\n📹 [VIDEO LINK]"},{"id":872,"s":"Heme Metabolism","t":"2,3-BPG and Oxygen Transport","u":"Fetal Haemoglobin Low 2,3-BPG Affinity High O2 Affinity","q":"Unloading of oxygen from maternal blood into fetal blood occurs because fetal haemoglobin has which of the following properties compared to adult haemoglobin?","o":["Higher affinity for 2,3-BPG","Higher affinity for CO₂","Lower affinity for 2,3-BPG","Lower affinity for oxygen","Same affinity for 2,3-BPG as adult"],"a":2,"p":"HbF (alpha₂ gamma₂): GAMMA CHAINS have lower affinity for 2,3-BPG than adult BETA CHAINS → HbF has HIGHER O₂ AFFINITY than HbA → O₂-Hb curve shifts LEFT for HbF → at placenta, maternal HbA releases O₂ → HbF captures it → O₂ transfers from mother to fetus. HbF replaced by HbA by 6 months of age.\n\n📹 [VIDEO LINK]"},{"id":873,"s":"Carbohydrate Chemistry and Metabolism","t":"Von Gierke Disease","u":"Von Gierke Gluconeogenesis Compromised G6Pase","q":"A 4-month-old baby with fasting hypoglycaemia, hepatomegaly, and seizures is diagnosed with Von Gierke disease. Which of the following pathways is compromised?","o":["Fatty acid oxidation","Gluconeogenesis","Glycogenesis","Glycolysis","Ketolysis"],"a":1,"p":"VON GIERKE (GSD Type I): GLUCOSE-6-PHOSPHATASE (G6Pase) deficiency → G6P cannot be converted to free glucose → BOTH glycogenolysis AND gluconeogenesis cannot release free glucose into blood → FASTING HYPOGLYCAEMIA. G6P accumulates → glycogen + fat accumulation → massive hepatomegaly. Lactic acidosis (glycolysis overactive), hyperuricaemia (PPP overactive), hyperlipidaemia.\n\n📹 [VIDEO LINK]"},{"id":874,"s":"Carbohydrate Chemistry and Metabolism","t":"Von Gierke Disease","u":"Von Gierke G6Pase Deficient Enzyme","q":"A 7-month-old female infant presents with abdominal distension, hepatomegaly, doll-like facies, fatty cheeks, and protuberant abdomen. Lab results show hypoglycaemia, lactic acidosis, hyperlipidaemia, and hyperuricaemia. Liver biopsy confirms Von Gierke disease. Which enzyme is deficient in her?","o":["Acid phosphatase","Alkaline phosphatase","Glucose-6-phosphatase","Glycogen phosphorylase","Protein phosphatase"],"a":2,"p":"VON GIERKE DISEASE (GSD Type Ia): GLUCOSE-6-PHOSPHATASE (G6Pase) deficiency. G6Pase catalyses: G6P → glucose + Pi (in liver, kidney, intestine). Without G6Pase → G6P accumulates → cannot exit hepatocytes → GLYCOGEN accumulates (hepatomegaly) + FAT accumulates + G6P → glycolysis → PYRUVATE → LACTATE (lactic acidosis) + G6P → PPP → ribose-5-P → PURINE SYNTHESIS → uric acid (hyperuricaemia). DOLL-LIKE FACIES from fat deposition. Treatment: frequent corn starch + continuous nocturnal feeds.\n\n📹 [VIDEO LINK]"},{"id":875,"s":"Carbohydrate Chemistry and Metabolism","t":"Von Gierke Disease","u":"Von Gierke Hypoglycaemia Glycolysis Overactive","q":"An 18-month-old with Von Gierke disease has raised uric acid levels and presents in a comatose state with hypoglycaemia. Hyperactivity of which of the following pathways is responsible for his lactic acidosis?","o":["Gluconeogenesis pathway","Glycogen synthesis pathway","Glycolysis pathway","Pentose phosphate pathway","Urea cycle"],"a":2,"p":"VON GIERKE: G6Pase deficiency → G6P cannot be exported as glucose → ACCUMULATES → forced into GLYCOLYSIS (overactive) → pyruvate → LACTATE → LACTIC ACIDOSIS. Also: G6P → pentose phosphate pathway → ribose-5-P → purines → uric acid (HYPERURICAEMIA). GLUCONEOGENESIS pathway = overactive (tries to produce glucose) but blocked at G6Pase step → G6P accumulates rather than being exported. Treatment: sodium bicarbonate for acidosis + glucose infusion for hypoglycaemia.\n\n📹 [VIDEO LINK]"},{"id":876,"s":"Carbohydrate Chemistry and Metabolism","t":"Pyruvate Kinase Deficiency","u":"PK Deficiency Non-Spherocytic Haemolytic Anaemia","q":"A 12-year-old boy presents with chronic non-spherocytic anaemia, jaundice, and increased red cell haemolysis. Red blood cell enzyme assay is done. Which of the following important enzymes of glycolysis is likely to have reduced activity?","o":["Glucokinase","Hexokinase","Phosphofructokinase","Phosphoglycerate mutase","Pyruvate kinase"],"a":4,"p":"PYRUVATE KINASE (PK) DEFICIENCY: most common glycolytic enzyme deficiency causing HEREDITARY NON-SPHEROCYTIC HAEMOLYTIC ANAEMIA (HNSHA). PK deficiency → cannot generate ATP from the last step of glycolysis (PEP → pyruvate + ATP) → RBCs depleted of ATP → Na⁺/K⁺-ATPase fails → RBC membrane disruption → haemolysis. JAUNDICE from increased bilirubin (haemolysis). 2,3-BPG ACCUMULATES (upstream of PK) → lower O₂ affinity of Hb → better O₂ delivery to tissues (partial compensation). Diagnosis: RBC enzyme assay showing reduced PK activity.\n\n📹 [VIDEO LINK]"},{"id":877,"s":"Lipid Chemistry","t":"Reactive Oxygen Species","u":"Radiation Generates ROS Cancer Treatment Breast","q":"A 53-year-old woman with early-stage breast cancer undergoes lumpectomy followed by radiation therapy. Radiation works, in part, by which of the following?","o":["Generating hypochlorous acid","Generating reactive oxygen species","Inhibiting NADH oxidase","Inhibiting the cytochrome bc1 complex","Stimulating glutathione production"],"a":1,"p":"RADIATION THERAPY → RADIOLYSIS OF WATER: H₂O → OH• (HYDROXYL RADICAL) + H• → REACTIVE OXYGEN SPECIES. ROS → DNA strand breaks → tumour cell death. Cancer cells are more sensitive because: faster cell division → less time for DNA repair + often deficient in repair mechanisms. Normal tissue damage = radiation side effects. GLUTATHIONE and antioxidants PROTECT normal cells (option D wrong — stimulating glutathione would be PROTECTIVE not the mechanism of radiation therapy). HYPOCHLOROUS ACID = produced by neutrophils (myeloperoxidase) — not radiation.\n\n📹 [VIDEO LINK]"},{"id":878,"s":"Enzymes","t":"Enzyme Classification","u":"Hydrolases Pancreatic Lipase TAG Digestion","q":"A medical student consumes a zinger burger. As the fatty burger enters his small intestine, the triacylglycerols are broken down to monoacylglycerols by the action of which of the following classes of enzymes?","o":["Hydrolases","Isomerases","Ligases","Oxidoreductases","Transferases"],"a":0,"p":"PANCREATIC LIPASE = HYDROLASE → cleaves ESTER BONDS in triacylglycerols by addition of water → monoacylglycerol (sn-2 position) + 2 FREE FATTY ACIDS. Requires COLIPASE. Products absorbed → reassembled into TAG → chylomicrons → lymphatics → blood. HYDROLASES cleave bonds with water. ISOMERASES = rearrange bonds. LIGASES = form bonds using ATP. OXIDOREDUCTASES = electron transfer. TRANSFERASES = move groups between molecules.\n\n📹 [VIDEO LINK]"},{"id":879,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"Cyanide Complex IV Inhibition Factory Worker","q":"A 28-year-old factory worker is found collapsed in a chemical plant after accidental release of hydrogen cyanide gas from a faulty storage tank. Inhibition of which of the following complexes of the electron transport chain led to his collapse?","o":["Complex I","Complex II","Complex III","Complex IV","Complex V"],"a":3,"p":"CYANIDE (CN⁻) → binds Fe³⁺ in haem a₃ of CYTOCHROME C OXIDASE (COMPLEX IV) → blocks electron transfer to O₂ → ETC arrested → ATP depletion → cellular asphyxia → collapse. Same inhibition site as CO (CO binds Fe²⁺, CN⁻ binds Fe³⁺). Cherry-red skin, bitter almonds smell, lactic acidosis, high venous pO₂ (O₂ cannot be utilised). Treatment: HYDROXOCOBALAMIN (first-line) or DICOBALT EDETATE + 100% O₂.\n\n📹 [VIDEO LINK]"},{"id":880,"s":"Enzymes","t":"Enzyme Inhibition","u":"Organophosphorus Inhibits Acetylcholinesterase Farmer","q":"A farmer is brought to the emergency department with miosis, excessive salivation, bronchospasm, muscle fasciculations, and urinary incontinence after spraying pesticide without protective equipment. The pesticide inhibits which of the following enzymes?","o":["Acetylcholinesterase","Adenylate cyclase","Creatine kinase","Monoamine oxidase","Phospholipase A2"],"a":0,"p":"ORGANOPHOSPHORUS COMPOUNDS (malathion, parathion, sarin) → IRREVERSIBLY inhibit ACETYLCHOLINESTERASE (AChE) by phosphorylating the serine residue in its active site → ACh accumulates at cholinergic synapses → MUSCARINIC effects: SLUDGE (Salivation, Lacrimation, Urination, Defecation, GI distress, Emesis) + NICOTINIC effects: muscle fasciculations, weakness, paralysis. Treatment: ATROPINE (blocks muscarinic receptors) + PRALIDOXIME (regenerates AChE if given early before ageing).\n\n📹 [VIDEO LINK]"},{"id":881,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Uncouplers of ETC","u":"Aspirin Salicylate Uncouples ETC Suicide Attempt","q":"A 22-year-old woman is brought to the emergency department after a suspected suicidal attempt by ingesting a large quantity of aspirin tablets. She presents with hyperpnoea, hyperthermia, and profuse sweating. At toxic concentrations, salicylate affects the electron transport chain by which of the following mechanisms?","o":["Inhibiting Complex I","Inhibiting Complex III","Inhibiting Complex IV","Inhibiting Complex V","Uncoupling"],"a":4,"p":"SALICYLATE at TOXIC CONCENTRATIONS = UNCOUPLER → lipid-soluble weak acid → shuttles protons across inner mitochondrial membrane → proton gradient DISSIPATED → electron transport runs unchecked → O₂ consumption increases → energy released as HEAT → HYPERTHERMIA + SWEATING. SALICYLATE TOXICITY acid-base: (1) Early: RESPIRATORY ALKALOSIS (stimulates respiratory centre → hyperventilation → CO₂ blown off). (2) Late: METABOLIC ACIDOSIS (uncoupling → lactic acidosis + salicylate itself is an organic acid). Uncoupling occurs at TOXIC DOSES ONLY. Therapeutic dose aspirin = COX inhibitor only.\n\n📹 [VIDEO LINK]"},{"id":882,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Clinical ETC Disorders","u":"MELAS Complex I Mitochondrial Inheritance TIA","q":"A 14-year-old girl presents with recurrent transient ischaemic attacks, lactic acidosis, and sensorineural hearing loss. Her mother has similar symptoms. Genetic testing reveals a mitochondrial DNA point mutation. She is diagnosed with MELAS syndrome. Which complex of the electron transport chain is most likely affected?","o":["Complex I","Complex II","Complex III","Complex IV","Complex V"],"a":0,"p":"MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis, Stroke-like episodes): most commonly caused by m.3243A>G point mutation in mtDNA (tRNA-Leu gene) → impaired synthesis of COMPLEX I subunits → COMPLEX I deficiency. Features: stroke-like episodes/TIAs before age 40, lactic acidosis, ragged red fibres on muscle biopsy, sensorineural hearing loss, diabetes, short stature. MATERNAL INHERITANCE — mitochondrial DNA inherited exclusively from mother → no male-to-offspring transmission.\n\n📹 [VIDEO LINK]"},{"id":883,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Clinical ETC Disorders","u":"Leigh Syndrome Bilateral Basal Ganglia Mitochondrial Mutation","q":"A 6-month-old infant presents with developmental regression, hypotonia, and abnormal eye movements. MRI shows bilateral symmetric lesions in the basal ganglia and brainstem. Plasma lactate is markedly elevated. Workup for mitochondrial mutation is underway. Which of the following is the most likely diagnosis?","o":["Gaucher disease","Krabbe disease","Leigh syndrome","MELAS syndrome","Wilson disease"],"a":2,"p":"LEIGH SYNDROME (subacute necrotising encephalomyelopathy): most commonly caused by COMPLEX I, IV, or V (PDH complex) deficiency → ATP depletion in high-energy-demand tissues → BILATERAL SYMMETRIC LESIONS in BASAL GANGLIA and BRAINSTEM on MRI (pathognomonic). Lactic acidosis. Features: developmental regression, hypotonia, respiratory failure, abnormal eye movements. Workup involves both mitochondrial and nuclear genome sequencing. Most common childhood mitochondrial disease.\n\n📹 [VIDEO LINK]"},{"id":884,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"Atractyloside ANT Inhibitor Shepherd Poisoning","q":"A 35-year-old shepherd in the Mediterranean is brought to the emergency department after consuming Atractylis gummifera, mistaking it for an edible herb. He develops rapid deterioration of consciousness and lactic acidosis. ATP synthesis stops despite intact electron transport. Atractyloside inhibits which of the following?","o":["ADP/ATP translocase","NADH dehydrogenase","Succinate dehydrogenase","Ubiquinol-cytochrome c reductase","Cytochrome c oxidase"],"a":0,"p":"ATRACTYLOSIDE → inhibits ADENINE NUCLEOTIDE TRANSLOCASE (ADP/ATP translocase, ANT) → ADP CANNOT ENTER mitochondria → ATP CANNOT EXIT → ATP synthesis abolished despite electron transport continuing initially. ANT: normal function = exchanges matrix ATP for cytosolic ADP. Without ANT: ADP depleted in matrix → ATP synthase stalls → proton gradient builds → electron transport slows → O₂ consumption decreases → lactic acidosis → cardiovascular collapse → death.\n\n[Note to editor: Insert diagram of ETC showing ANT in inner mitochondrial membrane circled in red. Show ADP blocked from entering matrix and ATP blocked from exiting.]\n\n📹 [VIDEO LINK]"},{"id":885,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"Bongkrekic Acid ANT Inhibitor Indonesia Poisoning","q":"A young man in Indonesia is brought to the emergency department after consuming bongkrek, a fermented coconut food contaminated with Burkholderia gladioli bacteria. He develops severe lactic acidosis and cardiovascular collapse. The toxin bongkrekic acid inhibits which of the following?","o":["ADP/ATP translocase","NADH dehydrogenase","Succinate dehydrogenase","Ubiquinol-cytochrome c reductase","Cytochrome c oxidase"],"a":0,"p":"BONGKREKIC ACID → inhibits ADP/ATP TRANSLOCASE (ANT) → same metabolic consequences as atractyloside → ATP synthesis abolished → cardiovascular collapse. KEY DIFFERENCE from atractyloside: bongkrekic acid LOCKS ANT in M-STATE (matrix-facing) while atractyloside locks ANT in C-STATE (cytosol-facing). Both inhibit ANT but at different conformational states → identical metabolic consequences. Bongkrek food poisoning = historically deadly outbreaks in Indonesia.\n\n📹 [VIDEO LINK]"},{"id":886,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"Amytal Complex I Inhibitor Marilyn Monroe","q":"The famous actress Marilyn Monroe died in 1962 from an overdose of amobarbital (Amytal). Amytal inhibits which of the following complexes of the electron transport chain?","o":["Complex I","Complex II","Complex III","Complex IV","Complex V"],"a":0,"p":"AMYTAL (amobarbital) and other barbiturates → inhibit COMPLEX I (NADH-Q oxidoreductase) → block electron transfer from NADH to CoQ. Same complex as ROTENONE and MPP⁺ (Parkinson toxin). Complex I inhibition → NADH accumulates → TCA slows → lactic acidosis → ATP depletion → CNS depression → respiratory arrest. Barbiturates at low doses = sedative/hypnotic (enhance GABA-A). At toxic doses = Complex I inhibition contributes to metabolic depression.\n\n📹 [VIDEO LINK]"},{"id":887,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"Doxorubicin Cardiomyopathy ETC ROS Hodgkin Lymphoma","q":"A 25-year-old woman treated for Hodgkin lymphoma with doxorubicin develops cardiomyopathy. Doxorubicin generates reactive oxygen species that damage the inner mitochondrial membrane. Which of the following best describes the effect on the electron transport chain in her cardiac mitochondria?","o":["Decreased electron transport and decreased oxygen consumption","Decreased electron transport and increased oxygen consumption","Increased electron transport and decreased oxygen consumption","Increased electron transport and increased ATP production","Uncoupled electron transport and increased thermogenesis"],"a":0,"p":"DOXORUBICIN → generates REACTIVE OXYGEN SPECIES (via semiquinone radical cycling) → oxidative damage to inner mitochondrial membrane → INHIBITS ETC components (particularly Complex I) → DECREASED ELECTRON TRANSPORT → DECREASED O₂ CONSUMPTION → NADH accumulates → TCA cycle slows → DECREASED CO₂ production → ATP production DECREASED → cardiomyocytes ATP-depleted → CARDIOMYOPATHY. Doxorubicin-induced cardiomyopathy is dose-dependent. Dexrazoxane (iron chelator) is cardioprotective.\n\n[Note to editor: Insert diagram of ETC showing Complexes I-IV and ATP synthase. Circle Complex I in red showing site of ROS-mediated damage by doxorubicin.]\n\n📹 [VIDEO LINK]"},{"id":888,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"Antimycin Complex III Poisoning Death","q":"A young man is brought to the emergency department as a case of poisoning. He dies later and the toxicology report confirms antimycin overdose. Inhibition of which complex of the electron transport chain is likely to have caused his death?","o":["Complex I","Complex II","Complex III","Complex IV","Complex V"],"a":2,"p":"ANTIMYCIN A → inhibits COMPLEX III (ubiquinol-cytochrome c reductase) → blocks electron transfer from CoQH₂ to cytochrome c → ETC arrested → O₂ consumption decreases → NADH and FADH₂ accumulate → TCA cycle inhibited → CO₂ decreases → ATP production abolished → lactic acidosis → death.\n\n[Note to editor: Insert diagram of ETC showing Complex III circled in red. Electrons backed up at CoQ. No electrons reaching cytochrome c, Complex IV, or O₂.]\n\n📹 [VIDEO LINK]"},{"id":889,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"Atractyloside Poisoning Complex V Death","q":"A young man is brought to the emergency department as a case of poisoning. He dies later and the toxicology report confirms atractyloside poisoning. Inhibition of which complex of the electron transport chain is likely to have caused his death?","o":["Complex I","Complex II","Complex III","Complex IV","Complex V"],"a":4,"p":"ATRACTYLOSIDE → inhibits ADP/ATP TRANSLOCASE (ANT) → ADP cannot enter mitochondria → ATP synthase (COMPLEX V) STALLS → proton gradient builds → back-pressure inhibits electron transport → eventually ALL complexes slow → ATP synthesis abolished → lactic acidosis → cardiovascular collapse → death. Primary target = ANT. Downstream effect = Complex V stops first, followed by inhibition of the entire chain.\n\n[Note to editor: Insert ETC diagram. Show ANT blocked (red X). Show proton gradient building. Show Complex V stalled.]\n\n📹 [VIDEO LINK]"},{"id":890,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Inhibitors of ETC","u":"Bongkrekate Poisoning Complex V Death","q":"A young man is brought to the emergency department as a case of poisoning. He dies later and the toxicology report confirms bongkrekate poisoning. Inhibition of which complex of the electron transport chain is likely to have caused his death?","o":["Complex I","Complex II","Complex III","Complex IV","Complex V"],"a":4,"p":"BONGKREKIC ACID → inhibits ADP/ATP TRANSLOCASE (ANT) → same mechanism as atractyloside → ATP synthase (COMPLEX V) stalls (no ADP substrate) → proton gradient builds → electron transport chain slows → ATP depletion → death. Bongkrekic acid locks ANT in m-state (matrix-facing). Atractyloside locks ANT in c-state (cytosol-facing). Both → Complex V arrest → lethal ATP depletion.\n\n📹 [VIDEO LINK]"},{"id":891,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Complex II Function Succinate to CoQ No Proton Pumping","q":"Which of the following correctly describes the function of Complex II in the electron transport chain?","o":["Pumps protons from matrix to intermembrane space","Transfers electrons from NADH to coenzyme Q","Transfers electrons from succinate to coenzyme Q","Transfers electrons from coenzyme Q to cytochrome c","Transfers electrons from cytochrome c to oxygen"],"a":2,"p":"FOUR COMPLEXES: Complex I (NADH-Q oxidoreductase) = NADH → CoQ + pumps H⁺. Complex II (succinate-Q reductase) = SUCCINATE → CoQ (does NOT pump H⁺ — important distinction). Complex III (Q-cytochrome c oxidoreductase) = CoQ → cytochrome c + pumps H⁺. Complex IV (cytochrome c oxidase) = cytochrome c → O₂ + pumps H⁺. Only Complexes I, III, IV pump protons. Complex II is the only complex that does NOT contribute to the proton gradient.\n\n📹 [VIDEO LINK]"},{"id":892,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Flavoproteins Two Electron Bridging Role ETC","q":"Which of the following correctly describes the role of flavoproteins in the electron transport chain?","o":["Direct proton pumping","One-electron transfer","Oxygen reduction","Single electron donation","Two-electron bridging"],"a":4,"p":"FLAVOPROTEINS (FMN in Complex I, FAD in Complex II) = TWO-ELECTRON CARRIERS — accept 2 electrons + 2 protons → FMNH₂ or FADH₂ → pass electrons ONE AT A TIME to iron-sulfur clusters. They bridge TWO-ELECTRON DONORS (NADH, FADH₂) and ONE-ELECTRON CARRIERS (iron-sulfur clusters, cytochromes). This two-to-one electron bridging function is unique to flavoproteins. Without flavoproteins, electrons from NADH and FADH₂ cannot be fed into the one-electron cytochrome chain.\n\n📹 [VIDEO LINK]"},{"id":893,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Iron Sulfur Proteins One Electron Transfer ETC","q":"Which of the following correctly describes the role of iron-sulfur proteins in the electron transport chain?","o":["Cytochrome reduction","NADH oxidation","One-electron transfer","Proton pumping","Two-electron transfer"],"a":2,"p":"IRON-SULFUR (Fe-S) PROTEINS contain Fe₂S₂ or Fe₄S₄ clusters → iron cycles between Fe²⁺ and Fe³⁺ → ONE-ELECTRON CARRIERS. Located in Complexes I, II, and III. Receive electrons one at a time from FMNH₂ (Complex I) or FADH₂ (Complex II) → pass to CoQ. Bridge between flavoproteins (two-electron donors) and CoQ (which receives one electron at a time in its semiquinone form).\n\n📹 [VIDEO LINK]"},{"id":894,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"CoQ Accepts Electrons from Complex I and II Convergence","q":"Coenzyme Q (ubiquinone) in the electron transport chain accepts electrons from which of the following?","o":["Complex I and Complex II","Complex I only","Complex II only","Complex III and Complex IV","Complex III only"],"a":0,"p":"CoQ (UBIQUINONE) = MOBILE ELECTRON CARRIER in the inner mitochondrial membrane. Accepts electrons from: (1) COMPLEX I (via FMN → Fe-S clusters → CoQ). (2) COMPLEX II (via FAD → Fe-S clusters → CoQ). Also from ETF-QO (fatty acid beta-oxidation) and glycerol-3-phosphate dehydrogenase. CoQ = CONVERGENCE POINT for electrons from multiple sources → passes electrons to Complex III. CoQ can accept 1 electron (semiquinone = CoQH•) or 2 electrons (ubiquinol = CoQH₂).\n\n📹 [VIDEO LINK]"},{"id":895,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Q Cycle CoQ Transfers Electrons to Cytochrome c Complex III","q":"During the Q cycle in Complex III, coenzyme Q transfers electrons ultimately to which of the following?","o":["Complex I","Complex IV","Cytochrome c","FADH₂","Oxygen"],"a":2,"p":"Q CYCLE (Mitchell Q cycle) in COMPLEX III: CoQH₂ (ubiquinol) → Complex III → electrons split: one electron → Fe-S cluster → cytochrome c1 → CYTOCHROME c (mobile carrier, exits to intermembrane space) → Complex IV. Second electron → cytochrome b → CoQ regenerated (recycled). NET RESULT: 2 CoQH₂ oxidised, 2 cytochrome c reduced, 4 H⁺ pumped to IMS. Q cycle DOUBLES the proton pumping efficiency of Complex III.\n\n📹 [VIDEO LINK]"},{"id":896,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Complex IV Cytochrome c Oxidised Oxygen Reduced","q":"In the electron transport chain, cytochrome c is oxidised and oxygen is reduced at which of the following complexes?","o":["Complex I","Complex II","Complex III","Complex IV","Complex V"],"a":3,"p":"COMPLEX IV (CYTOCHROME C OXIDASE): receives electrons from CYTOCHROME c (Fe²⁺ → Fe³⁺, cytochrome c OXIDISED) → electrons transferred to OXYGEN (O₂ → H₂O, oxygen REDUCED). Full 4-electron reduction: O₂ + 4H⁺ + 4e⁻ → 2H₂O. Pumps 4H⁺ per O₂ reduced. Contains haem a, haem a₃, and CuB centre. Inhibited by: CO (binds Fe²⁺), cyanide (binds Fe³⁺), azide, hydrogen sulfide.\n\n📹 [VIDEO LINK]"},{"id":897,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Proton Pumping Complexes I III IV Not Complex II","q":"Protons are pumped from the mitochondrial matrix to the intermembrane space by which of the following complexes?","o":["Complexes I and III only","Complexes I, II, and III","Complexes I, II, and IV","Complexes I, III, and IV","Complexes I, II, III, and IV"],"a":3,"p":"PROTON PUMPING COMPLEXES: I, III, and IV pump H⁺ from MATRIX to INTERMEMBRANE SPACE. COMPLEX II = does NOT pump protons (electrons enter at CoQ level, bypassing Complex I) — this is why FADH₂ produces less ATP (1.5) than NADH (2.5). Proton pumping creates PROTON MOTIVE FORCE → protons flow back through Complex V (ATP synthase) → ATP synthesis.\n\n📹 [VIDEO LINK]"},{"id":898,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"ADP Primary Factor Controlling Respiration Rate","q":"Which of the following is the PRIMARY factor controlling the rate of mitochondrial respiration under physiological conditions?","o":["Availability of ADP","Availability of CoQ","Concentration of cytochrome c","Oxygen partial pressure","Temperature"],"a":0,"p":"FIVE CONDITIONS controlling mitochondrial respiration rate (Chance and Williams): (1) ADP AVAILABILITY (primary physiological regulator — respiratory control). (2) Substrate availability (NADH, FADH₂). (3) O₂ availability. (4) Inorganic phosphate (Pi) availability. (5) Integrity of inner mitochondrial membrane. ADP is the PRINCIPAL REGULATOR because it directly reflects cellular ATP demand. High ADP → high respiration. Low ADP → low respiration.\n\n📹 [VIDEO LINK]"},{"id":899,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Oxidative Phosphorylation","u":"Malate Aspartate Shuttle Maintains Electrochemical Equilibrium","q":"The malate-aspartate shuttle uses which of the following transporters to maintain electrochemical equilibrium while transferring reducing equivalents into mitochondria?","o":["ANT translocase","Citrate carrier","Malate-aspartate shuttle","Pyruvate carrier","Uncoupling protein"],"a":2,"p":"MALATE-ASPARTATE SHUTTLE uses TWO ANTIPORTERS: (1) MALATE-ALPHA-KETOGLUTARATE ANTIPORTER: malate enters matrix, alpha-KG exits. (2) GLUTAMATE-ASPARTATE ANTIPORTER: glutamate enters matrix, aspartate exits (electrogenic — driven by membrane potential). Together these maintain electrochemical and osmotic equilibrium while transferring cytosolic NADH equivalents into mitochondria → 2.5 ATP per cytosolic NADH.\n\n📹 [VIDEO LINK]"},{"id":900,"s":"Bioenergetics and Oxidative Phosphorylation","t":"Electron Transport Chain","u":"Heart NADH Glycolysis Malate Aspartate Shuttle 2.5 ATP","q":"NADH generated during glycolysis cannot directly cross the inner mitochondrial membrane. Which of the following best describes the mechanism by which this NADH is utilised in heart muscle?","o":["Direct crossing","FADH₂ conversion","Glycerol-3-phosphate shuttle","Malate-aspartate shuttle","Outer membrane pore"],"a":3,"p":"HEART uses MALATE-ASPARTATE SHUTTLE → cytosolic NADH transferred as MALATE → mitochondrial NADH → Complex I → 2.5 ATP per cytosolic NADH. BRAIN uses GLYCEROL-3-PHOSPHATE SHUTTLE → 1.5 ATP per cytosolic NADH. NAD and NADH cannot directly cross the inner mitochondrial membrane. Heart prioritises maximum ATP yield.\n\n📹 [VIDEO LINK]"},{"id":901,"s":"Cell Membrane","t":"Membrane Structure","u":"Fluid Mosaic Model Singer Nicolson Lipid Bilayer","q":"The fluid mosaic model of cell membrane structure proposed by Singer and Nicolson describes the membrane as which of the following?","o":["A bilayer of proteins with lipids interspersed","A fixed rigid structure of phospholipids","A lipid bilayer with freely moving proteins embedded within it","A monolayer of amphipathic lipids","A protein scaffold coated with lipids"],"a":2,"p":"FLUID MOSAIC MODEL (Singer and Nicolson, 1972): cell membrane = PHOSPHOLIPID BILAYER (fluid, not rigid) with PROTEINS EMBEDDED WITHIN IT. Key features: lateral diffusion (rapid), flip-flop (very slow, requires flippase). INTEGRAL PROTEINS = span bilayer (transmembrane). PERIPHERAL PROTEINS = attached to surface. Cholesterol regulates fluidity. Membrane proteins: receptors, channels, enzymes, transporters.\n\n📹 [VIDEO LINK]"},{"id":902,"s":"Cell Membrane","t":"Membrane Structure","u":"Lipid Rafts Cholesterol Sphingomyelin Microdomains","q":"Lipid rafts are specialised microdomains within the cell membrane enriched in which of the following?","o":["Cardiolipin and phosphatidylserine","Cholesterol and sphingomyelin","Phosphatidylcholine and phosphatidylethanolamine","Phosphatidylinositol and phosphatidylserine","Triacylglycerol and cholesteryl esters"],"a":1,"p":"LIPID RAFTS = microdomains enriched in CHOLESTEROL and SPHINGOMYELIN → more ordered, less fluid (liquid-ordered phase). Functions: platform for signal transduction, membrane trafficking, pathogen entry (HIV, cholera toxin GM1). CAVEOLAE = flask-shaped invaginations of lipid rafts containing caveolin → endocytosis, signal transduction.\n\n📹 [VIDEO LINK]"},{"id":903,"s":"Cell Membrane","t":"Membrane Transport","u":"Na K ATPase Electrogenic 3 Na Out 2 K In","q":"The Na plus/K plus-ATPase pump is described as electrogenic because it pumps which of the following?","o":["2 Na plus out and 2 K plus in","2 Na plus out and 3 K plus in","3 Na plus in and 2 K plus out","3 Na plus out and 2 K plus in","Equal numbers in opposite directions"],"a":3,"p":"Na⁺/K⁺-ATPase: pumps 3 Na⁺ OUT and 2 K⁺ IN per ATP. ELECTROGENIC — net positive charge pumped OUT → contributes to negative resting membrane potential (~−10 mV). Inhibited by: OUABAIN, DIGOXIN (cardiac glycosides → ↑ intracellular Na⁺ → ↑ intracellular Ca²⁺ via Na⁺/Ca²⁺ exchanger → ↑ cardiac contractility). Mutations → familial hemiplegic migraine.\n\n📹 [VIDEO LINK]"},{"id":904,"s":"Cell Membrane","t":"Endocytosis","u":"Clathrin Coated Pit LDL Receptor Mediated Endocytosis","q":"A patient with familial hypercholesterolaemia has defective LDL receptors. Normally, LDL is internalised into cells by binding to LDL receptors concentrated in which of the following?","o":["Caveolae","Clathrin-coated pits","Lipid rafts","Macropinosomes","Phagosomes"],"a":1,"p":"RECEPTOR-MEDIATED ENDOCYTOSIS: LDL binds LDL receptor → complex clusters in CLATHRIN-COATED PITS → pit invaginates → clathrin-coated vesicle → clathrin shed → endosome → LYSOSOME → LDL degraded → cholesterol released → downregulates HMG-CoA reductase and LDL receptor synthesis. FAMILIAL HYPERCHOLESTEROLAEMIA: LDL receptor absent/defective → LDL cannot be internalised → accumulates in plasma → atherosclerosis.\n\n📹 [VIDEO LINK]"},{"id":905,"s":"Cell Membrane","t":"Glucose Transport","u":"GLUT4 Insulin Sensitive Muscle Adipose Tissue","q":"A type 2 diabetic patient has impaired glucose uptake by muscle and adipose tissue. Under normal conditions, insulin stimulates glucose uptake in these tissues by promoting translocation of which of the following?","o":["GLUT1","GLUT2","GLUT3","GLUT4","GLUT5"],"a":3,"p":"GLUT4 = INSULIN-SENSITIVE glucose transporter in MUSCLE and ADIPOSE TISSUE. Normally sequestered in intracellular vesicles → insulin → PI3K → Akt → GLUT4 vesicles translocate to plasma membrane → increased glucose uptake. In TYPE 2 DIABETES: insulin resistance → impaired GLUT4 translocation → reduced glucose uptake. GLUT1 = ubiquitous. GLUT2 = liver, pancreatic beta cells (glucose sensor). GLUT3 = neurons (high affinity). GLUT5 = fructose (intestine).\n\n📹 [VIDEO LINK]"},{"id":906,"s":"Cell Membrane","t":"Membrane Transport","u":"Donnan Equilibrium RBC Unequal Ion Distribution","q":"Red blood cells contain large negatively charged proteins that cannot cross the membrane. As a result, the intracellular concentration of chloride is lower than extracellular. This phenomenon is best explained by which of the following?","o":["Active transport","Donnan equilibrium","Osmosis","Primary active transport","Simple diffusion"],"a":1,"p":"DONNAN (GIBBS-DONNAN) EQUILIBRIUM: membrane permeable to small ions but NOT large charged molecules (proteins) → unequal distribution of diffusible ions across membrane. In RBCs: intracellular proteins (Hb, enzymes) negatively charged and cannot leave → attract cations (K⁺) and repel anions (Cl⁻) → [Cl⁻]inside < [Cl⁻]outside. Affects osmotic pressure and ion distribution.\n\n📹 [VIDEO LINK]"},{"id":907,"s":"Cell Membrane","t":"Signal Transduction","u":"Cholera Toxin Gsalpha ADP Ribosylation Increased cAMP","q":"A patient with cholera develops profuse watery diarrhoea. Cholera toxin ADP-ribosylates the Gsalpha subunit, locking it in the active state. Which of the following is the direct downstream consequence?","o":["Decreased adenylate cyclase activity","Decreased cAMP production","Increased cAMP production","Increased IP3 production","Increased PKC activity"],"a":2,"p":"CHOLERA TOXIN → ADP-ribosylates Gsα (arginine) → Gsα CANNOT hydrolyse GTP → PERMANENTLY ACTIVE → CONTINUOUS ACTIVATION of ADENYLATE CYCLASE → MASSIVELY ELEVATED cAMP → PKA activated → CFTR phosphorylated → massive Cl⁻ secretion → water follows osmotically → PROFUSE WATERY DIARRHOEA (rice-water stools). Compare: PERTUSSIS TOXIN → ADP-ribosylates Giα (cysteine) → Giα permanently inactive → cannot inhibit adenylate cyclase.\n\n📹 [VIDEO LINK]"},{"id":908,"s":"Cell Membrane","t":"Signal Transduction","u":"Insulin Receptor Tyrosine Kinase GLUT4 Translocation","q":"Insulin binds to its receptor on muscle cells. The insulin receptor belongs to which of the following classes of receptors?","o":["G protein-coupled receptor","Intracellular nuclear receptor","Ion channel-linked receptor","Receptor tyrosine kinase","Serine-threonine kinase receptor"],"a":3,"p":"INSULIN RECEPTOR = RECEPTOR TYROSINE KINASE (RTK): insulin binds alpha subunits → beta subunits (intrinsic tyrosine kinase) AUTOPHOSPHORYLATE → phosphorylate IRS-1 → PI3K → PIP3 → PDK1 → AKT (PKB) → GLUT4 translocation + glycogen synthesis (GSK-3β inhibition) + protein synthesis (mTOR). RTKs also include: EGF, PDGF, VEGF receptors. Compare: STEROID HORMONES = intracellular nuclear receptors.\n\n📹 [VIDEO LINK]"},{"id":909,"s":"Cytoskeleton","t":"Actin Dynamics","u":"Actin Treadmilling Cell Migration Profilin Mutation","q":"During cell migration, actin filaments grow at the leading edge and depolymerise at the trailing edge. A cell line is found to have impaired migration. A mutation in which of the following is most likely responsible?","o":["Capping proteins","Collagen","Dynein","Profilin","Spectrin"],"a":3,"p":"TREADMILLING drives cell migration. PROFILIN = promotes G-actin (ATP-actin) delivery and polymerisation onto BARBED ENDS (plus ends) at the leading edge → essential for treadmilling. Mutation in profilin → G-actin cannot polymerise at barbed end → treadmilling stops → migration impaired. COFILIN promotes depolymerisation at pointed end (also needed). CAPPING PROTEINS stabilise barbed ends. DYNEIN = microtubule motor. SPECTRIN = RBC membrane skeleton.\n\n📹 [VIDEO LINK]"},{"id":910,"s":"Cytoskeleton","t":"Intermediate Filaments","u":"Keratin Intermediate Filament Epidermolysis Bullosa Simplex","q":"A 25-year-old woman with epidermolysis bullosa simplex has blistering skin at sites of minor trauma. Genetic testing reveals a mutation in keratin. Keratin belongs to which of the following classes of cytoskeletal proteins?","o":["Actin filaments","Intermediate filaments","Microtubules","Motor proteins","Spectrin network"],"a":1,"p":"INTERMEDIATE FILAMENTS (10nm diameter): provide MECHANICAL STRENGTH. Types: KERATIN (epithelial) — mutation → epidermolysis bullosa simplex. VIMENTIN (mesenchymal). DESMIN (muscle). NEUROFILAMENTS (neurons). LAMIN A/C (nuclear lamina) — mutation → PROGERIA. GFAP (astrocytes). Unlike actin and microtubules, intermediate filaments are NOT involved in cell motility.\n\n📹 [VIDEO LINK]"},{"id":911,"s":"Cytoskeleton","t":"Microtubules","u":"Centrosome MTOC Centriole Microtubule Organising Centre","q":"Microtubules in animal cells radiate outward from which of the following organising centres?","o":["Centromere","Centrosome","Centriole","Kinetochore","Nucleolus"],"a":1,"p":"CENTROSOME = MICROTUBULE ORGANISING CENTRE (MTOC). Contains TWO CENTRIOLES arranged at right angles (each = 9 triplets of microtubules) + PERICENTRIOLAR MATERIAL (PCM, contains gamma-tubulin rings for nucleating microtubule polymerisation). During mitosis: centrosome duplicates → MITOTIC SPINDLE → chromosomes segregated. CENTROMERE = chromosome region where kinetochore attaches. KINETOCHORE = protein complex where spindle microtubules attach.\n\n📹 [VIDEO LINK]"},{"id":912,"s":"Cytoskeleton","t":"Cytoskeletal Poisons","u":"Colchicine Microtubule Depolymerisation Gout","q":"A 55-year-old man with acute gout is treated with colchicine. Colchicine relieves gout by which of the following mechanisms?","o":["Blocking prostaglandin synthesis","Decreasing uric acid production","Inhibiting microtubule polymerisation","Inhibiting neutrophil oxidative burst","Promoting uric acid excretion"],"a":2,"p":"COLCHICINE → binds tubulin dimers → PREVENTS MICROTUBULE POLYMERISATION → impairs NEUTROPHIL MIGRATION (chemotaxis requires intact microtubules) and DEGRANULATION → reduces neutrophil-mediated inflammation in gout. Does NOT decrease uric acid (allopurinol = xanthine oxidase inhibitor) or promote excretion (probenecid = uricosuric). Also used in: FMF, pericarditis. Side effects: GI toxicity, myopathy.\n\n📹 [VIDEO LINK]"},{"id":913,"s":"Cytoskeleton","t":"Cytoskeletal Poisons","u":"Vincristine Microtubule Spindle Poison ALL","q":"A 10-year-old boy with acute lymphoblastic leukaemia is being treated with vincristine. Vincristine exerts its anticancer effect by which of the following mechanisms?","o":["Alkylating DNA","Inhibiting dihydrofolate reductase","Inhibiting microtubule polymerisation","Inhibiting topoisomerase II","Stabilising microtubules"],"a":2,"p":"VINCRISTINE (Vinca alkaloid) → PREVENTS MICROTUBULE POLYMERISATION (binds tubulin, prevents assembly) → NO MITOTIC SPINDLE → cells arrested in METAPHASE → APOPTOSIS. Compare: TAXOL (paclitaxel) → STABILISES microtubules (prevents depolymerisation) → also arrests mitosis. Both arrest cells in M phase but by OPPOSITE mechanisms. Side effect of vincristine: PERIPHERAL NEUROPATHY (axonal transport requires microtubules).\n\n📹 [VIDEO LINK]"},{"id":914,"s":"Cytoskeleton","t":"Cilia and Flagella","u":"Primary Ciliary Dyskinesia Dynein Arms Kartagener Syndrome","q":"A 25-year-old man has recurrent sinusitis, bronchiectasis, situs inversus, and is investigated for infertility. His sperm are found to be immotile. This condition is caused by defects in which of the following?","o":["Actin filaments","Clathrin-coated vesicles","Dynein arms in cilia","Intermediate filaments","Spectrin network"],"a":2,"p":"PRIMARY CILIARY DYSKINESIA (KARTAGENER SYNDROME with situs inversus): defects in DYNEIN ARMS → cilia immotile → recurrent respiratory infections, male infertility (immotile sperm), situs inversus (random left-right positioning). Cilia: 9+2 microtubule arrangement + DYNEIN ARMS (ATP hydrolysis → sliding → ciliary beat). Without dynein: cilia immotile.\n\n📹 [VIDEO LINK]"},{"id":915,"s":"Extracellular Matrix","t":"Cell Adhesion","u":"Fibronectin RGD Integrin Binding Motif Cell Adhesion","q":"A mutation in the integrin binding motif of fibronectin is going to impair which of the following?","o":["Cell adhesion","Cell division","Gap junctions","Tight junctions","Vesicle transport"],"a":0,"p":"FIBRONECTIN RGD SEQUENCE (Arg-Gly-Asp) = integrin binding motif → binds integrins on cell surface → CELL-ECM ADHESION. Mutation in RGD → integrins cannot bind fibronectin → cells detach from ECM → ANOIKIS (apoptosis from loss of anchorage) → impaired wound healing, tumour suppression, tissue organisation. Integrins also transmit outside-in signals → cell survival, proliferation, migration.\n\n📹 [VIDEO LINK]"},{"id":916,"s":"Extracellular Matrix","t":"Basement Membrane","u":"Basal Lamina Collagen IV Laminin Composition","q":"The basal lamina of epithelial cells is composed primarily of which of the following?","o":["Collagen I and fibronectin","Collagen II and aggrecan","Collagen IV and laminin","Collagen V and perlecan","Elastin and fibrillin"],"a":2,"p":"BASAL LAMINA composition: COLLAGEN IV (network-forming, interrupted triple helix — unique to basement membranes) + LAMININ (cross-shaped glycoprotein, cell adhesion) + HEPARAN SULFATE PROTEOGLYCANS (perlecan, agrin) + ENTACTIN/NIDOGEN (links collagen IV to laminin). ALPORT SYNDROME = collagen IV (COL4A3/A4/A5) mutation → glomerular basement membrane defect → haematuria, renal failure, deafness. GOODPASTURE SYNDROME = autoantibodies to collagen IV alpha-3 chain.\n\n📹 [VIDEO LINK]"},{"id":917,"s":"Extracellular Matrix","t":"ECM Remodelling","u":"Matrix Metalloproteinases Zinc Dependent ECM Degradation RA","q":"A 60-year-old woman with rheumatoid arthritis has progressive joint destruction due to degradation of articular cartilage. Which of the following classes of enzymes is primarily responsible for ECM degradation in her joints?","o":["Elastases","Hyaluronidases","Lysyl oxidases","Matrix metalloproteinases","Serine proteases"],"a":3,"p":"MATRIX METALLOPROTEINASES (MMPs) = ZINC-DEPENDENT ENDOPEPTIDASES that degrade ALL ECM components: collagenases (MMP-1, 8, 13 — fibrillar collagens), gelatinases (MMP-2, 9 — collagen IV, gelatin), stromelysins (MMP-3 — proteoglycans, laminin). Regulated by TIMPS (tissue inhibitors of metalloproteinases). In RHEUMATOID ARTHRITIS: cytokines (TNF-α, IL-1) → MMP overexpression → cartilage destruction.\n\n📹 [VIDEO LINK]"},{"id":918,"s":"Extracellular Matrix","t":"Proteoglycans","u":"Aggrecan Compressive Resistance Articular Cartilage","q":"Articular cartilage can withstand significant compressive forces. Which of the following molecules is primarily responsible for this property?","o":["Aggrecan","Collagen I","Collagen IV","Elastin","Fibronectin"],"a":0,"p":"AGGRECAN = large aggregating proteoglycan in ARTICULAR CARTILAGE. Core protein + chondroitin sulfate and keratan sulfate chains → large aggregates with HYALURONAN. HIGHLY NEGATIVELY CHARGED → attracts cations → OSMOTIC SWELLING → draws water into cartilage → TURGOR PRESSURE → COMPRESSIVE RESISTANCE. Collagen II provides tensile strength. Aggrecan degradation (by MMPs, ADAMTS enzymes) → cartilage softening → osteoarthritis.\n\n📹 [VIDEO LINK]"},{"id":919,"s":"Cell Organelles","t":"Lysosomal Storage Diseases","u":"Fabry Disease Alpha Galactosidase A Globotriaosylceramide","q":"A 25-year-old man presents with episodes of severe pain in the extremities, angiokeratomas, and renal failure. Enzyme assay reveals deficiency of alpha-galactosidase A. Which of the following accumulates in his lysosomes?","o":["Ceramide","Galactocerebroside","Glucocerebroside","Globotriaosylceramide","Sphingomyelin"],"a":3,"p":"FABRY DISEASE: X-linked, ALPHA-GALACTOSIDASE A deficiency → GLOBOTRIAOSYLCERAMIDE (Gb3) ACCUMULATES in lysosomes of vascular endothelium, kidney, heart, and neurons. Features: ANGIOKERATOMAS, ACROPARESTHESIA (burning pain in extremities), corneal opacity (cornea verticillata), RENAL FAILURE, hypertrophic cardiomyopathy. X-linked: males severely affected. Treatment: enzyme replacement therapy (agalsidase).\n\n📹 [VIDEO LINK]"},{"id":920,"s":"Cell Organelles","t":"Lysosomal Storage Diseases","u":"Krabbe Disease Galactosylceramidase Galactocerebroside","q":"A 4-month-old infant presents with extreme irritability, peripheral neuropathy, and rapid neurological deterioration. Enzyme assay shows absent galactosylceramidase activity. Which of the following accumulates in his lysosomes?","o":["Galactocerebroside","Glucocerebroside","GM2 ganglioside","Sphingomyelin","Sulphatide"],"a":0,"p":"KRABBE DISEASE (globoid cell leukodystrophy): GALACTOSYLCERAMIDASE (GALC) deficiency → GALACTOCEREBROSIDE accumulates in lysosomes of oligodendrocytes → oligodendrocyte death → DEMYELINATION → globoid cells. Features: extreme irritability (hyperacusis), peripheral neuropathy, rapid neurological deterioration, optic atrophy. PSYCHOSINE (toxic metabolite) also accumulates → oligodendrocyte apoptosis. Treatment: HSCT if done before symptoms develop.\n\n📹 [VIDEO LINK]"},{"id":921,"s":"Cell Organelles","t":"Lysosomal Storage Diseases","u":"MLD Arylsulfatase A Sulphatide Demyelination","q":"A 2-year-old child presents with progressive gait disturbance, peripheral neuropathy, and cognitive decline. Urine shows metachromatic granules. Enzyme assay reveals arylsulfatase A deficiency. Which of the following accumulates?","o":["Galactocerebroside","Glucocerebroside","GM1 ganglioside","Sphingomyelin","Sulphatide"],"a":4,"p":"METACHROMATIC LEUKODYSTROPHY (MLD): ARYLSULFATASE A deficiency → SULPHATIDE (cerebroside sulfate) ACCUMULATES in lysosomes of oligodendrocytes and Schwann cells → DEMYELINATION of CNS and PNS. Metachromatic = sulphatide stains red-brown (metachromatic) with cresyl violet. Features: progressive gait disturbance, peripheral neuropathy, cognitive decline. Sulphatide = sulfated form of galactocerebroside (abundant in myelin).\n\n📹 [VIDEO LINK]"},{"id":922,"s":"Cell Organelles","t":"Lysosomal Storage Diseases","u":"Sandhoff Both Hexosaminidase A and B GM2 Globoside","q":"A 6-month-old infant presents with progressive neurodegeneration, cherry-red spot on the macula, and hepatosplenomegaly. Enzyme assay reveals deficiency of both hexosaminidase A and hexosaminidase B. Which of the following accumulates?","o":["Galactocerebroside and sulphatide","Glucocerebroside and ceramide","GM2 ganglioside and globoside","Sphingomyelin and ceramide","Sulphatide and galactocerebroside"],"a":2,"p":"SANDHOFF DISEASE: HEXB mutation → deficiency of BOTH hexosaminidase A (alpha+beta) AND hexosaminidase B (beta+beta) → BOTH GM2 GANGLIOSIDE AND GLOBOSIDE accumulate. Hepatosplenomegaly (globoside in visceral organs) distinguishes Sandhoff from isolated GM2 accumulation. Both hexosaminidase isoforms share the beta subunit — mutation in HEXB knocks out both enzymes simultaneously.\n\n📹 [VIDEO LINK]"},{"id":923,"s":"Cell Organelles","t":"Peroxisome Disorders","u":"Zellweger Spectrum PEX Genes All Peroxisomal Functions Lost","q":"A neonate presents with profound hypotonia, seizures, and craniofacial dysmorphism. Plasma shows elevated VLCFA and reduced plasmalogens. Liver biopsy shows absence of peroxisomes. Mutations in which of the following genes are responsible?","o":["ABCD1","HEXA","LIPA","PEX genes","SMPD1"],"a":3,"p":"ZELLWEGER SPECTRUM DISORDERS: mutations in PEX GENES (encoding peroxins — proteins required for peroxisome biogenesis) → absent/non-functional peroxisomes → ALL peroxisomal functions lost: VLCFA beta-oxidation (VLCFA accumulate), plasmalogen synthesis (plasmalogens deficient), bile acid synthesis (abnormal). ABCD1 = adrenoleukodystrophy (one peroxisomal transporter mutated — NOT all peroxisomal functions lost). Zellweger = most severe (neonatal lethal).\n\n📹 [VIDEO LINK]"},{"id":924,"s":"Enzymes","t":"Enzyme Classification","u":"LDH EC1 Oxidoreductase Class","q":"Lactate dehydrogenase belongs to which of the following enzyme classes?","o":["EC1 Oxidoreductases","EC2 Transferases","EC3 Hydrolases","EC4 Lyases","EC5 Isomerases"],"a":0,"p":"LDH = OXIDOREDUCTASE (EC1). Catalyses: lactate + NAD⁺ → pyruvate + NADH. EC1 = oxidation-reduction reactions. Other EC1 examples: succinate dehydrogenase, G6PD, cytochrome c oxidase, alcohol dehydrogenase. LDH elevated in MI (cardiac flip LDH1 > LDH2), liver disease, haemolysis.\\\n\\\n📹 [VIDEO LINK]"},{"id":925,"s":"Enzymes","t":"Enzyme Classification","u":"ALT EC2 Transferase Class","q":"ALT belongs to which of the following enzyme classes?","o":["EC1","EC2","EC3","EC4","EC5"],"a":1,"p":"ALT (ALANINE AMINOTRANSFERASE) = TRANSFERASE (EC2). Transfers amino group from alanine to alpha-ketoglutarate → pyruvate + glutamate. Requires PLP (pyridoxal phosphate). EC2 examples: kinases (phosphate transfer), glycosyl transferases (carbohydrate transfer), transaminases (amino group transfer). ALT = most sensitive and specific marker of hepatocellular damage.\\\n\\\n📹 [VIDEO LINK]"},{"id":926,"s":"Enzymes","t":"Enzyme Classification","u":"Pancreatic Lipase Esterase Subclass Hydrolase","q":"A student consumes a zinger burger. In his small intestine, triacylglycerols are digested to monoacylglycerols and fatty acids. The enzyme responsible belongs to which subclass of hydrolases?","o":["Amidases","Esterases","Glycosidases","Peptidases","Phosphatases"],"a":1,"p":"PANCREATIC LIPASE = ESTERASE (subclass of HYDROLASES, EC3). Esterases cleave ESTER BONDS by addition of water. TAGs contain ester bonds between glycerol and fatty acids. Lipase cleaves sn-1 and sn-3 positions → 2-monoacylglycerol + 2 free fatty acids. Compare: PEPTIDASES cleave peptide bonds. GLYCOSIDASES cleave glycosidic bonds. PHOSPHATASES cleave phosphate ester bonds.\\\n\\\n📹 [VIDEO LINK]"},{"id":927,"s":"Enzymes","t":"Enzyme Classification","u":"Aldolase EC4 Lyase Fructose Bisphosphate Cleavage","q":"During glycolysis in a marathon runner, fructose-1,6-bisphosphate is cleaved into two triose phosphates. Which class of enzyme catalyses this reaction?","o":["EC1 Oxidoreductases","EC2 Transferases","EC3 Hydrolases","EC4 Lyases","EC5 Isomerases"],"a":3,"p":"ALDOLASE = LYASE (EC4). Cleaves C-C bonds by means OTHER THAN hydrolysis or oxidation. EC4 lyases: ALDOLASES (cleave C-C bonds), DECARBOXYLASES (release CO₂), DEHYDRATASES (remove water to form double bond), SYNTHASES (favour C-C bond formation, e.g. citrate synthase). Aldolase cleaves fructose-1,6-bisphosphate → DHAP + glyceraldehyde-3-phosphate.\\\n\\\n📹 [VIDEO LINK]"},{"id":928,"s":"Enzymes","t":"Enzyme Classification","u":"Phosphoglucose Isomerase EC5 Isomerase Glycolysis","q":"During glycolysis, glucose-6-phosphate is converted to fructose-6-phosphate without addition or removal of atoms. Which class of enzyme catalyses this reaction?","o":["EC1 Oxidoreductases","EC2 Transferases","EC3 Hydrolases","EC4 Lyases","EC5 Isomerases"],"a":4,"p":"PHOSPHOGLUCOSE ISOMERASE = ISOMERASE (EC5). Rearranges bond structure without adding or removing atoms. Types: MUTASES (move phosphate between atoms), RACEMASES (interconvert D and L forms), EPIMERASES (interconvert epimers). Glucose-6-phosphate → fructose-6-phosphate = aldose → ketose conversion. No net change in molecular formula.\\\n\\\n📹 [VIDEO LINK]"},{"id":929,"s":"Enzymes","t":"Enzyme Classification","u":"Fatty Acyl CoA Synthetase EC6 Ligase Beta Oxidation","q":"During fatty acid activation before beta-oxidation, a fatty acid is joined to coenzyme A using energy from ATP hydrolysis. Which class of enzyme catalyses this bond formation?","o":["EC1 Oxidoreductases","EC2 Transferases","EC3 Hydrolases","EC4 Lyases","EC6 Ligases"],"a":4,"p":"FATTY ACYL CoA SYNTHETASE = LIGASE (EC6). Synthesises C-S bond COUPLED TO ATP HYDROLYSIS. EC6 types: CARBOXYLASES (biotin-dependent, add CO₂ — pyruvate carboxylase, ACC), SYNTHETASES (ATP-requiring — fatty acyl CoA synthetase), AMINOACYL-tRNA SYNTHETASES. Ligases differ from SYNTHASES (EC4) which do NOT require ATP.\\\n\\\n📹 [VIDEO LINK]"},{"id":930,"s":"Enzymes","t":"Enzyme Classification","u":"Luciferase EC1 Oxidoreductase Firefly Bioluminescence","q":"Luciferase catalyses the oxidation of luciferin in the presence of oxygen and ATP, enabling fireflies to emit light. Luciferase belongs to which of the following enzyme classes?","o":["EC1 Oxidoreductases","EC2 Transferases","EC3 Hydrolases","EC4 Lyases","EC5 Isomerases"],"a":0,"p":"LUCIFERASE = OXIDOREDUCTASE (EC1) — specifically a monooxygenase. Catalyses: luciferin + O₂ + ATP → oxyluciferin + CO₂ + AMP + PPi + LIGHT. O₂ reduced, luciferin oxidised = oxidoreductase. Used in reporter gene assays in molecular biology. The excited-state oxyluciferin releases energy as light upon returning to ground state.\\\n\\\n📹 [VIDEO LINK]"},{"id":931,"s":"Enzymes","t":"Enzyme Classification","u":"Propionyl CoA Carboxylase EC6 Ligase Odd Chain FA Starvation","q":"Before being trapped under rubble, a man's diet exclusively consisted of foods rich in odd-chain fatty acids. Now, as these fatty acids are being oxidised, propionyl CoA is converted to succinyl CoA, helping him survive. Which class of enzyme catalyses this carboxylation?","o":["EC1 Oxidoreductases","EC2 Transferases","EC3 Hydrolases","EC4 Lyases","EC6 Ligases"],"a":4,"p":"PROPIONYL CoA CARBOXYLASE = LIGASE (EC6, carboxylase subtype). Catalyses: propionyl CoA + CO₂ + ATP → methylmalonyl CoA. Requires BIOTIN (CO₂ carrier) and ATP. Methylmalonyl CoA → (vitamin B12-dependent mutase) → succinyl CoA → TCA cycle → gluconeogenesis. Allows odd-chain fatty acids to contribute to gluconeogenesis during starvation.\\\n\\\n📹 [VIDEO LINK]"},{"id":932,"s":"Enzymes","t":"Enzyme Classification","u":"LDH EC1 Oxidoreductase Lactic Acid Marathon Runner","q":"A marathon runner develops cramping leg pain from lactic acid accumulation. Which class of enzyme produced the compound that caused his pain?","o":["EC1 Oxidoreductases","EC2 Transferases","EC3 Hydrolases","EC4 Lyases","EC5 Isomerases"],"a":0,"p":"LACTATE DEHYDROGENASE (LDH) = OXIDOREDUCTASE (EC1). Pyruvate + NADH → LACTATE + NAD⁺ (anaerobic conditions). During intense exercise → muscle outstrips O₂ supply → anaerobic glycolysis → LACTIC ACID accumulation → cramping pain. LDH regenerates NAD⁺ → allows glycolysis to continue. CORI CYCLE: lactate → liver → gluconeogenesis → glucose returned to muscle.\\\n\\\n📹 [VIDEO LINK]"},{"id":943,"s":"Enzymes","t":"Enzyme Inhibition","u":"Suicide Inhibition Mechanism Based Enzyme Converts Drug","q":"Which of the following best describes the situation when an enzyme converts a drug to a transition-state analogue that irreversibly binds to its own active site?","o":["Competitive inhibition","Irreversible inhibition","Mixed inhibition","Non-competitive inhibition","Suicide inhibition"],"a":4,"p":"SUICIDE INHIBITION (mechanism-based inhibition): the ENZYME ITSELF converts the drug to a reactive intermediate that COVALENTLY and IRREVERSIBLY attacks its own active site — the enzyme commits suicide. Examples: ALLOPURINOL → xanthine oxidase converts to OXYPURINOL → irreversible inactivation. PENICILLIN → glycopeptidyl transpeptidase forms covalent penicilloyl-serine intermediate. FLUOROURACIL → thymidylate synthase converted to active metabolite → covalent inhibition.\\\n\\\n📹 [VIDEO LINK]"},{"id":944,"s":"Enzymes","t":"Enzyme Inhibition","u":"Holoenzyme Active Enzyme with Cofactor","q":"Which of the following terms best describes an active enzyme together with its non-protein cofactor?","o":["Abzyme","Apoenzyme","Cosubstrate","Holoenzyme","Isoenzyme"],"a":3,"p":"HOLOENZYME = APOENZYME + COFACTOR = COMPLETE, ACTIVE ENZYME. APOENZYME = protein part without cofactor = INACTIVE. COSUBSTRATE = transiently associating coenzyme (NAD⁺, FAD) — released after reaction. PROSTHETIC GROUP = permanently bound coenzyme (FAD in succinate dehydrogenase, haem in Hb). ABZYME = catalytic antibody. ISOENZYME = different molecular forms of same enzyme (e.g. LDH1–5).\\\n\\\n📹 [VIDEO LINK]"},{"id":945,"s":"Enzymes","t":"Enzyme Inhibition","u":"Cosubstrate Transiently Associates Released After Reaction","q":"Which of the following terms best describes coenzymes that transiently associate with the enzyme and are released after each reaction cycle?","o":["Abzyme","Cosubstrate","Cofactor","Holoenzyme","Prosthetic group"],"a":1,"p":"COSUBSTRATE = coenzyme that TRANSIENTLY ASSOCIATES with the enzyme — binds, participates in reaction, released in modified form, regenerated separately. Examples: NAD⁺/NADH, FAD/FADH₂, CoA, ATP. Compare: PROSTHETIC GROUP = PERMANENTLY bound coenzyme that does NOT dissociate (FAD in succinate dehydrogenase, haem in cytochromes, PLP in some enzymes). COFACTOR = general term for any non-protein helper molecule.\\\n\\\n📹 [VIDEO LINK]"},{"id":946,"s":"Enzymes","t":"Enzyme Inhibition Clinical","u":"Allopurinol Xanthine Oxidase Inhibition Gout Kata Kat","q":"A 50-year-old man presents with severe throbbing pain in his right great toe after eating kata kat. His doctor prescribes allopurinol. Which enzyme is inhibited by this drug?","o":["Cytochrome oxidase","Glucose oxidase","Lysyl oxidase","Monoamine oxidase","Xanthine oxidase"],"a":4,"p":"ALLOPURINOL → inhibits XANTHINE OXIDASE → decreases uric acid production. Mechanism: SUICIDE INHIBITOR — xanthine oxidase converts allopurinol to OXYPURINOL → oxypurinol covalently binds to reduced molybdenum (Mo⁴⁺) at active site → IRREVERSIBLE inactivation. Without xanthine oxidase: hypoxanthine and xanthine cannot be converted to uric acid → urate levels fall → gout prevented. Kata kat (offal) = rich in purines → uric acid → urate crystals in joints → gout.\\\n\\\n📹 [VIDEO LINK]"},{"id":947,"s":"Enzymes","t":"Enzyme Inhibition Clinical","u":"Allopurinol Oxypurinol Covalent Bond Suicide Inhibition","q":"A 50-year-old man with gouty arthritis is started on allopurinol therapy. Xanthine oxidase in his cells commits suicide through irreversible attachment to oxypurinol via which of the following?","o":["Covalent bond","Electrostatic bond","Hydrogen bond","Hydrophobic interaction","Ionic bond"],"a":0,"p":"ALLOPURINOL → xanthine oxidase converts it to OXYPURINOL → oxypurinol forms a COVALENT BOND with the molybdenum cofactor at the active site → IRREVERSIBLE inhibition (suicide inhibition). COVALENT BOND = strongest type of chemical bond → cannot be broken under physiological conditions → permanent enzyme inactivation. Compare: competitive inhibitors bind via non-covalent interactions (hydrogen bonds, ionic bonds, hydrophobic interactions) → reversible.\\\n\\\n📹 [VIDEO LINK]"},{"id":948,"s":"Enzymes","t":"Enzyme Inhibition Clinical","u":"Statins Competitive Inhibition HMG CoA Reductase Cholesterol","q":"A 45-year-old man with raised cholesterol and triglycerides is discharged on statin therapy. HMG-CoA reductase in his cells is likely to undergo which of the following types of inhibition?","o":["Competitive","Irreversible","Mixed","Suicide","Uncompetitive"],"a":0,"p":"STATINS (atorvastatin, simvastatin, rosuvastatin) = COMPETITIVE INHIBITORS of HMG-CoA REDUCTASE. Statins structurally resemble HMG-CoA → compete for active site → REVERSIBLE competitive inhibition. Effect: apparent Km INCREASED, Vmax UNCHANGED. Sustained inhibition → ↓ cholesterol synthesis → ↑ LDL receptor expression → ↓ plasma LDL.\\\n\\\n📹 [VIDEO LINK]"},{"id":949,"s":"Enzymes","t":"Enzyme Inhibition Clinical","u":"Statins Target HMG CoA Reductase Cholesterol Synthesis","q":"A 55-year-old man with chest pain on exertion and dyslipidaemia is prescribed statins. Which of the following enzyme does this drug target?","o":["Acetyl CoA carboxylase","Dihydrofolate reductase","Fatty acyl CoA reductase","HMG CoA reductase","Ribonucleotide reductase"],"a":3,"p":"STATINS target HMG-CoA REDUCTASE (3-hydroxy-3-methylglutaryl CoA reductase) = RATE-LIMITING enzyme of the MEVALONATE PATHWAY (cholesterol synthesis). Reaction inhibited: HMG-CoA → mevalonate. Without mevalonate: cannot synthesise cholesterol, dolichol, ubiquinone (CoQ), isoprenoids. Located in SMOOTH ER of hepatocytes.\\\n\\\n📹 [VIDEO LINK]"},{"id":950,"s":"Enzymes","t":"Enzyme Inhibition Clinical","u":"Aspirin Acetylation COX1 Serine Irreversible Headache","q":"A first-year medical student takes aspirin dissolved in water. Aspirin relieves his headache by which of the following alterations to his COX-1 enzymes in scalp blood vessels?","o":["Acetylation","Farnesylation","Methylation","Phosphorylation","Prenylation"],"a":0,"p":"ASPIRIN (acetylsalicylic acid) → IRREVERSIBLY ACETYLATES SERINE-530 in the active site of COX-1 → permanently blocks the substrate channel → cannot produce prostaglandins → ↓ PGE2 in scalp blood vessels → vasodilation reversed → headache relieved. ACETYLATION = transfer of acetyl group (CH₃CO−) from aspirin to the serine hydroxyl group → COVALENT modification → IRREVERSIBLE. Unlike ibuprofen (reversible competitive inhibitor). Aspirin's irreversibility = basis of antiplatelet effect (platelets cannot make new COX).\n\n📹 [VIDEO LINK]"},{"id":951,"s":"Enzymes","t":"Enzyme Inhibition Clinical","u":"Penicillin Transition State Analog Glycopeptidyl Transpeptidase","q":"An 11-year-old boy is prescribed penicillin for a bacterial infection. Penicillin is an analogue of which of the following?","o":["Enzyme","Enzyme-substrate complex","Product","Substrate","Transition state"],"a":4,"p":"PENICILLIN = TRANSITION STATE ANALOGUE of glycopeptidyl transpeptidase (bacterial enzyme cross-linking peptidoglycan in cell wall). Penicillin mimics the TRANSITION STATE of the transpeptidase reaction → binds tightly to active site serine → forms COVALENT penicilloyl-serine intermediate → IRREVERSIBLE inactivation (SUICIDE INHIBITION). Without functional transpeptidase → bacteria cannot synthesise cell wall → osmotic lysis → death.\\\n\\\n📹 [VIDEO LINK]"},{"id":952,"s":"Enzymes","t":"Enzyme Inhibition Clinical","u":"Methotrexate Competitive Inhibition DHFR Ectopic Pregnancy","q":"A 32-year-old female with ectopic pregnancy is given methotrexate to terminate the pregnancy. Which enzyme is inhibited in the embryo by this drug?","o":["Dihydrofolate reductase","Dihydropteridine reductase","Glutathione reductase","Ribonucleotide reductase","Thioredoxin reductase"],"a":0,"p":"METHOTREXATE = COMPETITIVE INHIBITOR of DIHYDROFOLATE REDUCTASE (DHFR). DHFR reduces DHF → THF → one-carbon transfers → nucleotide synthesis. Methotrexate structurally resembles DHF → binds DHFR with very high affinity → NO THF → NO nucleotide synthesis → rapidly dividing cells (embryo, cancer cells) cannot proliferate. Folinic acid (leucovorin) rescue protects normal cells.\\\n\\\n📹 [VIDEO LINK]"},{"id":953,"s":"Enzymes","t":"Enzyme Inhibition Clinical","u":"Lisinopril Competitive Inhibition ACE Hypertension","q":"A 50-year-old hypertensive man is prescribed lisinopril, which decreases Angiotensin II by competing with Angiotensin I on angiotensin-converting enzyme. Which biochemical mechanism does this drug use?","o":["Competitive inhibition","Irreversible inhibition","Mixed inhibition","Suicide inhibition","Uncompetitive inhibition"],"a":0,"p":"LISINOPRIL = COMPETITIVE INHIBITOR of ACE → reversible → ↓ Angiotensin II → ↓ vasoconstriction + ↓ aldosterone → ↓ blood pressure. ACE also degrades bradykinin → ACE inhibitors → ↑ bradykinin → COUGH (major side effect). Apparent Km increased, Vmax unchanged.\\\n\\\n📹 [VIDEO LINK]"},{"id":954,"s":"Enzymes","t":"Enzyme Inhibition Clinical","u":"Malathion Irreversible Inhibition Acetylcholinesterase Toddler","q":"A 3-year-old boy drinks malathion insecticide and develops vomiting, abdominal cramps, salivation, and uncontrollable defecation. Which type of enzyme inhibition is causing his symptoms?","o":["Competitive","Irreversible","Mixed","Non-competitive","Uncompetitive"],"a":1,"p":"MALATHION (organophosphorus) → IRREVERSIBLY PHOSPHORYLATES serine in acetylcholinesterase active site → AChE permanently inactivated → ACh accumulates → MUSCARINIC effects (SLUDGE: Salivation, Lacrimation, Urination, Defecation, GI distress, Emesis) + NICOTINIC effects (muscle fasciculations, paralysis). Treatment: ATROPINE + PRALIDOXIME.\\\n\\\n📹 [VIDEO LINK]"},{"id":955,"s":"Enzymes","t":"Enzyme Inhibition Clinical","u":"Pyridostigmine Reversible Competitive AChE Myasthenia Gravis","q":"A 38-year-old female takes pyridostigmine for myasthenia gravis. Pyridostigmine affects her acetylcholinesterase through which of the following mechanisms?","o":["Competitive inhibition","Irreversible inhibition","Mixed inhibition","Non-competitive inhibition","Suicide inhibition"],"a":0,"p":"PYRIDOSTIGMINE = REVERSIBLE COMPETITIVE INHIBITOR of AChE → ACh accumulates at NMJ → more nicotinic receptor stimulation → improved muscle contraction in MYASTHENIA GRAVIS. REVERSIBLE — unlike organophosphorus compounds which are IRREVERSIBLE. When pyridostigmine dissociates, AChE activity returns. Compare: NEOSTIGMINE (short-acting, reversal of muscle relaxants). PHYSOSTIGMINE (crosses BBB, used in glaucoma).\\\n\\\n📹 [VIDEO LINK]"},{"id":956,"s":"Enzymes","t":"Enzyme Inhibition Clinical","u":"Fluoroacetate Suicide Inhibition Aconitase Lethal Synthesis Suicidal Intent","q":"A 20-year-old female, with suicidal intent, ingests rat poison (fluoroacetate) and dies from convulsions and cardiac failure. The biochemical reason for her death was inhibition of aconitase by fluorocitrate through which of the following mechanisms?","o":["Competitive inhibition","Irreversible inhibition","Non-competitive inhibition","Reversible inhibition","Suicide inhibition"],"a":4,"p":"FLUOROACETATE → FLUOROACETYL CoA (acetyl CoA synthetase) → FLUOROCITRATE (citrate synthase — LETHAL SYNTHESIS) → fluorocitrate is a SUICIDE INHIBITOR of ACONITASE → TCA cycle blocked → citrate accumulates → ATP depletion → cardiac failure + seizures. Aconitase converts fluorocitrate to a reactive carbanion intermediate that COVALENTLY BINDS to the active site iron-sulfur cluster → irreversible inactivation. Fluoroacetate = compound 1080 rat poison. LETHAL SYNTHESIS = body synthesises toxic compound from harmless precursor.\n\n📹 [VIDEO LINK]"},{"id":957,"s":"Enzymes","t":"Enzyme Inhibition Clinical","u":"Lead Ferrochelatase Irreversible Inhibition Sideroblastic Anaemia","q":"A 55-year-old woman consuming ayurvedic medicines develops anaemia. Lead in the medicine covalently alters ferrochelatase, an enzyme of haem synthesis. Which type of inhibition is causing her anaemia?","o":["Competitive","Irreversible","Mixed","Non-competitive","Reversible"],"a":1,"p":"LEAD → IRREVERSIBLY INHIBITS FERROCHELATASE by forming COVALENT BONDS with SH groups (cysteine residues) → cannot incorporate Fe²⁺ into protoporphyrin IX → haem synthesis blocked → SIDEROBLASTIC ANAEMIA (iron accumulates in RBC mitochondria → ring sideroblasts). ZINC PROTOPORPHYRIN rises (zinc replaces iron). Lead also inhibits ALA dehydratase. Chelation therapy: DMSA, EDTA.\\\n\\\n📹 [VIDEO LINK]"},{"id":958,"s":"Enzymes","t":"Enzyme Inhibition Clinical","u":"HIV Protease Inhibitor Transition State Analog","q":"A 30-year-old with AIDS is prescribed an HIV protease inhibitor which binds a tetrahedral intermediate formed during action of HIV protease. This drug is an analogue of which of the following?","o":["Enzyme","Enzyme-substrate complex","Product","Substrate","Transition state"],"a":4,"p":"HIV PROTEASE INHIBITORS (saquinavir, ritonavir, lopinavir) = TRANSITION STATE ANALOGUES. During HIV protease catalysis, a TETRAHEDRAL INTERMEDIATE (transition state) is formed at the cleavage site. HIV protease inhibitors mimic this tetrahedral transition state → bind active site with very high affinity (picomolar Kd) → COMPETITIVE inhibition. Prevents viral polyprotein processing → immature, non-infectious viral particles.\\\n\\\n📹 [VIDEO LINK]"},{"id":959,"s":"Enzymes","t":"Enzyme Inhibition Clinical","u":"Fomepizole Active Site Alcohol Dehydrogenase Ethylene Glycol","q":"A 22-year-old female ingests ethylene glycol in a suicide attempt. Fomepizole is administered, which competes with ethylene glycol at the binding pocket of alcohol dehydrogenase. Which of the following best describes this binding pocket?","o":["Active site","Allosteric site","Effector site","Regulatory site","Signal peptide site"],"a":0,"p":"FOMEPIZOLE (4-methylpyrazole) = COMPETITIVE INHIBITOR of ALCOHOL DEHYDROGENASE (ADH). Occupies the ACTIVE SITE (= substrate binding site = catalytic site — all the same location) → ethylene glycol CANNOT be metabolised to toxic glycolic acid and oxalic acid → acidosis and renal failure prevented. Active site ≠ allosteric site (regulatory, on a DIFFERENT domain from the catalytic site).\\\n\\\n📹 [VIDEO LINK]"},{"id":965,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"Copper Cofactor Cytochrome Oxidase","q":"Copper is a cofactor for which of the following enzymes?","o":["Carbonic anhydrase","Cytochrome oxidase","Glutathione peroxidase","Pyruvate kinase","Ribonucleotide reductase"],"a":1,"p":"COPPER (Cu) cofactors: CYTOCHROME C OXIDASE (Complex IV — CuA and CuB centres), LYSYL OXIDASE (collagen and elastin crosslinking — Menkes disease when deficient), CERULOPLASMIN (ferroxidase, copper transport), Cu-Zn SOD, DOPAMINE BETA-HYDROXYLASE (noradrenaline synthesis), TYROSINASE (melanin synthesis). ZINC = carbonic anhydrase. SELENIUM = glutathione peroxidase. IRON = ribonucleotide reductase. MAGNESIUM = pyruvate kinase.\\\n\\\n📹 [VIDEO LINK]"},{"id":966,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"Iron and Copper Both Cofactors Cytochrome Oxidase","q":"Both iron and copper act as cofactors for which of the following enzymes?","o":["Arginase","Carbonic anhydrase","Cytochrome oxidase","Glutathione peroxidase","Xanthine oxidase"],"a":2,"p":"CYTOCHROME C OXIDASE (COMPLEX IV) requires BOTH IRON (haem a and haem a₃ — Fe²⁺ ↔ Fe³⁺) AND COPPER (CuA — receives electrons from cytochrome c, and CuB — paired with haem a₃ for O₂ reduction). Both essential for sequential 4-electron reduction of O₂ → H₂O. XANTHINE OXIDASE = iron-sulfur + molybdenum. ARGINASE = manganese. CARBONIC ANHYDRASE = zinc. GLUTATHIONE PEROXIDASE = selenium.\\\n\\\n📹 [VIDEO LINK]"},{"id":967,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"Iron Cofactor Peroxidase Haem Enzyme","q":"Which of the following minerals is required for the proper functioning of peroxidase?","o":["Copper","Iron","Magnesium","Selenium","Zinc"],"a":1,"p":"PEROXIDASE (myeloperoxidase, horseradish peroxidase, thyroid peroxidase) = IRON-containing HAEM enzyme. MYELOPEROXIDASE: H₂O₂ + Cl⁻ → HYPOCHLOROUS ACID (HOCl) → kills bacteria. THYROID PEROXIDASE: oxidises iodide → thyroid hormone synthesis. CATALASE also iron-haem enzyme (H₂O₂ → H₂O + O₂). Compare: glutathione peroxidase = SELENIUM (not iron — different enzyme).\\\n\\\n📹 [VIDEO LINK]"},{"id":968,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"Potassium Activating Cofactor Pyruvate Kinase Alkali Metal","q":"A 3-year-old boy in the emergency department needs replacement therapy. His doctor notes pyruvate kinase in RBCs requires an alkali metal ion cofactor for activation. Which of the following is this cofactor?","o":["Calcium","Magnesium","Potassium","Sodium","Zinc"],"a":2,"p":"PYRUVATE KINASE requires POTASSIUM (K⁺) as an activating cofactor — binds to enzyme and stabilises the active conformation. PK also requires Mg²⁺ for ATP-Mg²⁺ complex (different role from K⁺). K⁺ = ALKALI METAL ION (Group 1). PK deficiency → hereditary non-spherocytic haemolytic anaemia → 2,3-BPG accumulates upstream → lower O₂ affinity of Hb → better tissue O₂ delivery (partial compensation for anaemia).\\\n\\\n📹 [VIDEO LINK]"},{"id":969,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"Magnesium Cofactor Hexokinase Pyruvate Kinase G6Pase","q":"Which of the following minerals is required for the proper functioning of pyruvate kinase, hexokinase, and glucose-6-phosphatase?","o":["Copper","Iron","Magnesium","Potassium","Selenium"],"a":2,"p":"MAGNESIUM (Mg²⁺) chelates PHOSPHATE GROUPS of ATP → Mg²⁺-ATP complex = TRUE SUBSTRATE for all kinases and phosphatases. Required by: ALL KINASES (hexokinase, pyruvate kinase, PFK, adenylate kinase), ALL PHOSPHATASES (glucose-6-phosphatase, alkaline phosphatase), ATPases (Na⁺/K⁺-ATPase), DNA and RNA polymerases. Mg²⁺ deficiency → muscle cramps, cardiac arrhythmias, hypokalaemia.\\\n\\\n📹 [VIDEO LINK]"},{"id":970,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"Manganese Cofactor Arginase MnSOD","q":"Which of the following minerals is required for the proper functioning of arginase and manganese superoxide dismutase?","o":["Copper","Iron","Magnesium","Manganese","Selenium"],"a":3,"p":"MANGANESE (Mn²⁺): ARGINASE (urea cycle — Mn²⁺ at active site, cleaves arginine → ornithine + urea), MANGANESE SUPEROXIDE DISMUTASE (MnSOD, mitochondria — converts O₂•⁻ → H₂O₂). NOTE: RIBONUCLEOTIDE REDUCTASE requires IRON (Fe-S cluster), NOT manganese — correction from source material. Mn deficiency = rare (impaired bone formation, reproductive dysfunction).\\\n\\\n📹 [VIDEO LINK]"},{"id":971,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"Selenium Cofactor Glutathione Peroxidase Selenocysteine","q":"Which of the following minerals is required for the proper functioning of glutathione peroxidase?","o":["Copper","Iron","Magnesium","Manganese","Selenium"],"a":4,"p":"SELENIUM (as SELENOCYSTEINE) = active-site residue of GLUTATHIONE PEROXIDASE (GPx). GPx: H₂O₂ + 2GSH → GSSG + 2H₂O (detoxifies H₂O₂ using reduced glutathione). Selenocysteine has selenol (−SeH) — more nucleophilic than cysteine (−SH) → better catalyst. Selenium deficiency → impaired GPx → oxidative stress → KESHAN DISEASE (cardiomyopathy, endemic in China). GPx also protects RBCs from oxidative damage.\\\n\\\n📹 [VIDEO LINK]"},{"id":972,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"Zinc Cofactor Carbonic Anhydrase","q":"Which of the following minerals is required for the proper functioning of carbonic anhydrase?","o":["Copper","Iron","Magnesium","Manganese","Zinc"],"a":4,"p":"ZINC (Zn²⁺) required for: CARBONIC ANHYDRASE (CO₂ + H₂O → HCO₃⁻ + H⁺), ALCOHOL DEHYDROGENASE, CARBOXYPEPTIDASE, MATRIX METALLOPROTEINASES (MMPs), ACE, DNA polymerase, RNA polymerase. ZINC DEFICIENCY → impaired wound healing, growth retardation, hypogonadism, skin lesions, loss of taste (ageusia) and smell (anosmia).\\\n\\\n📹 [VIDEO LINK]"},{"id":973,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"Tetrahydrofolate One Carbon Group Transfer","q":"Which of the following coenzymes is responsible for the transfer of one-carbon groups?","o":["Ascorbate","Dihydrolipoate","Pyridoxal phosphate","Tetrahydrofolate","Thiamine pyrophosphate"],"a":3,"p":"TETRAHYDROFOLATE (THF, from FOLIC ACID/vitamin B9) = carrier of ONE-CARBON GROUPS (formyl, methylene, methyl). Functions: (1) Purine synthesis (C2 and C8 of purine ring). (2) Thymidylate synthesis (dUMP → dTMP by thymidylate synthase). (3) Methionine synthesis (5-methyl-THF + homocysteine → methionine). FOLATE DEFICIENCY → megaloblastic anaemia (no thymidylate → no DNA synthesis). Methotrexate inhibits DHFR → functional folate deficiency.\\\n\\\n📹 [VIDEO LINK]"},{"id":974,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"Pyridoxal Phosphate Amino Group Transfer Transaminases","q":"Which of the following coenzymes is responsible for the transfer of amino groups?","o":["Ascorbate","Dihydrolipoate","Pyridoxal phosphate","Tetrahydrofolate","Thiamine pyrophosphate"],"a":2,"p":"PYRIDOXAL PHOSPHATE (PLP, active form of VITAMIN B6) = coenzyme for ALL TRANSAMINASES (aminotransferases) — transfers amino groups from amino acids to alpha-keto acids. Also required for: ALA SYNTHASE (haem synthesis), GLYCOGEN PHOSPHORYLASE, SERINE HYDROXYMETHYLTRANSFERASE, CBS (homocysteine metabolism), DOPA decarboxylase (dopamine synthesis). PLP deficiency: sideroblastic anaemia, peripheral neuropathy, convulsions. ISONIAZID = PLP antagonist.\\\n\\\n📹 [VIDEO LINK]"},{"id":975,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"NAD Hydride Ion Transfer Dehydrogenases","q":"Which of the following coenzymes is responsible for the transfer of hydride ions?","o":["Flavin adenine dinucleotide","Flavin mononucleotide","Nicotinamide adenine dinucleotide","Pyridoxal phosphate","Thiamine pyrophosphate"],"a":2,"p":"NAD⁺ (NICOTINAMIDE ADENINE DINUCLEOTIDE) = carrier of HYDRIDE IONS (H⁻ = two electrons + one proton). NAD⁺ + 2H → NADH + H⁺. The nicotinamide ring accepts the hydride ion at the C4 position. Used by ALL DEHYDROGENASES in TCA cycle and glycolysis (GAPDH, pyruvate DH). NADH → Complex I → ATP generation. NADPH = reductive biosynthesis and antioxidant defence (different role from NADH).\\\n\\\n📹 [VIDEO LINK]"},{"id":976,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"FAD Electron Transfer Flavoproteins ETC","q":"Which of the following coenzymes is responsible for the transfer of electrons in flavoproteins of the electron transport chain?","o":["Flavin adenine dinucleotide","Nicotinamide adenine dinucleotide","Nicotinamide adenine dinucleotide phosphate","Pyridoxal phosphate","Thiamine pyrophosphate"],"a":0,"p":"FAD (FLAVIN ADENINE DINUCLEOTIDE) = carrier of ELECTRONS in flavoproteins. FAD → FADH₂ → electrons enter ETC at CoQ (COMPLEX II, bypasses Complex I → 1.5 ATP). FAD required for: succinate dehydrogenase (Complex II), fatty acyl CoA dehydrogenase (beta-oxidation), alpha-KG DH complex, MAO. Derived from RIBOFLAVIN (vitamin B2). Riboflavin deficiency → cheilosis, glossitis, seborrhoeic dermatitis.\\\n\\\n📹 [VIDEO LINK]"},{"id":977,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"Thiamine Pyrophosphate TPP Coenzyme Wernicke Alcoholic","q":"A 44-year-old chronic alcoholic with poor appetite, confusion, and early congestive heart failure presents with blood glucose of 40mg/dL. Which coenzyme should be part of his therapy?","o":["Flavin adenine dinucleotide","Flavin mononucleotide","Nicotinamide adenine dinucleotide","Nicotinamide adenine dinucleotide phosphate","Thiamine pyrophosphate"],"a":4,"p":"CHRONIC ALCOHOLISM + CONFUSION + CARDIAC FAILURE + HYPOGLYCAEMIA = THIAMINE (B1) DEFICIENCY → WERNICKE'S ENCEPHALOPATHY. THIAMINE PYROPHOSPHATE (TPP) required for: PDH (pyruvate → acetyl-CoA), ALPHA-KG DH (TCA cycle), TRANSKETOLASE (pentose phosphate pathway), BCKAD (branched-chain ketoacid DH). Without TPP: pyruvate accumulates → lactic acidosis. Treatment: IMMEDIATE IV THIAMINE (before glucose, to avoid precipitating Wernicke's).\\\n\\\n📹 [VIDEO LINK]"},{"id":978,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"Abzyme Catalytic Antibody Transition State Variable Region","q":"Which of the following is a catalytic antibody with amino acid side chains in its variable region arranged similar to an enzyme active site in the transition state?","o":["Abzyme","Apoenzyme","Coenzyme","Holoenzyme","Isoenzyme"],"a":0,"p":"ABZYMES (catalytic antibodies): antibodies raised against TRANSITION STATE ANALOGUES → variable region (antigen-binding site) develops arrangement resembling enzyme active site at TRANSITION STATE → can catalyse chemical reactions. Example: COCAINE ESTERASE ABZYME — raised against transition state analogue of cocaine ester hydrolysis → destroys cocaine → decreases cocaine addiction. Nobel Prize technology: Lerner and Schultz.\\\n\\\n📹 [VIDEO LINK]"},{"id":979,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"Zymogen Activation Proteolytic Cleavage Chymotrypsinogen","q":"Conversion of a zymogen to the active enzyme involves which of the following?","o":["Acetylation","Cleavage","Condensation","Methylation","Phosphorylation"],"a":1,"p":"ZYMOGEN (proenzyme) → active enzyme by PROTEOLYTIC CLEAVAGE (removal of inhibitory propeptide). Examples: CHYMOTRYPSINOGEN → (trypsin cleaves Arg15-Ile16 bond) → alpha-chymotrypsin. PEPSINOGEN → (acid, pH < 5) → pepsin. PROTHROMBIN → (Factor Xa + Va) → thrombin. Zymogens prevent premature enzyme activity in the wrong location (e.g. pancreatic zymogens prevent autodigestion).\\\n\\\n📹 [VIDEO LINK]"},{"id":980,"s":"Enzymes","t":"Cofactors and Coenzymes","u":"Glycogen Synthase Covalent Modification Phosphorylation","q":"Which of the following enzymes is regulated by covalent modification?","o":["Aldolase","Citrate synthase","Glycogen synthase","Hexokinase","Malate dehydrogenase"],"a":2,"p":"GLYCOGEN SYNTHASE regulated by REVERSIBLE COVALENT MODIFICATION: PHOSPHORYLATION (by PKA, glycogen synthase kinase-3) → INACTIVATES. DEPHOSPHORYLATION (protein phosphatase-1) → ACTIVATES. Also allosterically activated by glucose-6-phosphate. Compare: glycogen phosphorylase ALSO regulated by covalent modification (phosphorylation → active form). Aldolase, citrate synthase, hexokinase, malate dehydrogenase = NOT regulated by covalent modification.\\\n\\\n📹 [VIDEO LINK]"},{"id":981,"s":"Enzymes","t":"Cardiac Biomarkers","u":"CKMB Rises at 4 Hours After MI Cardiac Specific","q":"A 55-year-old man with crushing chest pain of 4 hours duration is diagnosed with myocardial infarction. Which of the following biomarker is likely to start rising in him at this point in time?","o":["ALP","AST","CKMB","LDH","Myoglobin"],"a":2,"p":"CARDIAC BIOMARKER TIMING after MI: MYOGLOBIN rises 1-3h (first to rise, NOT cardiac-specific). CKMB rises 4-8h, PEAKS at 24h, returns to baseline 48-72h. TnT/TnI rises 4-6h, PEAKS at 8-28h, remains elevated 3-14 DAYS. LDH rises 24-48h, PEAKS at 3-6 days. AST rises 8-12h. At 4 HOURS: CKMB is the first CARDIAC-SPECIFIC marker to start rising. CKMB (CK2) = myocardial muscle is the ONLY tissue with >5% of total CK activity as CK2.\n\n📹 [VIDEO LINK]"},{"id":982,"s":"Enzymes","t":"Cardiac Biomarkers","u":"TnT Rising at 7 Hours After MI Most Sensitive Specific","q":"A 65-year-old obese woman with crushing chest pain of 7 hours duration is diagnosed with myocardial infarction. Which of the following biomarker is also starting to rise in her at this point in time?","o":["AST","CKMB","LDH","Myoglobin","TnT"],"a":4,"p":"At 7 HOURS after MI: CKMB already rising (onset 4-8h) AND TnT NOW RISING (onset 4-6h, so at 7 hours it is rising). TnT = most sensitive and specific cardiac biomarker. Peaks 8-28h, remains elevated 3-14 DAYS. Elevated troponins MORE PREDICTIVE of adverse outcomes in unstable angina and MI than CKMB.\\\n\\\n📹 [VIDEO LINK]"},{"id":983,"s":"Enzymes","t":"Cardiac Biomarkers","u":"TnT Most Sensitive Specific Biomarker 12 Hours After MI","q":"A patient in CCU was diagnosed with myocardial infarction 12 hours ago. Which of the following marker will be most sensitive and specific at this point in time?","o":["AST","CKMB","LDH","Myoglobin","TnT"],"a":4,"p":"At 12 HOURS after MI: TnT = MOST SENSITIVE and SPECIFIC cardiac marker. Rising since 4-6h, well underway at 12h. More cardiac-specific than CKMB. Will remain elevated 3-14 DAYS → useful for both early and late presentations. LDH not yet risen (onset 24-48h). Myoglobin = non-specific (also rises in skeletal muscle damage). CKMB also rising but LESS SPECIFIC than TnT.\n\n📹 [VIDEO LINK]"},{"id":984,"s":"Enzymes","t":"Cardiac Biomarkers","u":"CKMB Peaks at 24 Hours After MI Prognosis","q":"A 55-year-old MI patient is admitted in CCU. His house officer needs to take a sample for cardiac biomarkers to determine prognosis. Levels of which biomarker will be at peak in this patient 24 hours after the onset of MI?","o":["AST","CKMB","LDH","Myoglobin","TnT"],"a":1,"p":"At 24 HOURS after MI: CKMB PEAKS at approximately 24h → OPTIMAL TIME to sample CKMB for diagnosis and prognostic value. CKMB appears 4-8h after onset, PEAKS at 24h, returns to BASELINE by 48-72h. A second CKMB peak at 24-48h suggests REINFARCTION. TnT also rising at 24h but does not specifically peak at this time point.\\\n\\\n📹 [VIDEO LINK]"},{"id":985,"s":"Enzymes","t":"Cardiac Biomarkers","u":"TnT Gold Standard Reliably Elevated 6 Days After MI","q":"An 88-year-old female is admitted in CCU six days after acute inferior wall MI. Which of the following is the gold standard cardiac biomarker that will still be reliably elevated in her?","o":["AST","CKMB","CK total","LDH","TnT"],"a":4,"p":"At 6 DAYS after MI: TnT = GOLD STANDARD cardiac biomarker — remains elevated for 3-14 DAYS. CKMB = returned to baseline by day 3. AST = normal by day 4. CK total = normal by day 3-4. LDH may still be elevated (returns normal by day 8-14). TnT is the ONLY reliably elevated, cardiac-SPECIFIC biomarker at 6 days. TnT/TnI = gold standard for MI diagnosis and prognostication.\n\n📹 [VIDEO LINK]"},{"id":986,"s":"Enzymes","t":"Cardiac Biomarkers","u":"TnT Elevated 12 Days After MI Follow Up","q":"A 70-year-old male with history of myocardial infarction 12 days ago comes to OPD for follow-up. Levels of which cardiac biomarker will still be elevated in him?","o":["AST","CKMB","CK total","LDH","TnT"],"a":4,"p":"At 12 DAYS after MI: TnT STILL ELEVATED (up to 3-14 days, sometimes longer with large infarcts). CKMB = normal by day 3. AST = normal by day 4. CK total = normal by day 3-4. LDH borderline at 12 days (normal by day 8-14). TnT = GOLD STANDARD for MI diagnosis and prognostication. Serial troponin measurements used to confirm MI and assess its size.\\\n\\\n📹 [VIDEO LINK]"},{"id":987,"s":"Enzymes","t":"Serum Enzymes in Diagnosis","u":"Acid Phosphatase Prostate Cancer Historical Marker PSA","q":"A 72-year-old man presents with difficulty urinating, bone pain, and weight loss. Digital rectal examination reveals a hard irregular prostate. PSA is markedly elevated. Which of the following serum enzymes was historically used for the diagnosis of this condition before PSA became available?","o":["Acid phosphatase","Alanine aminotransferase","Alkaline phosphatase","Amylase","Aspartate aminotransferase"],"a":0,"p":"ACID PHOSPHATASE (prostatic acid phosphatase, PAP) = historically used marker for PROSTATE CANCER before PSA era. Elevated in metastatic prostate cancer (bone metastases). Now replaced by PSA (prostate-specific antigen) for screening and monitoring. Acid phosphatase active at acid pH (optimum ~5). Alkaline phosphatase active at alkaline pH (~9-10) → bone disease and cholestatic liver disease.\n\n📹 [VIDEO LINK]"},{"id":988,"s":"Enzymes","t":"Serum Enzymes in Diagnosis","u":"Alkaline Phosphatase Bone Disease Diagnosis","q":"Which of the following serum enzymes is used for the diagnosis of bone disease?","o":["Acid phosphatase","Alanine aminotransferase","Alkaline phosphatase","Amylase","Aspartate aminotransferase"],"a":2,"p":"ALKALINE PHOSPHATASE (ALP) = elevated in BONE DISEASE (Paget's disease, bone metastases, rickets, hyperparathyroidism) and CHOLESTATIC LIVER DISEASE (biliary obstruction, primary biliary cirrhosis). Osteoblasts produce large amounts of ALP → elevated with increased bone turnover. Isoenzyme analysis separates bone ALP (heat-stable) from liver ALP (heat-labile). Also elevated in: pregnancy (placental isoenzyme), growing children.\\\n\\\n📹 [VIDEO LINK]"},{"id":989,"s":"Enzymes","t":"Serum Enzymes in Diagnosis","u":"Amylase Acute Pancreatitis Diagnosis Mid-Abdominal Pain","q":"A 50-year-old minister presents with sudden onset mid-abdominal pain radiating to his back and vomiting. Labs reveal raised serum amylase. He is likely to be suffering from which of the following?","o":["Acute appendicitis","Acute cholangitis","Acute cholecystitis","Acute pancreatitis","Acute pyelonephritis"],"a":3,"p":"SERUM AMYLASE = primary marker for ACUTE PANCREATITIS. Rises within 2-12h, peaks 24h, returns to normal in 3-5 days. Classic: epigastric pain radiating to BACK + vomiting + raised amylase and lipase. LIPASE = more specific than amylase for pancreatitis. Causes of pancreatitis: GETS SMASHED (Gallstones, Ethanol, Trauma, Steroids, Mumps, Autoimmune, Scorpion, Hyperlipidaemia, ERCP, Drugs).\\\n\\\n📹 [VIDEO LINK]"},{"id":990,"s":"Enzymes","t":"Serum Enzymes in Diagnosis","u":"Lipase More Specific than Amylase for Pancreatitis","q":"Which of the following serum enzymes is used for the diagnosis of pancreatitis and is more specific than amylase?","o":["Acid phosphatase","Alanine aminotransferase","Alkaline phosphatase","Amylase","Lipase"],"a":4,"p":"SERUM LIPASE = more SENSITIVE and SPECIFIC for acute pancreatitis than amylase. Rises 4-8h, peaks 24h, remains elevated 8-14 DAYS (longer than amylase → better for delayed presentations). More specific because: (1) Pancreatic lipase = primary source. (2) Less affected by renal disease and other conditions. Current guidelines recommend lipase over amylase for diagnosis of acute pancreatitis.\\\n\\\n📹 [VIDEO LINK]"},{"id":991,"s":"Enzymes","t":"Serum Enzymes in Diagnosis","u":"ALT Acute Liver Disease Most Sensitive Specific","q":"Acute liver disorder can be diagnosed by increase in serum level of which of the following?","o":["Alanine aminotransferase","Alkaline phosphatase","Amylase","Creatine kinase","Trypsin"],"a":0,"p":"ALT (ALANINE AMINOTRANSFERASE, SGPT) = most SENSITIVE and SPECIFIC marker for HEPATOCELLULAR DAMAGE. Located primarily in hepatocyte cytoplasm. ALT > AST = viral hepatitis (ALT:AST > 1). AST > ALT = alcoholic hepatitis (AST:ALT > 2), MI, haemolysis. VERY HIGH ALT (>1000 U/L) = acute viral hepatitis, drug-induced liver injury, ischaemic hepatitis.\\\n\\\n📹 [VIDEO LINK]"},{"id":993,"s":"Enzymes","t":"Serum Enzymes in Diagnosis","u":"CK Cardiac Brain Muscle Disease Isoenzymes","q":"Which of the following enzymes is used for diagnosis of cardiac disease, muscle damage, and brain injury?","o":["Acid phosphatase","Alanine aminotransferase","Alkaline phosphatase","Creatine kinase","Lactate dehydrogenase"],"a":3,"p":"CREATINE KINASE (CK) ISOENZYMES: CK-BB (CK1) = BRAIN. CK-MB (CK2) = MYOCARDIUM (>5% of total CK = cardiac-specific). CK-MM (CK3) = SKELETAL MUSCLE (predominant in normal serum). Total CK elevated in: MI (CK-MB), rhabdomyolysis (CK-MM), muscular dystrophy (CK-MM), dermatomyositis, brain injury (CK-BB). CK-MB = first cardiac-specific marker to rise (4-8h), peaks 24h, baseline 48-72h.\\\n\\\n📹 [VIDEO LINK]"},{"id":994,"s":"Enzymes","t":"Serum Enzymes in Diagnosis","u":"LDH Cardiac Liver Malignancy Diagnosis Isoenzymes","q":"Which of the following enzymes is used for diagnosis of cardiac disease, liver damage, and certain malignancies?","o":["Alanine aminotransferase","Alkaline phosphatase","Amylase","Creatine kinase","Lactate dehydrogenase"],"a":4,"p":"LDH (LACTATE DEHYDROGENASE) elevated in: MI (cardiac flip — LDH1 > LDH2), LIVER DISEASE (LDH5 from liver), HAEMOLYSIS (LDH1 from RBCs), MALIGNANCIES (lymphoma, leukaemia, testicular germ cell tumours — LDH correlates with tumour burden), PULMONARY EMBOLISM, RENAL INFARCTION. Non-specific marker used with other tests. LDH1 > LDH2 = CARDIAC FLIP (confirms MI). Rises 24-48h after MI, peaks 3-6 days, elevated 8-14 days.\\\n\\\n📹 [VIDEO LINK]"},{"id":995,"s":"Enzymes","t":"Serum Enzymes in Diagnosis","u":"Streptokinase Thrombolytic Treatment MI Coronary Clot","q":"A patient diagnosed with acute MI has a clot in his coronary artery. The enzyme preparation given intravenously to dissolve the clot is which of the following?","o":["Glucocerebrosidase","Hyaluronidase","Papain","Streptokinase","Urokinase"],"a":3,"p":"STREPTOKINASE (from Streptococcus) = THROMBOLYTIC. Streptokinase + plasminogen → complex → converts plasminogen → PLASMIN → dissolves fibrin clot → reperfusion. Used in STEMI when primary PCI unavailable within 120 minutes. Side effects: bleeding, allergic reactions (foreign protein — antibodies may develop). UROKINASE = another thrombolytic (human-derived → less antigenic). Heparin = anticoagulant (NOT thrombolytic — prevents new clot but does NOT dissolve existing clots).\n\n📹 [VIDEO LINK]"},{"id":996,"s":"Enzymes","t":"Regulatory Enzymes","u":"PFK1 Allosteric Activation Fructose 2,6-Bisphosphate Sprinting","q":"A sprinting athlete needs rapid ATP generation. Which of the following is allosterically activating PFK-1 in him?","o":["ATP","Citrate","Fructose-1-phosphate","Fructose-2,6-bisphosphate","Glucose-6-phosphate"],"a":3,"p":"PFK-1 ALLOSTERIC ACTIVATORS: AMP (signals low energy), FRUCTOSE-2,6-BISPHOSPHATE (most potent activator — opposes ATP inhibition, separate binding site). During sprinting: AMP rises + F2,6-BP from PFK-2 activity → PFK-1 maximally activated → glycolysis accelerated → rapid ATP generation. INHIBITORS: ATP (high energy state), CITRATE (TCA cycle full), H⁺. F2,6-BP produced by PFK-2 (insulin stimulates, glucagon inhibits via cAMP/PKA → falls in fasting).\n\n📹 [VIDEO LINK]"},{"id":997,"s":"Enzymes","t":"Regulatory Enzymes","u":"Trypanosomiasis Irreversible Inhibition Trypanosomal Enzyme","q":"A soldier with African sleeping sickness is treated with a drug that covalently inhibits the enzyme used by trypanosomes to switch their protein coats. Which biochemical mechanism is responsible for the drug's success?","o":["Competitive inhibition","Irreversible inhibition","Mixed inhibition","Non-competitive inhibition","Uncompetitive inhibition"],"a":1,"p":"TRYPANOSOMIASIS treatment: drug IRREVERSIBLY INHIBITS trypanosomal enzyme (eflornithine irreversibly inhibits ORNITHINE DECARBOXYLASE — required for polyamine synthesis and VSG coat switching) via COVALENT inhibition. IRREVERSIBLE = enzyme permanently inactivated. Cannot be overcome by increasing substrate concentration — more effective than competitive/reversible inhibition for antiparasitic drugs. Trypanosoma brucei escapes immune system by switching VARIANT SURFACE GLYCOPROTEINS (VSGs).\\\n\\\n📹 [VIDEO LINK]"},{"id":998,"s":"Enzymes","t":"Regulatory Enzymes","u":"Malonate Competitive Inhibitor Succinate Dehydrogenase Km Vmax","q":"Malonate is a competitive inhibitor of succinate dehydrogenase of the citric acid cycle. Which of the following correctly describes the effect of malonate on succinate dehydrogenase kinetics?","o":["Decreased Km and decreased Vmax","Decreased Km and unchanged Vmax","Increased Km and decreased Vmax","Increased Km and unchanged Vmax","Unchanged Km and decreased Vmax"],"a":3,"p":"MALONATE = COMPETITIVE INHIBITOR of succinate dehydrogenase (both malonate and succinate are dicarboxylic acids — structural similarity). COMPETITIVE INHIBITION: INCREASED APPARENT Km (more succinate needed to reach ½Vmax), VMAX UNCHANGED (at very high succinate concentration, malonate is outcompeted). Lineweaver-Burk: same y-intercept (1/Vmax), different x-intercept (−1/Km shifts toward zero). Historical significance: malonate used to prove TCA cycle is cyclic (Krebs).\\\n\\\n[Note to editor: Insert Michaelis-Menten curve showing right shift with malonate, and Lineweaver-Burk showing same y-intercept, different x-intercept.]\\\n\\\n📹 [VIDEO LINK]"},{"id":999,"s":"Haemoglobin and Oxygen Transport","t":"Haem Structure","u":"Iron Porphyrin Complex is Heme","q":"Which of the following best describes the iron porphyrin complex?","o":["Coproporphyrin","Heme","Porphyrin","Porphyrinogen","Uroporphyrin"],"a":1,"p":"HEME = IRON (Fe²⁺) + PROTOPORPHYRIN IX. Iron must be in FERROUS state (Fe²⁺) for O₂ binding. PORPHYRIN = the ring without iron. PORPHYRINOGEN = the reduced (colourless) precursor of porphyrin. COPROPORPHYRIN and UROPORPHYRIN = biosynthetic intermediates formed during haem synthesis. Haem is the functional prosthetic group of haemoglobin, myoglobin, cytochromes, catalase, and peroxidase.\\\n\\\n📹 [VIDEO LINK]"},{"id":1000,"s":"Haemoglobin and Oxygen Transport","t":"Haem Structure","u":"O2 Binds Ferrous Fe2 Before Nitrite Exposure","q":"A patient is brought to the emergency department with methaemoglobinaemia after nitrite exposure. Before nitrate exposure, oxygen was binding to which oxidation state of iron in his haemoglobin?","o":["Plus 1","Plus 2","Plus 3","Plus 4","Plus 5"],"a":1,"p":"O₂ binds ONLY to FERROUS IRON (Fe²⁺, +2). The hydrophobic haem pocket prevents spontaneous oxidation of Fe²⁺ → Fe³⁺. METHAEMOGLOBIN REDUCTASE (NADH-dependent, cytochrome b5 reductase) = keeps iron in Fe²⁺ state in normal RBCs. Nitrites OXIDISE Fe²⁺ → Fe³⁺ → methaemoglobin → O₂ cannot bind → tissue hypoxia.\\\n\\\n📹 [VIDEO LINK]"},{"id":10001,"s":"Haemoglobin and Oxygen Transport","t":"Haem Structure","u":"Iron in Ferric Fe3 State After Nitrite Exposure Methaemoglobin","q":"A patient is brought to the emergency department with methaemoglobinaemia after nitrite exposure. After nitrate exposure, oxygen can no longer bind his haemoglobin. Iron is now in which oxidation state?","o":["Plus 1","Plus 2","Plus 3","Plus 4","Plus 5"],"a":2,"p":"NITRITES oxidise Fe²⁺ → FERRIC IRON (Fe³⁺, +3) → METHAEMOGLOBIN → CANNOT BIND O₂ → CHOCOLATE-BROWN BLOOD. Treatment: METHYLENE BLUE (donates electrons via NADPH-metHb reductase → reduces Fe³⁺ → Fe²⁺). Causes of methaemoglobinaemia: nitrites, dapsone, primaquine, local anaesthetics (benzocaine, prilocaine). Pulse oximetry falsely reads ~85% (metHb absorbs at both 660nm and 940nm wavelengths used by oximeter).\\\n\\\n📹 [VIDEO LINK]"},{"id":1001,"s":"Haemoglobin and Oxygen Transport","t":"Haem Structure","u":"Porphyrin Ring Four Pyrrole Rings Methenyl Bridges","q":"Heme consists of a planar porphyrin ring composed of which of the following?","o":["Four pyrrole rings linked by methenyl bridges","One pyrrole ring","Three pyrrole rings linked by methyl bridges","Two pyrrole rings linked by methylene bridges","Two pyrrole rings linked by vinyl bridges"],"a":0,"p":"PROTOPORPHYRIN IX: FOUR PYRROLE RINGS linked by FOUR METHENYL BRIDGES (alpha, beta, gamma, delta methene bridges) in a cyclic arrangement. The four NITROGEN ATOMS at the centre coordinate with Fe²⁺ (four equatorial coordination bonds). Fe²⁺ has two additional coordination positions: 5th = PROXIMAL HISTIDINE (protein), 6th = O₂ (when oxygenated). Heme = protoporphyrin IX + Fe²⁺.\\\n\\\n📹 [VIDEO LINK]"},{"id":1002,"s":"Haemoglobin and Oxygen Transport","t":"Haem Structure","u":"Nitrogen Atoms Centre Porphyrin Ring Bind Ferrous Iron","q":"Heme consists of a planar porphyrin ring composed of pyrrole rings linked by methenyl bridges. The nitrogen atoms at the centre bind which of the following?","o":["Carbon","Ferric iron","Ferrous iron","Nitrogen","Oxygen"],"a":2,"p":"The four NITROGEN ATOMS of the four pyrrole rings coordinate with FERROUS IRON (Fe²⁺) at four equatorial positions (four of the six coordination bonds of iron). The fifth coordination bond = PROXIMAL HISTIDINE (His-F8 of globin chain). The sixth coordination bond = O₂ (in oxyhaemoglobin) or empty (in deoxyhaemoglobin). FERRIC IRON (Fe³⁺) = present in methaemoglobin → cannot bind O₂.\\\n\\\n📹 [VIDEO LINK]"},{"id":1003,"s":"Haemoglobin and Oxygen Transport","t":"Haem Structure","u":"Heme Lies Between E and F Helices Hydrophobic Pocket","q":"Heme lies in a crevice between which of the following alpha helices in the globin fold?","o":["A and B","B and C","C and D","D and E","E and F"],"a":4,"p":"HEME lies in a CREVICE between the E and F HELICES (the EF pocket) of each globin subunit. This HYDROPHOBIC POCKET prevents oxidation of Fe²⁺ to Fe³⁺ by excluding water. Key residues: PROXIMAL HISTIDINE = F8 (His-87 in alpha chain, His-92 in beta chain) — coordinates directly with Fe²⁺. DISTAL HISTIDINE = E7 (His-58 in alpha chain, His-63 in beta chain) — forms hydrogen bond with O₂ → stabilises O₂ binding and reduces CO affinity.\\\n\\\n📹 [VIDEO LINK]"},{"id":1004,"s":"Haemoglobin and Oxygen Transport","t":"Haem Structure","u":"Oxygen Binding Pocket is Hydrophobic Prevents Fe2 Oxidation","q":"A medical student studying haemoglobin structure notes that haem must be protected from oxidation to maintain iron in the ferrous state. The alpha helices of a globin subunit form an oxygen binding pocket with which of the following property?","o":["Hydrophilic","Hydrophobic","Ionic","Polar","Positively charged"],"a":1,"p":"The OXYGEN BINDING POCKET is HYDROPHOBIC. This hydrophobic environment: (1) EXCLUDES WATER → prevents oxidation of Fe²⁺ to Fe³⁺ (water is needed for spontaneous Fe²⁺ oxidation → methaemoglobin formation). (2) Allows O₂ to bind reversibly to Fe²⁺. (3) Forces CO to bind at an angle (distal histidine tilts CO → reduces CO affinity from theoretical 25,000× to ~250× greater than O₂). Without the hydrophobic pocket: Fe²⁺ would be rapidly oxidised to Fe³⁺ → methaemoglobin → no O₂ transport.\\\n\\\n📹 [VIDEO LINK]"},{"id":1005,"s":"Haemoglobin and Oxygen Transport","t":"Haem Structure","u":"Proximal Histidine Coordinates Fe2 Directly","q":"Which of the following coordinates with ferrous iron directly in the haem pocket?","o":["Distal histidine","Lateral histidine","Proximal histidine","Terminal histidine","Valine residue"],"a":2,"p":"PROXIMAL HISTIDINE (His-F8): coordinates DIRECTLY with Fe²⁺ via the 5th coordination bond (below the porphyrin plane). In deoxyHb: Fe²⁺ sits ABOVE the porphyrin plane (large atomic radius) → proximal His pulled out of plane. Upon O₂ binding: Fe²⁺ moves INTO porphyrin plane (smaller covalent radius) → proximal His moves with it → conformational change propagates through the tetramer → T→R transition (molecular basis of cooperativity).\\\n\\\n📹 [VIDEO LINK]"},{"id":1006,"s":"Haemoglobin and Oxygen Transport","t":"Haem Structure","u":"Distal Histidine Stabilises O2 via Hydrogen Bond","q":"Which of the following stabilises oxygen binding in the haem pocket via a hydrogen bond with oxygen?","o":["Distal histidine","Lateral histidine","Proximal histidine","Terminal histidine","Valine residue"],"a":0,"p":"DISTAL HISTIDINE (His-E7): forms a HYDROGEN BOND with O₂ after it binds to Fe²⁺. Does NOT coordinate with Fe²⁺ directly (that is the proximal histidine). Functions: (1) STABILISES O₂ binding via hydrogen bonding. (2) REDUCES CO AFFINITY by forcing CO to bind at an angle (CO normally binds Fe²⁺ linearly with very high affinity; distal His tilts CO → reduces affinity from 25,000× to ~250× greater than O₂). This discrimination between O₂ and CO is physiologically critical.\\\n\\\n📹 [VIDEO LINK]"},{"id":1007,"s":"Haemoglobin and Oxygen Transport","t":"Haem Structure","u":"DeoxyHb Fe2 Large Atomic Radius Above Porphyrin Plane","q":"In the absence of oxygen, ferrous iron sits above the plane of the porphyrin ring because it has a large which of the following?","o":["Atomic radius","Covalent radius","Ionic radius","Metallic radius","Van der Waals radius"],"a":0,"p":"In DEOXYHAEMOGLOBIN: Fe²⁺ has a large ATOMIC RADIUS → too large to fit in the plane of the porphyrin ring → sits ABOVE the plane (proximal side) → porphyrin ring is DOMED (puckered) → proximal histidine is pulled out of plane. Upon O₂ BINDING: shared electrons → Fe²⁺ gets slightly smaller COVALENT RADIUS → moves INTO the porphyrin plane → proximal histidine moves with it → conformational change → T→R transition. This iron movement is the MOLECULAR BASIS of cooperativity in haemoglobin.\\\n\\\n📹 [VIDEO LINK]"},{"id":1008,"s":"Haemoglobin and Oxygen Transport","t":"Haem Structure","u":"O2 Binding Fe2 Moves Into Plane Smaller Covalent Radius","q":"Upon oxygen binding to ferrous iron, the iron moves into the plane of the porphyrin ring because the shared electrons result in a slightly smaller which of the following?","o":["Atomic radius","Covalent radius","Ionic radius","Metallic radius","Van der Waals radius"],"a":1,"p":"Upon O₂ BINDING: O₂ shares electrons with Fe²⁺ → COVALENT RADIUS of Fe²⁺ decreases slightly → now fits within the porphyrin ring plane → iron MOVES INTO the plane → PROXIMAL HISTIDINE (F8) is pulled → CONFORMATIONAL CHANGE propagates through the subunit → transmitted to adjacent subunits via alpha1-beta2 interface → COOPERATIVITY. This movement is why O₂ binding to one subunit increases O₂ affinity of remaining subunits (sigmoidal curve).\\\n\\\n📹 [VIDEO LINK]"},{"id":1009,"s":"Haemoglobin and Oxygen Transport","t":"Haem Structure","u":"Glycine Substrate for Heme Synthesis ALA Synthase","q":"Which of the following amino acids is a substrate for haem synthesis?","o":["Alanine","Glycine","Proline","Serine","Valine"],"a":1,"p":"ALA SYNTHASE (rate-limiting enzyme of haem synthesis): SUCCINYL-CoA + GLYCINE → DELTA-AMINOLEVULINIC ACID (ALA) + CO₂. Requires PYRIDOXAL PHOSPHATE (PLP, vitamin B6). GLYCINE provides the carbon backbone of ALA. Glycine = simplest amino acid (H as side chain) — the only amino acid that is not chiral. Succinyl-CoA = from TCA cycle. Both substrates converge in mitochondria for the first step of haem synthesis.\\\n\\\n📹 [VIDEO LINK]"},{"id":1010,"s":"Haemoglobin and Oxygen Transport","t":"Haem Structure","u":"Ferrochelatase Adds Iron to Protoporphyrin IX to Form Heme","q":"Heme is formed by addition of iron into which of the following?","o":["Coproporphyrinogen","Porphobilinogen","Protoporphyrin IX","Protoporphyrinogen","Uroporphyrinogen III"],"a":2,"p":"FERROCHELATASE (last enzyme of haem synthesis, located in MITOCHONDRIA): PROTOPORPHYRIN IX + Fe²⁺ → HEME. Ferrochelatase is inhibited by LEAD (Pb²⁺) → zinc replaces iron → zinc protoporphyrin accumulates → diagnostic of lead poisoning. The immediate precursor of haem = PROTOPORPHYRIN IX (not protoporphyrinogen IX, which must first be oxidised by protoporphyrinogen oxidase to protoporphyrin IX before iron is inserted).\\\n\\\n📹 [VIDEO LINK]"},{"id":1011,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Structure","u":"Alpha Beta Dimers Held by Hydrophobic Interactions","q":"The alpha and beta polypeptide chains of haemoglobin are held together to form dimers by which of the following?","o":["Covalent interactions","Hydrophilic interactions","Hydrophobic interactions","Ionic interactions","Van der Waals interactions"],"a":2,"p":"Within each ALPHA-BETA DIMER: alpha and beta chains are held together by HYDROPHOBIC INTERACTIONS (at the alpha1-beta1 interface — STABLE, does NOT change during T→R transition). The two dimers (alpha1-beta1 and alpha2-beta2) interact at the ALPHA1-BETA2 and ALPHA2-BETA1 interfaces — these DO change during T→R transition (dimers rotate ~15° relative to each other upon oxygenation). This rotation is the structural basis of cooperative O₂ binding.\\\n\\\n📹 [VIDEO LINK]"},{"id":1012,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Structure","u":"Two Dimers Held Together by Ionic Interactions Salt Bridges","q":"Two alpha-beta dimers of haemoglobin are held together by which of the following?","o":["Covalent interactions","Hydrophobic interactions","Ionic interactions","Peptide bonds","Van der Waals interactions"],"a":2,"p":"The two ALPHA-BETA DIMERS interact through IONIC INTERACTIONS (salt bridges) at the alpha1-beta2 and alpha2-beta1 interfaces. In the T (TAUT/DEOXY) STATE: salt bridges INTACT → low O₂ affinity. Upon O₂ binding: salt bridges RUPTURE → R (RELAXED/OXY) STATE → high O₂ affinity. 2,3-BPG stabilises the T state by binding to and REINFORCING the salt bridges between the two beta chains in the central cavity.\\\n\\\n📹 [VIDEO LINK]"},{"id":1013,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Structure","u":"T Form Two Dimers Interact via Ionic Bonds and Hydrogen Bonds","q":"In the T form, the two alpha-beta dimers interact through a network of hydrogen bonds and which of the following?","o":["Covalent bonds","Hydrophobic bonds","Ionic bonds","Non-covalent bonds","Peptide bonds"],"a":2,"p":"T (TAUT/DEOXY) FORM: stabilised by a NETWORK OF HYDROGEN BONDS AND IONIC BONDS (salt bridges) between the two alpha-beta dimers at alpha1-beta2 and alpha2-beta1 interfaces. Salt bridges involve charged amino acid residues including C-terminal residues (Arg-141alpha, His-146beta) and 2,3-BPG binding sites. 2,3-BPG fits into the CENTRAL CAVITY of deoxyHb → forms multiple ionic bonds with BOTH BETA CHAINS → stabilises T state → ↓ O₂ affinity.\\\n\\\n📹 [VIDEO LINK]"},{"id":1014,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Structure","u":"T Form Low O2 Affinity Taut Deoxy State","q":"Compared to the relaxed form, the taut form of haemoglobin has which of the following?","o":["Equal oxygen affinity","High oxygen affinity","Low oxygen affinity","No oxygen affinity","Variable oxygen affinity"],"a":2,"p":"T (TAUT) FORM = DEOXYHAEMOGLOBIN: salt bridges intact, central cavity open (2,3-BPG binds) → LOW O₂ AFFINITY. R (RELAXED) FORM = OXYHAEMOGLOBIN: salt bridges ruptured, central cavity closes → HIGH O₂ AFFINITY. COOPERATIVITY: binding of first O₂ → partial T→R transition → subsequent subunits have progressively higher O₂ affinity → SIGMOIDAL O₂ dissociation curve. T→R transition: dimers rotate ~15° relative to each other. R state has ~500× higher O₂ affinity than T state.\\\n\\\n📹 [VIDEO LINK]"},{"id":1015,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Structure","u":"R Form High O2 Affinity Relaxed Oxy State","q":"Compared to the taut form, the relaxed form of haemoglobin has which of the following?","o":["Equal oxygen affinity","High oxygen affinity","Low oxygen affinity","No oxygen affinity","Variable oxygen affinity"],"a":1,"p":"R (RELAXED) FORM = OXYHAEMOGLOBIN: salt bridges ruptured, central cavity collapsed (2,3-BPG expelled) → HIGH O₂ AFFINITY. Oxygenation of one subunit → partial T→R transition → adjacent subunits easier to oxygenate (cooperative O₂ binding). R state has ~500× HIGHER O₂ AFFINITY than T state. All four subunits in R state = fully saturated haemoglobin. This allosteric mechanism underlies the SIGMOIDAL shape of the O₂ dissociation curve.\\\n\\\n📹 [VIDEO LINK]"},{"id":1016,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Structure","u":"O2 Binding Ruptures Salt Bridges T to R Transition","q":"The binding of oxygen to haemoglobin leads to rupture of which of the following?","o":["Covalent bridges","Disulfide bridges","Hydrophobic bridges","Peptide bridges","Salt bridges"],"a":4,"p":"O₂ BINDING → Fe²⁺ moves into porphyrin plane → proximal His moves → conformational change propagates → SALT BRIDGES (ionic bonds) at the alpha1-beta2 interface RUPTURE → T→R TRANSITION. This rupture: (1) Collapses the central cavity. (2) Expels 2,3-BPG. (3) Expels H⁺ (Bohr effect — releases protons on oxygenation). (4) Increases O₂ affinity of remaining subunits. The Bohr effect = H⁺ released on oxygenation → promotes CO₂ transport from tissues.\\\n\\\n📹 [VIDEO LINK]"},{"id":1017,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Structure","u":"Eight Alpha Helices in Each Globin Subunit Labelled A to H","q":"The number of alpha helices in the globin fold of each subunit is which of the following?","o":["2","4","6","7","8"],"a":4,"p":"Each GLOBIN SUBUNIT has EIGHT ALPHA HELICES, labelled A through H. The GLOBIN FOLD (helix-loop-helix motif) is the characteristic tertiary structure of ALL GLOBINS (Hb alpha and beta chains, myoglobin, neuroglobin, cytoglobin). Heme sits in the EF pocket (between E and F helices). PROXIMAL HISTIDINE = F8 (8th residue of F helix). DISTAL HISTIDINE = E7 (7th residue of E helix). The A-H labelling system: helices separated by non-helical corner segments.\\\n\\\n📹 [VIDEO LINK]"},{"id":1018,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Structure","u":"Haemoglobin Has Quaternary Structure Most Distinctive Feature","q":"Which of the following best describes haemoglobin structure?","o":["Both secondary and tertiary only","Primary structure only","Quaternary structure","Secondary structure only","Tertiary structure only"],"a":2,"p":"HAEMOGLOBIN has ALL FOUR LEVELS of protein structure — primary, secondary, tertiary, and QUATERNARY. The MOST DISTINCTIVE feature is QUATERNARY STRUCTURE (assembly of 4 subunits: 2 alpha + 2 beta chains into tetramer). MYOGLOBIN = only primary, secondary, and tertiary (monomer — NO quaternary structure → no cooperativity → HYPERBOLIC O₂ binding curve). Quaternary structure gives Hb its cooperative O₂ binding (sigmoidal curve) and allosteric regulation properties (2,3-BPG, H⁺, CO₂, CO effects).\\\n\\\n📹 [VIDEO LINK]"},{"id":1019,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Structure","u":"Iron Bound to Histidine in Haemoglobin Proximal","q":"In the haemoglobin molecule, iron is bound to which of the following amino acids?","o":["Histidine","Isoleucine","Leucine","Lysine","Valine"],"a":0,"p":"In haemoglobin: Fe²⁺ is bound to the PROXIMAL HISTIDINE (His-F8) via the 5th coordination bond. Iron is also coordinated at four equatorial positions by the NITROGEN ATOMS of the four pyrrole rings of protoporphyrin IX. The 6th coordination position = O₂ (when oxygenated). HISTIDINE (imidazole side chain, pKa ~6) plays this structural role uniquely — no other amino acid can coordinate with transition metals in this way at physiological pH.\\\n\\\n📹 [VIDEO LINK]"},{"id":1020,"s":"Haemoglobin and Oxygen Transport","t":"Oxygen Binding","u":"Haemoglobin Binds 4 O2 Per Tetramer One Per Heme","q":"Per tetramer, haemoglobin binds which of the following?","o":["Four molecules of oxygen","One molecule of oxygen","Three molecules of oxygen","Two molecules of oxygen","Zero molecules of oxygen"],"a":0,"p":"HAEMOGLOBIN TETRAMER (2 alpha + 2 beta chains + 4 haem groups): binds FOUR O₂ MOLECULES (one per haem group). Per HAEM = ONE O₂. 100% SATURATION = all 4 haem groups occupied by O₂ = 4 O₂ per Hb tetramer. Compare: MYOGLOBIN (monomer, 1 haem group) = ONE O₂ per molecule. The tetrameric structure → COOPERATIVITY (binding of first O₂ increases affinity for subsequent O₂ → SIGMOIDAL curve). Myoglobin = no cooperativity → HYPERBOLIC curve.\\\n\\\n📹 [VIDEO LINK]"},{"id":1021,"s":"Haemoglobin and Oxygen Transport","t":"Oxygen Binding","u":"Myoglobin Binds One O2 Hyperbolic Curve Monomer","q":"Myoglobin can bind which of the following?","o":["Four molecules of oxygen","One molecule of oxygen","Three molecules of oxygen","Two molecules of oxygen","Variable number"],"a":1,"p":"MYOGLOBIN (monomer, 1 haem group): binds ONE O₂ molecule. O₂ binding = HYPERBOLIC curve (NOT sigmoidal) because myoglobin is a MONOMER → no cooperativity → each molecule binds O₂ independently. MYOGLOBIN has HIGHER O₂ AFFINITY than HbA (curve shifted LEFT relative to HbA) → functions as O₂ STORAGE in muscle. Releases O₂ only when pO₂ is very low (during intense exercise). Brown colour of meat = myoglobin.\\\n\\\n📹 [VIDEO LINK]"},{"id":1022,"s":"Haemoglobin and Oxygen Transport","t":"Oxygen Binding","u":"Haemoglobin 100 Percent Saturated All 4 Sites Occupied","q":"Haemoglobin is said to be 100 percent saturated when binding sites are occupied by which of the following?","o":["Four molecules of oxygen","One molecule of oxygen","Three molecules of oxygen","Two molecules of oxygen","Zero molecules of oxygen"],"a":0,"p":"100% SATURATION = ALL FOUR HAEM GROUPS occupied by O₂ = 4 O₂ per tetramer. In arterial blood (pO₂ ~100 mmHg): Hb is approximately 97-98% saturated. In venous blood (pO₂ ~40 mmHg): Hb is approximately 75% saturated. Cooperativity means the transition from 0% to 100% saturation is steep (sigmoidal) → Hb loads O₂ efficiently in lungs AND unloads efficiently in tissues.\\\n\\\n📹 [VIDEO LINK]"},{"id":1023,"s":"Haemoglobin and Oxygen Transport","t":"Oxygen Binding","u":"Cooperativity O2 Affinity Increases as Saturation Increases","q":"Which of the following statements concerning oxygen binding by haemoglobin is correct?","o":["2,3-BPG increases O₂ affinity","Bohr effect results in lower affinity at higher pH","CO₂ increases O₂ affinity","O₂ affinity increases as percentage saturation increases","OxyHb and deoxyHb have same affinity for H plus"],"a":3,"p":"COOPERATIVITY: as saturation increases, O₂ AFFINITY INCREASES. Binding of first O₂ (partial T→R transition) → second subunit more easily oxygenated → third → fourth. BASIS of SIGMOIDAL ODC. Other options WRONG: Bohr effect = LOWER affinity at LOWER pH (higher H⁺ — not higher pH). CO₂ DECREASES O₂ affinity (forms carbamino-Hb → allosteric change to T state). DeoxyHb has HIGHER affinity for H⁺ than oxyHb (Bohr effect). 2,3-BPG DECREASES O₂ affinity.\\\n\\\n📹 [VIDEO LINK]"},{"id":1024,"s":"Haemoglobin and Oxygen Transport","t":"Allosteric Regulation","u":"2,3-BPG Increases Altitude Acclimatisation Gilgit","q":"A farmer living in the mountainous region of Gilgit is acclimatised to low atmospheric pO₂. He is likely to have increased levels of which of the following?","o":["2,3-BPG","H plus","HCO₃ minus","pCO₂","pO₂"],"a":0,"p":"HIGH ALTITUDE ACCLIMATISATION: ↓ atmospheric pO₂ → ↓ arterial pO₂ → tissue hypoxia → compensatory response → INCREASED 2,3-BPG in RBCs within 24-48 hours. ↑ 2,3-BPG → binds deoxyHb T state → stabilises T state → SHIFTS ODC RIGHT → ↓ O₂ affinity → MORE O₂ RELEASED TO TISSUES at same pO₂. Long-term: ↑ RBC production (EPO). Hyperventilation → ↓ pCO₂ → respiratory ALKALOSIS (↓ H⁺) — the opposite of what 2,3-BPG does.\\\n\\\n📹 [VIDEO LINK]"},{"id":1025,"s":"Haemoglobin and Oxygen Transport","t":"Allosteric Regulation","u":"2,3-BPG Responsible for Right Shift ODC Hilly Area","q":"A woman moves from a city to a hilly area. Increased levels of which of the following are responsible for the shift of her oxyhaemoglobin dissociation curve?","o":["2,3-BPG","H plus","HCO₃ minus","pH","pO₂"],"a":0,"p":"ALTITUDE ACCLIMATISATION → INCREASED 2,3-BPG → ODC shifts RIGHT. 2,3-BPG binds to the CENTRAL CAVITY of deoxyHb (between the two beta chains) → STABILISES T STATE → ↓ O₂ affinity → more O₂ released to tissues at lower pO₂. 2,3-BPG produced by BISPHOSPHOGLYCERATE MUTASE in RBCs (from 1,3-BPG in glycolysis). RBCs contain HIGH concentrations of 2,3-BPG (~5mM = equimolar with Hb).\\\n\\\n📹 [VIDEO LINK]"},{"id":1026,"s":"Haemoglobin and Oxygen Transport","t":"Allosteric Regulation","u":"HbF Binds 2,3-BPG Less Strongly Higher O2 Affinity","q":"Which of the following correctly describes fetal haemoglobin?","o":["Binds 2,3-BPG equally strongly as adult Hb","Binds 2,3-BPG less strongly than adult Hb","Binds 2,3-BPG more strongly than adult Hb","High O₂ affinity does not facilitate O₂ transfer across placenta","Is composed of two zeta and two gamma chains"],"a":1,"p":"HbF DOES encounter 2,3-BPG in RBCs (same intracellular environment as HbA) but binds it LESS STRONGLY because gamma chains have SERINE at position 143 (instead of HISTIDINE in beta chains — His-143 is a key residue for 2,3-BPG binding in the central cavity). HbF is NOT free of 2,3-BPG — it simply binds it with lower affinity → HbF stays in R state more easily → HIGHER O₂ AFFINITY → captures O₂ from maternal HbA at the placenta. Option E wrong: HbF = alpha₂ GAMMA₂ (not zeta₂ gamma₂ = Hb Portland).\\\n\\\n📹 [VIDEO LINK]"},{"id":1027,"s":"Haemoglobin and Oxygen Transport","t":"Allosteric Regulation","u":"HbF Higher O2 Affinity Because Decreased 2,3-BPG Affinity Correct Statement","q":"Which of the following statements concerning haemoglobins is correct?","o":["Fetal blood has higher O₂ affinity because HbF has decreased affinity for 2,3-BPG","HbA1c differs by genetically determined amino acid substitution","HbA2 appears early in fetal life","HbF composition is alpha₂ delta₂","Purified HbF has higher O₂ affinity than purified HbA"],"a":0,"p":"CORRECT STATEMENT: Fetal blood has higher O₂ affinity because HbF has DECREASED AFFINITY for 2,3-BPG (gamma chains lack His-143 → reduced 2,3-BPG binding → HbF stays in R state more easily). PURIFIED HbF stripped of 2,3-BPG has SAME intrinsic O₂ affinity as purified HbA — the difference is ENTIRELY due to 2,3-BPG binding. HbF = alpha₂ GAMMA₂ (not delta₂). HbA1c = GLYCATED Hb (non-enzymatic glycation, NOT genetic substitution). HbA2 appears AFTER birth.\\\n\\\n📹 [VIDEO LINK]"},{"id":1028,"s":"Haemoglobin and Oxygen Transport","t":"Allosteric Regulation","u":"Hb Bart Highest O2 Affinity Among Hb Types","q":"Which of the following biomolecules has the highest affinity for oxygen?","o":["Hb A","Hb Bart","Hb F","Hb Gower 1","Hb Gower 2"],"a":1,"p":"O₂ AFFINITY RANKING: Hb BART (gamma₄) > HbF (alpha₂gamma₂) > HbA (alpha₂beta₂). Hb BART has EXTREMELY HIGH O₂ affinity: (1) gamma₄ tetramer has NO BOHR EFFECT. (2) NO useful cooperativity. (3) Cannot bind 2,3-BPG effectively → always in R-like state. RESULT: Hb Bart CANNOT release O₂ to fetal tissues → fatal HYDROPS FETALIS. HbF has HIGHER O₂ affinity than HbA (lower 2,3-BPG affinity). HbA = reference for normal adult O₂ delivery. Hb Gower 1 (zeta₂epsilon₂) and Gower 2 (alpha₂epsilon₂) = embryonic, have intermediate affinities.\\\n\\\n📹 [VIDEO LINK]"},{"id":1029,"s":"Haemoglobin and Oxygen Transport","t":"Oxygen Dissociation Curve","u":"CO Poisoning ODC Shifts Left Carboxyhaemoglobin","q":"A man falls asleep in a poorly ventilated room with the gas heater on and is found dead the following morning from carbon monoxide poisoning. His oxygen dissociation curve was shifted to which of the following?","o":["Down","Left","Right","Unchanged","Up"],"a":1,"p":"CO POISONING → ODC shifts LEFT for TWO REASONS: (1) CO binds Hb with 250× greater affinity than O₂ → CARBOXYHAEMOGLOBIN (cannot carry O₂). (2) CO binds remaining Hb subunits → allosteric change → T→R shift in remaining subunits → INCREASED O₂ affinity → O₂ bound MORE TIGHTLY → LESS O₂ released to tissues. The combination of REDUCED O₂ carrying capacity + LEFT SHIFTED ODC = DOUBLE HYPOXIA. Curve also becomes MORE HYPERBOLIC. Treatment: 100% O₂ or hyperbaric O₂.\\\n\\\n📹 [VIDEO LINK]"},{"id":1030,"s":"Haemoglobin and Oxygen Transport","t":"Oxygen Dissociation Curve","u":"CO Poisoning Comatose Man ODC Left Shift Gas Heater","q":"A 33-year-old man forgot to turn off his gas heater and was found comatose the next morning. His oxyhaemoglobin dissociation curve is likely to be which of the following?","o":["Polynomial","Shifted to left","Shifted to right","Sigmoidal","Unchanged"],"a":1,"p":"CO POISONING → LEFT SHIFT of ODC. CO → carboxyHb (cannot carry O₂) + allosteric change in remaining Hb subunits → ↑ O₂ affinity → O₂ not released to tissues → tissue hypoxia despite adequate blood flow. COMA from cerebral hypoxia. ODC shifts LEFT AND loses sigmoidal character (becomes more hyperbolic as CO replaces O₂). TREATMENT: 100% O₂ (competitive displacement of CO) → hyperbaric O₂ if severe.\\\n\\\n📹 [VIDEO LINK]"},{"id":1031,"s":"Haemoglobin and Oxygen Transport","t":"Oxygen Dissociation Curve","u":"CO Poisoning Murree Snowfall Blocked Exhaust ODC Left","q":"In 2022, heavy snowfall in Murree blocked car exhaust pipes, causing carbon monoxide poisoning deaths. Before their deaths, the oxyhaemoglobin dissociation curves of the victims were likely shifted to which of the following?","o":["Hyperbolic","Left","Linear","Right","Sigmoidal"],"a":1,"p":"CO from blocked exhaust → carboxyhaemoglobin → ODC shifts LEFT. CO has 250× higher affinity for Hb than O₂ → rapidly displaces O₂. Even at low CO concentrations: 50% carboxyHb → potentially fatal. LEFT SHIFT: high O₂ affinity of remaining Hb → O₂ not released to tissues → hypoxia at tissue level despite adequate O₂ in blood. Symptoms: headache, confusion, cherry-red skin (carboxyHb colour), coma, death. Prevention: CO detectors, proper vehicle ventilation.\\\n\\\n📹 [VIDEO LINK]"},{"id":1032,"s":"Haemoglobin and Oxygen Transport","t":"Oxygen Dissociation Curve","u":"COPD Acidosis Right Shift ODC Bohr Effect","q":"A 50-year-old chronic smoker diagnosed with COPD has blood gas analysis showing acidosis. His oxyhaemoglobin dissociation curve is likely to have shifted to which of the following?","o":["Hyperbolic","Left","Linear","Right","Sigmoidal"],"a":3,"p":"COPD + ACIDOSIS (↑ H⁺) → BOHR EFFECT → ODC shifts RIGHT. BOHR EFFECT: ↑ H⁺ (acidosis) → H⁺ binds deoxyHb (His-146beta, Val-1alpha) → STABILISES T STATE → ↓ O₂ affinity → MORE O₂ RELEASED TO TISSUES. Also: ↑ CO₂ → carbamino-Hb formed → also shifts ODC RIGHT. COPD → hypercapnia + hypoxia → acidosis → RIGHT SHIFT = physiological compensation to improve O₂ delivery to hypoxic tissues. Right shift causes: acidosis, ↑ CO₂, ↑ 2,3-BPG, ↑ temperature.\\\n\\\n📹 [VIDEO LINK]"},{"id":1033,"s":"Haemoglobin and Oxygen Transport","t":"Oxygen Dissociation Curve","u":"Fetal Hb Left Shift Low 2,3-BPG Affinity Placental O2 Transfer","q":"A 24-year-old lady visited a prenatal clinic at 28 weeks of pregnancy. The fetus was able to receive oxygen from maternal blood because of which of the following biochemical reasons?","o":["Fetal Hb has high affinity for 2,3-BPG","Fetal Hb has low affinity for 2,3-BPG","Fetal Hb has same affinity for 2,3-BPG as maternal Hb","Maternal Hb has high affinity for 2,3-BPG","Maternal Hb has low affinity for 2,3-BPG"],"a":1,"p":"HbF has LOW AFFINITY for 2,3-BPG (gamma chains lack His-143) → HbF stays in R state → HIGHER O₂ AFFINITY → HbF curve is LEFT SHIFTED relative to HbA. At the PLACENTA: maternal HbA RELEASES O₂ (right-shifted by 2,3-BPG, acidosis, CO₂) → HbF CAPTURES O₂ (higher affinity) → O₂ transferred from mother to fetus. This O₂ gradient is maintained because HbF always has higher O₂ affinity than maternal HbA in the placental environment.\\\n\\\n📹 [VIDEO LINK]"},{"id":1034,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Variants","u":"HbA Composition Alpha2 Beta2 Adult Haemoglobin","q":"Which of the following is the composition of HbA?","o":["Alpha₂ beta₂","Alpha₂ delta₂","Alpha₂ epsilon₂","Alpha₂ gamma₂","Alpha₂ zeta₂"],"a":0,"p":"HbA (ADULT HAEMOGLOBIN) = ALPHA₂ BETA₂. Most abundant Hb in adults (>96%). Beta chain gene on CHROMOSOME 11. HbA2 = alpha₂ DELTA₂ (1.5-3.5% of adult Hb — elevated in beta-thalassaemia trait). HbF = alpha₂ GAMMA₂ (predominant in fetus, <1% in adults). Hb Gower 1 = ZETA₂ EPSILON₂ (earliest embryonic Hb). Hb Gower 2 = ALPHA₂ EPSILON₂. Hb Portland = ZETA₂ GAMMA₂.\\\n\\\n📹 [VIDEO LINK]"},{"id":1035,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Variants","u":"HbF Composition Alpha2 Gamma2 Fetal Haemoglobin","q":"Which of the following is the composition of HbF?","o":["Alpha₂ beta₂","Alpha₂ delta₂","Alpha₂ epsilon₂","Alpha₂ gamma₂","Alpha₂ zeta₂"],"a":3,"p":"HbF (FETAL HAEMOGLOBIN) = ALPHA₂ GAMMA₂. Gamma chain gene on CHROMOSOME 11 (same cluster as beta and delta). HbF predominates from 10-36 weeks of gestation → replaced by HbA after birth (gamma→beta switch). In adults: <1% HbF. Elevated in BETA-THALASSAEMIA MAJOR and SICKLE CELL DISEASE (HbF is PROTECTIVE — gamma chains do not polymerise with HbS). HbF has LOWER affinity for 2,3-BPG → HIGHER O₂ affinity → facilitates O₂ transfer from mother to fetus.\\\n\\\n📹 [VIDEO LINK]"},{"id":1036,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Variants","u":"HbA2 Composition Alpha2 Delta2 Thalassaemia Trait Marker","q":"Which of the following is the composition of HbA2?","o":["Alpha₂ beta₂","Alpha₂ delta₂","Alpha₂ epsilon₂","Alpha₂ gamma₂","Alpha₂ zeta₂"],"a":1,"p":"HbA2 = ALPHA₂ DELTA₂. Normal adult level: 1.5-3.5% of total Hb. ELEVATED HbA2 (>3.5%) = DIAGNOSTIC of BETA-THALASSAEMIA TRAIT. Delta chain = similar to beta chain but produced in smaller amounts. HbA2 appears after birth. Measured by HPLC or CAPILLARY ELECTROPHORESIS for thalassaemia screening. In beta-thalassaemia trait: ↓ beta chains → alpha chains pair with DELTA chains → ↑ HbA2.\\\n\\\n📹 [VIDEO LINK]"},{"id":1037,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Variants","u":"Hb Gower1 Composition Zeta2 Epsilon2 Earliest Embryonic Hb","q":"Which of the following is the composition of Hb Gower 1?","o":["Alpha₂ beta₂","Alpha₂ delta₂","Alpha₂ epsilon₂","Zeta₂ epsilon₂","Zeta₂ gamma₂"],"a":3,"p":"Hb GOWER 1 = ZETA₂ EPSILON₂ (EARLIEST embryonic haemoglobin, appearing at 3-4 weeks of gestation). EMBRYONIC HAEMOGLOBINS: Hb Gower 1 (zeta₂epsilon₂), Hb Gower 2 (alpha₂epsilon₂), Hb Portland (zeta₂gamma₂). ZETA (ζ) chains = embryonic equivalent of alpha chains (chromosome 16). EPSILON (ε) chains = embryonic equivalent of beta/gamma chains (chromosome 11). All replaced by adult Hb types after birth.\\\n\\\n📹 [VIDEO LINK]"},{"id":1038,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Variants","u":"Hb Bart Gamma4 Alpha Thalassaemia Major Hydrops Fetalis","q":"A pregnant woman's fetus has mutations in all four copies of the alpha-globin gene. Mutational analysis shows hydrops fetalis. Which of the following Hb is present in this fetus?","o":["Hb Bart","Hb H","HbA","HbA2","HbSC"],"a":0,"p":"ALPHA-THALASSAEMIA MAJOR (all 4 alpha-globin genes deleted): NO alpha chains → gamma chains form TETRAMERS → Hb BART (gamma₄). Hb Bart has EXTREMELY HIGH O₂ affinity (no Bohr effect, no cooperativity) → CANNOT release O₂ to fetal tissues → severe fetal hypoxia → HYDROPS FETALIS (generalised oedema, ascites) → stillbirth. Alpha-globin gene = CHROMOSOME 16 (2 genes per chromosome = 4 copies total). Hb H = beta₄ (3 alpha genes deleted, 1 remaining).\\\n\\\n📹 [VIDEO LINK]"},{"id":1039,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Variants","u":"Beta Globin Gene Chromosome 11 Thalassaemia Major","q":"A baby with beta-thalassaemia major awaits transfusion. The mutated gene in this baby is located on which of the following chromosomes?","o":["Chromosome 11","Chromosome 13","Chromosome 16","Chromosome 17","Chromosome 20"],"a":0,"p":"BETA-GLOBIN GENE = CHROMOSOME 11 (beta-globin gene cluster: epsilon → Gy → Ay → delta → beta). ALPHA-GLOBIN GENE = CHROMOSOME 16. Beta-thalassaemia MAJOR = BOTH beta-globin alleles mutated → no HbA → compensatory ↑ HbF and HbA2 → severe microcytic hypochromic anaemia requiring lifelong transfusions → iron overload (treatment: desferrioxamine).\\\n\\\n📹 [VIDEO LINK]"},{"id":1040,"s":"Haemoglobin and Oxygen Transport","t":"Haemoglobin Variants","u":"Alpha Globin Gene Chromosome 16 Hydrops Fetalis","q":"A neonate presents with hydrops fetalis and Hb Bart on electrophoresis. The mutated genes responsible for his condition are located on which of the following chromosomes?","o":["Chromosome 11","Chromosome 13","Chromosome 16","Chromosome 17","Chromosome 20"],"a":2,"p":"ALPHA-GLOBIN GENE = CHROMOSOME 16. TWO alpha-globin genes per chromosome 16 = FOUR alpha-globin genes total. Alpha-thalassaemia spectrum: (1) 1 deleted = silent carrier. (2) 2 deleted = alpha-thal trait (mild anaemia). (3) 3 deleted = Hb H disease (moderate haemolytic anaemia). (4) 4 deleted = alpha-thal MAJOR → Hb BART → HYDROPS FETALIS. Hb Bart = gamma₄ (gamma chains form tetramers in absence of ALL alpha chains).\\\n\\\n📹 [VIDEO LINK]"},{"id":1041,"s":"Haemoglobin and Oxygen Transport","t":"Sickle Cell Disease","u":"HbS Valine Substitution Beta6 Glu Val Sickle Cell","q":"A 10-year-old boy with shortness of breath, fatigue, and bone pain has sickle-shaped RBCs on peripheral smear. Which of the following amino acid substitutions is responsible for his symptoms?","o":["Glycine","Lysine","Proline","Serine","Valine"],"a":4,"p":"SICKLE CELL DISEASE: GLUTAMATE (Glu-6, charged, hydrophilic) in beta chain REPLACED BY VALINE (Val, uncharged, hydrophobic) → HbS. Val-6 creates a HYDROPHOBIC STICKY PATCH on deoxyHbS surface → HbS polymerises in deoxygenated state → long fibres → RBC SICKLING → vaso-occlusion, haemolysis, organ damage. In oxygenated state: HbS is SOLUBLE (R state → sticky patch concealed). Single nucleotide substitution: GAG → GTG (Glu → Val).\\\n\\\n📹 [VIDEO LINK]"},{"id":1042,"s":"Haemoglobin and Oxygen Transport","t":"Sickle Cell Disease","u":"HbS Polymerisation Dehydration Hot Weather Vaso-Occlusive","q":"A 12-year-old boy with sickle cell disease presents with shortness of breath, fatigue, and bone pain after playing in hot weather without drinking water. Polymerisation of which of the following is responsible for his symptoms?","o":["HbA","HbC","HbF","HbS","HbSC"],"a":3,"p":"HbS POLYMERISATION: deoxyHbS → Val-6 hydrophobic patch → polymerises → sickle-shaped RBCs → vaso-occlusion. DEHYDRATION: ↑ HbS concentration per RBC → ↑ polymerisation tendency. Hot weather → ↑ O₂ consumption → ↑ deoxyHbS → polymerisation. HbF is PROTECTIVE (gamma chains do NOT polymerise with HbS). HYDROXYUREA increases HbF → reduces sickling crises.\\\n\\\n📹 [VIDEO LINK]"},{"id":1043,"s":"Haemoglobin and Oxygen Transport","t":"Sickle Cell Disease","u":"Sickle Cell Crisis Precipitated by Dehydration Travellers Diarrhoea","q":"A 25-year-old woman with sickle cell disease presents with vaso-occlusive pain crisis during an episode of traveller's diarrhoea. Which of the following stress is likely to have precipitated this crisis?","o":["Dehydration","Oxidative","Osmotic","Redox","Reductive"],"a":0,"p":"TRAVELLER'S DIARRHOEA → DEHYDRATION → ↑ HbS concentration in RBCs → ↑ HbS polymerisation → VASO-OCCLUSIVE CRISIS. DEHYDRATION is the MOST COMMON PRECIPITANT. Other precipitants: hypoxia (altitude, infection), acidosis, cold (vasoconstriction → stasis → hypoxia → sickling), fever, stress. Treatment: IV hydration, O₂, analgesia (opioids), exchange transfusion for severe crises.\\\n\\\n📹 [VIDEO LINK]"},{"id":1044,"s":"Haemoglobin and Oxygen Transport","t":"Sickle Cell Disease","u":"Sickle Cell Haemolytic Jaundice Bilirubin Production Greater 300mg Day","q":"A 12-year-old boy presents with jaundice and recurrent acute bone pains after a school trip to northern areas of Pakistan. Blood picture shows normocytic anaemia with sickle-shaped RBCs. Bilirubin production in this boy is likely to be which of the following?","o":["Equal to normal 300mg per day","Greater than 300mg per day","Greater than 3000mg per day","Less than 250mg per day","Less than 300mg per day"],"a":1,"p":"SICKLE CELL DISEASE → HAEMOLYTIC ANAEMIA → ↑ RBC destruction → bilirubin production GREATER THAN 300mg/day (normal = ~300mg/day). UNCONJUGATED HYPERBILIRUBINAEMIA → jaundice. Liver has capacity to excrete >3000mg/day — usually handles the excess. However in SEVERE haemolytic crises, bilirubin production may temporarily exceed liver capacity → elevated unconjugated bilirubin. Elevated URINARY UROBILINOGEN.\\\n\\\n📹 [VIDEO LINK]"},{"id":1045,"s":"Haemoglobin and Oxygen Transport","t":"Thalassaemia","u":"HbA2 Elevated Diagnoses Beta Thalassaemia Trait","q":"A bride-to-be with mild anaemia since childhood has hypochromic microcytic cells with low MCV and MCH. Which test would diagnose thalassaemia trait?","o":["HbA","HbA1c","HbA2","HbF","HbH"],"a":2,"p":"BETA-THALASSAEMIA TRAIT diagnosis: ELEVATED HbA2 (>3.5%). Normal = 1.5-3.5%. Mechanism: ↓ beta chains → alpha chains pair with DELTA chains → ↑ HbA2. DIAGNOSIS: HPLC or CAPILLARY ELECTROPHORESIS. Iron deficiency must be EXCLUDED first (can lower HbA2 → falsely normal → missed diagnosis). Give iron first, then repeat Hb analysis.\\\n\\\n📹 [VIDEO LINK]"},{"id":1046,"s":"Haemoglobin and Oxygen Transport","t":"Thalassaemia","u":"Capillary Electrophoresis Best for HbA2 Quantification","q":"A woman with hypochromic microcytic cells and low MCV is being screened for thalassaemia trait. Which of the following techniques would be preferred for HbA2 estimation?","o":["Capillary electrophoresis","Gel electrophoresis","Immunoelectrophoresis","Protein electrophoresis","Zone electrophoresis"],"a":0,"p":"CAPILLARY ELECTROPHORESIS = preferred for HbA2 QUANTIFICATION in thalassaemia screening. High resolution, automated, accurate. HPLC (ion-exchange) = also gold standard. Zone electrophoresis and gel electrophoresis = qualitative/semi-quantitative (less precise for HbA2). Immunoelectrophoresis = for paraprotein detection, not Hb variants.\\\n\\\n📹 [VIDEO LINK]"},{"id":1047,"s":"Haemoglobin and Oxygen Transport","t":"Thalassaemia","u":"Iron Supplement Avoid in Thalassaemia Trait Iron Overload","q":"A woman is diagnosed with beta-thalassaemia trait. Which of the following supplements should she avoid?","o":["Calcium","Folic acid","Iron","Sodium","Zinc"],"a":2,"p":"IRON should be AVOIDED in thalassaemia trait. Thalassaemia → ↑ intestinal iron absorption (stimulated erythropoiesis) → iron accumulates → haemosiderosis → organ damage (liver, heart, pancreas, gonads). Give FOLIC ACID instead. NEVER give iron without confirmed iron deficiency (serum ferritin low). Thalassaemia trait ≠ iron deficiency — both cause microcytic hypochromic anaemia but differ in ferritin level.\\\n\\\n📹 [VIDEO LINK]"},{"id":1048,"s":"Haemoglobin and Oxygen Transport","t":"Thalassaemia","u":"Folic Acid Supplement Recommended Thalassaemia Trait","q":"Which of the following supplements should be recommended for a woman diagnosed with beta-thalassaemia trait?","o":["Calcium","Folic acid","Iron","Sodium","Zinc"],"a":1,"p":"FOLIC ACID recommended in thalassaemia trait. ↑ RBC turnover (haemolysis, ineffective erythropoiesis) → ↑ demand for folate (required for DNA synthesis in rapidly dividing erythroid precursors) → FOLATE DEFICIENCY → megaloblastic change ON TOP of thalassaemia → worsening anaemia. Compare: IRON should be AVOIDED (unless confirmed iron deficiency on ferritin level).\\\n\\\n📹 [VIDEO LINK]"},{"id":1049,"s":"Haemoglobin and Oxygen Transport","t":"Thalassaemia","u":"HbF Raised in Thalassaemia Major Infant Compensatory","q":"A 6-month-old baby presents with failure to thrive, breathlessness, and irritability. Blood picture shows severe microcytic hypochromic anaemia with anisocytosis. Which of the following Hb types is likely to be raised in this infant with thalassaemia major?","o":["HbA","HbA1c","HbA2","HbF","HbS"],"a":3,"p":"THALASSAEMIA MAJOR (homozygous beta-thalassaemia): NO beta chains → NO HbA → compensatory ↑ HbF (alpha₂gamma₂) — gamma→beta switch reversed. HbF may constitute 90-95% of total Hb. However HbF has high O₂ affinity (left-shifted ODC) → O₂ release to tissues impaired → tissue hypoxia despite elevated HbF. Diagnosis confirmed by: elevated HbF on HPLC, absent or very low HbA, elevated HbA2.\\\n\\\n📹 [VIDEO LINK]"},{"id":1050,"s":"Haemoglobin and Oxygen Transport","t":"Thalassaemia","u":"HbA2 Raised Thalassaemia Trait Diagnostic Hallmark","q":"A bride-to-be diagnosed with thalassaemia trait has which of the following type of Hb raised?","o":["HbA","HbA2","HbC","HbF","HbS"],"a":1,"p":"BETA-THALASSAEMIA TRAIT: ELEVATED HbA2 (>3.5%) = DIAGNOSTIC HALLMARK. Mechanism: ↓ beta chain production → remaining alpha chains pair with delta chains → ↑ HbA2 (alpha₂delta₂). HbA reduced (less beta chains) but not absent. HbA2 elevation used for CARRIER SCREENING before marriage.\\\n\\\n📹 [VIDEO LINK]"},{"id":1051,"s":"Haemoglobin and Oxygen Transport","t":"Thalassaemia","u":"CVS Heterozygous Mutation Baby is Carrier Continue Pregnancy","q":"A pregnant woman with thalassaemia trait undergoes chorionic villous sampling. The result shows a heterozygous mutation. Which of the following best describes the baby in utero?","o":["Carrier of thalassaemia trait","Has thalassaemia intermedia","Has thalassaemia major","Normal with no mutations","Will require immediate transfusion at birth"],"a":0,"p":"HETEROZYGOUS MUTATION on CVS = THALASSAEMIA TRAIT (CARRIER). ONE mutated + ONE normal beta-globin allele → carrier, NOT thalassaemia major (which requires TWO mutated alleles). ADVICE: CONTINUE PREGNANCY — baby is carrier only. Genetic counselling: carrier × carrier = 25% risk major, 50% trait, 25% normal in each pregnancy.\\\n\\\n📹 [VIDEO LINK]"},{"id":1052,"s":"Haemoglobin and Oxygen Transport","t":"Methaemoglobinaemia","u":"Methaemoglobinaemia Fe3 Brown Blood Cyanosis Nitrates","q":"A 67-year-old man on isosorbide dinitrate and nitroglycerin presents with blue-grey discolouration of face and extremities. Blood obtained for analysis is brown. Which of the following is the most likely diagnosis?","o":["Beta-thalassaemia","Carboxyhaemoglobinaemia","Haemoglobin SC disease","Methemoglobinaemia","Sickle cell anaemia"],"a":3,"p":"METHAEMOGLOBINAEMIA: NITRATES (isosorbide dinitrate, nitroglycerin) → oxidise Fe²⁺ → Fe³⁺ → METHAEMOGLOBIN → CANNOT BIND O₂ → CHOCOLATE-BROWN BLOOD. CYANOSIS (blue-grey discolouration) despite normal pO₂. Pulse oximetry falsely reads ~85%. TREATMENT: METHYLENE BLUE (via NADPH-metHb reductase → reduces Fe³⁺ → Fe²⁺) or ascorbic acid.\\\n\\\n📹 [VIDEO LINK]"},{"id":1053,"s":"Heme Metabolism","t":"Porphyrias","u":"AIP King George III Delirium ALA Accumulation Neurotoxic","q":"King George III suffered from excruciating stomach pain and episodes of delirium. Based on a diagnosis of acute intermittent porphyria, accumulation of which of the following best explains the cause of his delirium?","o":["Delta-aminolevulinate","Heme","Hydroxymethylbilane","Porphobilinogen","Porphyrin"],"a":0,"p":"AIP (PBG deaminase/HMBS deficiency) → BOTH ALA (DELTA-AMINOLEVULINATE) AND PBG accumulate. DELIRIUM and NEUROPSYCHIATRIC SYMPTOMS caused specifically by DELTA-AMINOLEVULINATE (ALA): ALA is structurally similar to GABA → inhibits GABA receptors in CNS → disinhibition → DELIRIUM, confusion, psychosis, seizures, peripheral neuropathy. PBG accumulates and is excreted in urine (Watson-Schwartz test positive) but is NOT directly neurotoxic. ARSENIC (used by King George's physicians) → induces ALA SYNTHASE → MORE ALA and PBG produced → worsened symptoms. NO photosensitivity (AIP = NEUROLOGICAL porphyria).\\\n\\\n📹 [VIDEO LINK]"},{"id":1054,"s":"Heme Metabolism","t":"Porphyrias","u":"PCT Vampire Poland Glowing Teeth Porphyrin Accumulation Fluorescence","q":"Skeletal remains of a woman in Poland, believed to be a vampire, were found buried with a sickle across her throat. Her teeth were hypothesised to appear to glow, contributing to the belief that she drank blood. Accumulation of which of the following best explains the underlying biochemical cause?","o":["Delta-aminolevulinate","Heme","Hydroxymethylbilane","Porphobilinogen","Porphyrin"],"a":4,"p":"PORPHYRIA (PCT/CEP): PORPHYRINS accumulate in skin and teeth. Porphyrins FLUORESCE under ultraviolet/wood light → TEETH APPEAR TO GLOW (erythrodontia — pink-red fluorescence visible as glowing in wooden lamp light in medieval times). Additional vampire myth features: PHOTOSENSITIVITY (aversion to sunlight), DARK URINE (porphyrins → suspected to drink blood), HIRSUTISM + SCARRING (werewolf-like appearance from chronic blistering), GUM RECESSION → teeth resemble fangs, AVERSION TO GARLIC (sulfur in garlic precipitates porphyria attacks).\\\n\\\n📹 [VIDEO LINK]"},{"id":1055,"s":"Heme Metabolism","t":"Porphyrias","u":"AIP Hydroxymethylbilane Synthase Deficiency Diagnosis ALA PBG Urine","q":"A 24-year-old patient presents with acute abdominal pain and confusion. Urine analysis reveals elevated delta-aminolevulinic acid and porphobilinogen. Enzyme assay shows hydroxymethylbilane synthase deficiency. Which of the following is the likely diagnosis?","o":["Acute intermittent porphyria","Congenital erythropoietic porphyria","Erythropoietic protoporphyria","Porphyria cutanea tarda","Variegate porphyria"],"a":0,"p":"ACUTE INTERMITTENT PORPHYRIA (AIP): PBG DEAMINASE (hydroxymethylbilane synthase, HMBS) DEFICIENCY → ALA and PBG accumulate in urine (Watson-Schwartz test POSITIVE for PBG). Features: NEUROLOGICAL (abdominal pain, peripheral neuropathy, psychiatric symptoms, seizures), NO PHOTOSENSITIVITY. Triggers: barbiturates, OCP, fasting, alcohol. Treatment: HAEMATIN (suppresses ALA synthase), glucose loading, avoid triggers.\\\n\\\n📹 [VIDEO LINK]"},{"id":1056,"s":"Heme Metabolism","t":"Jaundice","u":"Pre-Hepatic Haemolytic Jaundice Lab Pattern UCB Dominant","q":"A 35-year-old patient presents with jaundice and anaemia. Total bilirubin is 10mg/dL, unconjugated bilirubin is 9mg/dL, conjugated bilirubin is 1mg/dL, Hb is 9g/dL, urobilinogen is raised, and stools are dark brown. Which of the following disorders is he likely suffering from?","o":["Hepatic jaundice","Obstructive jaundice","Post-hepatic jaundice","Pre-hepatic jaundice","Viral hepatitis"],"a":3,"p":"LAB PATTERN → PRE-HEPATIC (HAEMOLYTIC) JAUNDICE: UNCONJUGATED bilirubin 9mg/dL (dominant fraction), LOW Hb (haemolytic anaemia), RAISED UROBILINOGEN (↑ bilirubin → ↑ intestinal urobilinogen → ↑ urobilinogen in urine), DARK BROWN STOOLS (↑ stercobilin from ↑ urobilinogen), NO BILIRUBINURIA (unconjugated bilirubin NOT water-soluble → does not appear in urine). Causes: haemolytic anaemia (sickle cell, G6PD deficiency, thalassaemia, autoimmune haemolytic anaemia).\\\n\\\n📹 [VIDEO LINK]"},{"id":1057,"s":"Heme Metabolism","t":"Jaundice","u":"Hepatic Jaundice Bilirubinuria Tea Coloured Urine Conjugated","q":"A 55-year-old patient presents with yellowish discolouration of skin and sclera and tea-coloured urine. Urine chemistry shows bilirubinuria. Which of the following disorders is he likely suffering from?","o":["Crigler-Najjar syndrome","Gilbert syndrome","Haemolytic jaundice","Hepatic jaundice","Pre-hepatic jaundice"],"a":3,"p":"BILIRUBINURIA = CONJUGATED (water-soluble) bilirubin in urine → indicates HEPATIC or POST-HEPATIC JAUNDICE. In HEPATIC JAUNDICE (hepatitis, cirrhosis): ↓ conjugation capacity AND ↑ regurgitation of conjugated bilirubin back into blood → bilirubinuria. In PRE-HEPATIC jaundice: NO bilirubinuria (unconjugated only — not water-soluble). Gilbert and Crigler-Najjar = unconjugated only → no bilirubinuria.\\\n\\\n📹 [VIDEO LINK]"},{"id":1058,"s":"Heme Metabolism","t":"Jaundice","u":"Only Conjugated Bilirubin Present in Urine Water Soluble","q":"A patient with jaundice has a urine sample. Which of the following types of bilirubin can be present in a urine sample?","o":["Both as both are water insoluble","Both as both are water soluble","Conjugated bilirubin as it is water soluble","Neither as bilirubin is always albumin bound","Unconjugated bilirubin as it is water soluble"],"a":2,"p":"ONLY CONJUGATED (DIRECT) BILIRUBIN can appear in urine. Conjugated bilirubin = bilirubin diglucuronide = WATER-SOLUBLE → filtered by kidney → BILIRUBINURIA. UNCONJUGATED bilirubin = lipid-soluble + tightly bound to ALBUMIN → too large to be filtered → CANNOT appear in urine. BILIRUBINURIA = dark urine + bilirubin dipstick positive → indicates hepatic or post-hepatic jaundice. In haemolytic jaundice: UROBILINOGEN elevated in urine (NOT bilirubin).\\\n\\\n📹 [VIDEO LINK]"},{"id":1059,"s":"Heme Metabolism","t":"Jaundice","u":"Liver Capacity Conjugate Excrete Greater 3000mg Bilirubin Per Day","q":"A child with sickle cell disease develops severe haemolytic crisis. His doctor explains that the liver can handle bilirubin production up to which of the following amounts per day before jaundice becomes inevitable?","o":["1000mg per day","2000mg per day","3000mg per day","4000mg per day","5000mg per day"],"a":2,"p":"LIVER CONJUGATION CAPACITY: GREATER THAN 3000mg bilirubin/day. Normal production = ~300mg/day. 10× excess capacity allows liver to respond to increased haemolysis with corresponding ↑ conjugation and ↑ secretion of conjugated bilirubin. In SEVERE haemolysis (sickle cell crisis): bilirubin production may temporarily exceed even this large capacity → unconjugated bilirubin rises → jaundice.\\\n\\\n📹 [VIDEO LINK]"},{"id":1060,"s":"Heme Metabolism","t":"Jaundice","u":"Neonatal Physiological Jaundice Definition First Week of Life","q":"Jaundice occurring in young infants in the first week of life is known as which of the following?","o":["Hepatic jaundice","Neonatal jaundice","Obstructive jaundice","Post-hepatic jaundice","Pre-hepatic jaundice"],"a":1,"p":"NEONATAL JAUNDICE (physiological jaundice of the newborn): occurs in first week of life. Causes: (1) Immature UGT1A1 (reaches adult levels by 4 weeks). (2) ↑ RBC turnover (fetal Hb replacement → haemolysis). (3) Short RBC lifespan in neonates. (4) Relative dehydration (poor feeding). UNCONJUGATED hyperbilirubinaemia. Treatment: PHOTOTHERAPY → converts unconjugated bilirubin to water-soluble LUMIRUBIN → excreted without conjugation. Severe cases: exchange transfusion (kernicterus risk).\\\n\\\n📹 [VIDEO LINK]"},{"id":1061,"s":"Anemias","t":"Anaemia Classification","u":"Megaloblastic Anaemia Macrocytic Normochromic Folate Deficiency Pregnancy","q":"A 22-year-old pregnant woman with no antenatal care presents with a baby with lumbosacral swelling. Her Hb is 10g/dL, MCV is 110fL, peripheral smear shows megaloblasts and polysegmented neutrophils. Which of the following categories does her anaemia fall into?","o":["Macrocytic hypochromic","Macrocytic normochromic","Microcytic hypochromic","Normocytic hypochromic","Normocytic normochromic"],"a":1,"p":"MEGALOBLASTIC ANAEMIA (folate deficiency): MCV 110fL = MACROCYTIC. Cells are NORMOCHROMIC (normal Hb concentration per RBC — each large cell has normal Hb). NOT hypochromic. PERIPHERAL SMEAR: MEGALOBLASTS + POLYSEGMENTED NEUTROPHILS (5+ lobes = pathognomonic) + macro-ovalocytes. NEURAL TUBE DEFECT (lumbosacral swelling = myelomeningocele) = hallmark of folate deficiency in early pregnancy. FOLATE required for DNA synthesis → deficiency → impaired cell division → nuclear enlargement without cytoplasmic growth.\\\n\\\n📹 [VIDEO LINK]"},{"id":1062,"s":"Anemias","t":"Anaemia Classification","u":"Normocytic Normochromic Anaemia Acute Blood Loss MCV Normal","q":"A 42-year-old man presents with massive blood loss after a road traffic accident. His Hb is 10g/dL, RBC count is 4 million per mm³, MCV is 80fL. Which of the following categories does his anaemia fall into?","o":["Macrocytic hypochromic","Macrocytic normochromic","Microcytic hypochromic","Normocytic hypochromic","Normocytic normochromic"],"a":4,"p":"ACUTE BLOOD LOSS ANAEMIA: MCV 80fL = NORMAL (80-100fL = normocytic). All RBCs lost equally → remaining cells are normal size and colour → NORMOCYTIC NORMOCHROMIC. Compare: CHRONIC IRON DEFICIENCY (repeated small blood losses) → initially normocytic → then MICROCYTIC HYPOCHROMIC (insufficient iron for Hb synthesis). Acute loss: no time for iron depletion → cells produced are normal. Treatment: volume replacement → blood transfusion if Hb <7g/dL or haemodynamically unstable.\\\n\\\n📹 [VIDEO LINK]"},{"id":1063,"s":"Amino Acid Disorders","t":"Maple Syrup Urine Disease","u":"MSUD Alpha Ketoacid Dehydrogenase Ketoacidosis Burnt Sugar Smell","q":"A 3-day-old newborn develops vomiting, lethargy, irritability, and a burnt sugar smell in diapers. Investigations reveal severe ketoacidosis. Which enzyme is affected?","o":["Alpha-ketoacid dehydrogenase complex","Branched-chain aminotransferase","Homogentisate oxidase","Phenylalanine hydroxylase","Propionyl CoA carboxylase"],"a":0,"p":"MAPLE SYRUP URINE DISEASE (MSUD): BRANCHED-CHAIN KETOACID DEHYDROGENASE (BCKAD) COMPLEX = ALPHA-KETOACID DEHYDROGENASE COMPLEX DEFICIENCY → cannot decarboxylate alpha-keto acids from leucine, isoleucine, valine. KETO ACIDS ACCUMULATE → SEVERE KETOACIDOSIS. BURNT SUGAR SMELL = SOTOLON (from isoleucine metabolite). MENTAL RETARDATION from LEUCINE accumulation (neurotoxic). TREATMENT: dietary restriction of branched-chain amino acids + thiamine supplementation (BCKAD requires TPP). REQUIRES NEWBORN SCREENING.\\\n\\\n📹 [VIDEO LINK]"},{"id":1064,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"Monosaccharides Absorbed Most Rapidly Blood Sugar","q":"A 45-year-old diabetic patient wants to know which type of carbohydrate is most rapidly absorbed into the bloodstream, causing the quickest rise in blood sugar. Which of the following is the correct answer?","o":["Disaccharides","Monosaccharides","Oligosaccharides","Polysaccharides","Trisaccharides"],"a":1,"p":"MONOSACCHARIDES (glucose, fructose, galactose) = MOST RAPIDLY ABSORBED — require NO digestion → directly absorbed from intestinal lumen into enterocytes via specific transporters (SGLT1 for glucose/galactose, GLUT5 for fructose) → portal circulation. DISACCHARIDES require ONE enzymatic cleavage (lactase, sucrase, maltase). POLYSACCHARIDES require extensive digestion (amylase → dextrins → maltose → glucose). Monosaccharides cause the most rapid rise in blood glucose → highest glycaemic index.\\\n\\\n📹 [VIDEO LINK]"},{"id":1065,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"Sucrose is a Disaccharide","q":"Sucrose (table sugar) is classified as which type of carbohydrate?","o":["Disaccharide","Monosaccharide","Oligosaccharide","Polysaccharide","Trisaccharide"],"a":0,"p":"SUCROSE = DISACCHARIDE = GLUCOSE + FRUCTOSE (alpha-1,beta-2 glycosidic bond). Cleaved by SUCRASE (intestinal brush border enzyme). Sucrose intolerance = sucrase-isomaltase deficiency. Other disaccharides: LACTOSE (glucose + galactose, beta-1,4 bond), MALTOSE (glucose + glucose, alpha-1,4 bond), TREHALOSE (glucose + glucose, alpha-1,alpha-1 bond — found in mushrooms). Sucrose is NON-REDUCING (no free anomeric carbon) while maltose and lactose are REDUCING sugars.\\\n\\\n📹 [VIDEO LINK]"},{"id":1066,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"Starch Primary Polysaccharide in Pasta","q":"A group of athletes preparing for a marathon consume pasta the night before for energy reserves. Which of the following primary polysaccharides is present in pasta?","o":["Cellulose","Chitin","Glycogen","Pectin","Starch"],"a":4,"p":"STARCH = primary storage polysaccharide in PLANTS (pasta, bread, rice, potatoes). Two forms: AMYLOSE (unbranched, alpha-1,4 linkages, ~20%) and AMYLOPECTIN (branched, alpha-1,4 + alpha-1,6 at branch points, ~80%). Digested by SALIVARY and PANCREATIC AMYLASE → limit dextrins + maltose → glucose. GLYCOGEN = animal equivalent (more branched than amylopectin). CELLULOSE = structural polysaccharide (beta-1,4 linkages, indigestible by humans). CHITIN = structural polysaccharide in fungi and insect exoskeletons.\\\n\\\n📹 [VIDEO LINK]"},{"id":1068,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"Galactose Glucose Epimers Differ at C4","q":"Galactose and glucose are examples of which of the following?","o":["Anomers","Diastereomers","Enantiomers","Epimers","Isomers"],"a":3,"p":"GALACTOSE and GLUCOSE = EPIMERS — differ in configuration at ONLY ONE CARBON (C4). EPIMERS = monosaccharides that differ at a single specific carbon only. Compare: ENANTIOMERS = mirror images (differ at ALL chiral centres — e.g. D-glucose and L-glucose). ANOMERS = differ at C1 (anomeric carbon) only — e.g. alpha-glucose and beta-glucose. DIASTEREOMERS = stereoisomers that are NOT mirror images (includes epimers and anomers). The C4 difference between glucose and galactose = basis of galactosaemia.\\\n\\\n📹 [VIDEO LINK]"},{"id":1069,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"Glycosidic Bond in Sugar and Starch","q":"A recipe involves mixing sugar with starch. Which of the following bonds are present in both of these ingredients?","o":["Glycosidic bond","Hydrogen bond","Ionic bond","Peptide bond","Phosphodiester bond"],"a":0,"p":"GLYCOSIDIC BONDS = covalent bonds linking monosaccharide units in disaccharides (sucrose, lactose, maltose) and polysaccharides (starch, glycogen, cellulose). Formed by CONDENSATION REACTION (loss of water) between the anomeric carbon of one sugar and a hydroxyl of another. Types: alpha-1,4 (starch, glycogen, maltose), beta-1,4 (cellulose, lactose), alpha-1,2 (sucrose). Glycosidic bonds are ACID-HYDROLYSED and enzymatically cleaved by GLYCOSIDASES. IONIC BONDS = between charged groups (not present in sugar/starch). PEPTIDE BONDS = in proteins only.\\\n\\\n📹 [VIDEO LINK]"},{"id":1070,"s":"Carbohydrate Chemistry and Metabolism","t":"Carbohydrate Classification","u":"Pancreatic Amylase Breaks Starch into Maltose and Limit Dextrins","q":"Which of the following enzyme is responsible for breaking down starch into limit dextrins?","o":["Glucoamylase","Lactase","Maltase","Pancreatic amylase","Trehalase"],"a":3,"p":"PANCREATIC AMYLASE (secreted by exocrine pancreas into duodenum) = PRIMARY enzyme that breaks starch into MALTOSE + LIMIT DEXTRINS. LIMIT DEXTRINS = branched oligosaccharides formed where amylase cannot cleave alpha-1,6 branch points. Subsequent digestion: GLUCOAMYLASE (brush border) cleaves terminal alpha-1,4 bonds of limit dextrins. MALTASE (brush border) cleaves maltose → 2 glucose. ISOMALTASE (alpha-dextrinase) cleaves alpha-1,6 bonds of limit dextrins. TREHALASE = cleaves trehalose (mushroom sugar) → 2 glucose. LACTASE = cleaves lactose → glucose + galactose.\\\n\\\n📹 [VIDEO LINK]"},{"id":1071,"s":"Carbohydrate Chemistry and Metabolism","t":"Fructose Metabolism","u":"Essential Fructosuria Fructokinase Deficiency Benign Reducing Sugar Urine","q":"A 20-year-old male has a positive reducing sugar but negative glucose oxidase test in his urine during a routine visa medical checkup. He has no cataracts, no jaundice, no hepatomegaly, and is otherwise completely well. Which enzyme is most likely deficient in him?","o":["Aldolase B","Fructokinase","Galactokinase","Glucokinase","Hexokinase"],"a":1,"p":"ESSENTIAL FRUCTOSURIA = FRUCTOKINASE DEFICIENCY. DISTINGUISH from other causes of non-glucose reducing sugar in urine: GALACTOKINASE DEFICIENCY also causes reducing sugar (galactose in urine) but presents with CATARACTS (galactitol accumulates in lens). GALT DEFICIENCY (classic galactosaemia) = jaundice, hepatomegaly, cataracts. This patient: NO cataracts + NO jaundice + NO hepatomegaly + ASYMPTOMATIC = FRUCTOKINASE deficiency (essential fructosuria). Fructokinase deficiency → fructose not phosphorylated → FRUCTOSE excreted in urine. BENIGN — no treatment needed. POSITIVE CLINITEST + NEGATIVE GLUCOSE OXIDASE = non-glucose reducing sugar.\\\n\\\n📹 [VIDEO LINK]"},{"id":1072,"s":"Carbohydrate Chemistry and Metabolism","t":"Fructose Metabolism","u":"HFI Aldolase B Deficiency Fruit Juice Hepatomegaly Vomiting","q":"A 6-month-old boy develops vomiting, night sweats, and tremors after fruit juices are introduced during weaning. Physical examination reveals hepatomegaly. Which enzyme is deficient in this child?","o":["Aldolase A","Aldolase B","Aldolase C","Fetal aldolase","Fructokinase"],"a":1,"p":"HEREDITARY FRUCTOSE INTOLERANCE (HFI) = ALDOLASE B DEFICIENCY. Aldolase B: FRUCTOSE-1-PHOSPHATE → DHAP + GLYCERALDEHYDE (in LIVER). Deficiency → fructose-1-P ACCUMULATES → (1) Inhibits GLYCOGEN PHOSPHORYLASE → no glycogenolysis → hypoglycaemia. (2) Inhibits ALDOLASE A → no gluconeogenesis → worsens hypoglycaemia. (3) Sequesters PHOSPHATE → ↓ ATP → cell damage. Result: HYPOGLYCAEMIA + HEPATOMEGALY (fructose-1-P toxic to hepatocytes) + VOMITING + AVERSION TO SWEET FOODS. Compare: ALDOLASE A = muscle (glycolysis), ALDOLASE C = brain. Treatment: eliminate fructose and sucrose from diet.\\\n\\\n📹 [VIDEO LINK]"},{"id":1073,"s":"Carbohydrate Chemistry and Metabolism","t":"Fructose Metabolism","u":"HFI Fructose 1-Phosphate Accumulates Fruit Diet","q":"A 1-year-old girl develops crankiness, sweating, dizziness, and lethargy after eating fruit. The problems are most likely due to accumulation of which of the following?","o":["Fructose-1-phosphate","Fructose-6-phosphate","Galactose-1-phosphate","Glucose-1-phosphate","Glucose-6-phosphate"],"a":0,"p":"HEREDITARY FRUCTOSE INTOLERANCE (HFI): ALDOLASE B DEFICIENCY → FRUCTOSE-1-PHOSPHATE ACCUMULATES. Mechanism of hypoglycaemia: fructose-1-P → (1) INHIBITS GLYCOGEN PHOSPHORYLASE → no glycogenolysis. (2) INHIBITS ALDOLASE A → no gluconeogenesis. (3) SEQUESTERS Pi → ↓ ATP synthesis. SYMPTOMS: hypoglycaemia (sweating, dizziness, lethargy) + crankiness (neuroglycopenia) after fructose ingestion. FRUCTOSE-6-PHOSPHATE is an intermediate of glycolysis — not formed in HFI. GALACTOSE-1-PHOSPHATE accumulates in GALACTOSAEMIA (different condition — onset with milk, not fruit).\\\n\\\n📹 [VIDEO LINK]"},{"id":1074,"s":"Carbohydrate Chemistry and Metabolism","t":"Fructose Metabolism","u":"HFI Hypoglycaemia Biochemical Cause","q":"A 7-month-old child develops vomiting, night sweats, and tremors after fruit juice is introduced during weaning. Which of the following biochemical abnormality explains his symptoms?","o":["Hyperglycaemia","Hyperkalaemia","Hypernatraemia","Hypoglycaemia","Hyponatraemia"],"a":3,"p":"HFI → FRUCTOSE-1-PHOSPHATE ACCUMULATION → HYPOGLYCAEMIA. Mechanism: (1) Fructose-1-P SEQUESTERS INORGANIC PHOSPHATE (Pi) → rapid Pi depletion → ↓ ATP synthesis → energy failure. (2) Fructose-1-P INHIBITS ALDOLASE A → GLUCONEOGENESIS BLOCKED (aldolase A cleaves fructose-1,6-bisphosphate in gluconeogenesis → without it, no new glucose can be made). Result: BLOOD GLUCOSE FALLS → AUTONOMIC SYMPTOMS (sweating, tremors, tachycardia = night sweats) + NEUROGLYCOPENIC symptoms (confusion, coma if severe). NIGHT SWEATS = nocturnal hypoglycaemia (glucose nadir during sleep after evening fruit intake). Treatment: IV DEXTROSE if acute, eliminate dietary fructose.\n\n📹 [VIDEO LINK]"},{"id":1075,"s":"Carbohydrate Chemistry and Metabolism","t":"Fructose Metabolism","u":"HFI Hyperuricaemia ATP Depletion Purine Degradation","q":"A 6-month-old boy develops vomiting and hepatomegaly after fruit juice is introduced. Which biochemical abnormality will be present in addition to hypoglycaemia?","o":["Hyperammonaemia","Hyperglycaemia","Hyperkalaemia","Hypernatraemia","Hyperuricaemia"],"a":4,"p":"HFI HYPERURICAEMIA MECHANISM: FRUCTOSE-1-PHOSPHATE SEQUESTERS INORGANIC PHOSPHATE (Pi). Mechanism step by step: ↓ Pi in cell → RAPID ATP DEGRADATION (ATP → ADP → AMP). AMP is then deaminated: AMP → (AMP deaminase) → IMP → inosine → hypoxanthine → (xanthine oxidase) → xanthine → (xanthine oxidase) → URIC ACID. Additionally: fructose-1-P directly STIMULATES AMP DEAMINASE. In HFI: this occurs RAPIDLY within minutes of fructose ingestion. HYPERURICAEMIA → risk of acute GOUT attacks. Also: HYPOGLYCAEMIA (fructose-1-P inhibits glycogen phosphorylase and aldolase A), elevated transaminases, coagulopathy (liver damage from fructose-1-P toxicity).\\\n\\\n📹 [VIDEO LINK]"},{"id":1076,"s":"Carbohydrate Chemistry and Metabolism","t":"Fructose Metabolism","u":"HFI Fructose in Urine Reducing Sugar Weaning","q":"A 6-month-old boy develops vomiting and hepatomegaly after weaning and introduction of fruit juices. Tests on his urine are positive for reducing sugar but negative for glucose. Which of the following sugar is most likely present in his urine?","o":["Fructose","Galactose","Mannose","Ribose","Sucrose"],"a":0,"p":"HFI → FRUCTOSE-1-PHOSPHATE accumulation → excess FRUCTOSE in blood (backed up before aldolase B) → exceeds renal threshold → FRUCTOSE excreted in urine. FRUCTOSE = REDUCING SUGAR (free ketone group at C2 in open chain form) → POSITIVE CLINITEST (reduces Cu²⁺ → Cu₂O) → NEGATIVE GLUCOSE OXIDASE (enzyme-specific for glucose only). DISTINGUISH: GALACTOSE in urine = galactosaemia (onset with MILK, not fruit juice). FRUCTOSE in urine = HFI or essential fructosuria (onset with FRUIT JUICE or SUCROSE). The clinical history — weaning + fruit juice introduction — points to FRUCTOSE.\\\n\\\n📹 [VIDEO LINK]"},{"id":1077,"s":"Carbohydrate Chemistry and Metabolism","t":"Fructose Metabolism","u":"HFI Fructose Defective Metabolism Aversion to Sweet Foods","q":"A 6-month-old boy develops nausea, vomiting, sweating, and dizziness shortly after fruit juice is introduced during weaning. His mother notes he cries and refuses sweet foods. Urine shows a strong reducing substance that does not react to glucose oxidase. Defective metabolism of which of the following monosaccharide is most likely causing these symptoms?","o":["Fructose","Galactose","Glucose","Sorbitol","Sucrose"],"a":0,"p":"FRUCTOSE METABOLISM DEFECT (HFI — Aldolase B deficiency) → fructose-1-phosphate accumulates → HYPOGLYCAEMIA → vomiting, sweating, dizziness. AVERSION TO SWEET FOODS = a unique protective feature of HFI — patients UNCONSCIOUSLY AVOID fructose-containing foods (patients learn that eating sweet things causes pain and hypoglycaemia). URINE: reducing sugar positive + glucose oxidase negative = FRUCTOSE in urine. Compare: GALACTOSAEMIA = galactose in urine (positive reducing sugar, negative glucose oxidase) but onset with MILK (not fruit), and presents with jaundice, cataracts, hepatomegaly.\\\n\\\n📹 [VIDEO LINK]"},{"id":1078,"s":"Carbohydrate Chemistry and Metabolism","t":"Fructose Metabolism","u":"Hereditary Fructose Intolerance Disorder Name Aldolase B Deficiency","q":"A 6-month-old boy develops vomiting, sweating, and irritability shortly after weaning and introduction of fruit juices and sugary foods. His parents note he cries and refuses sweet feeds. Urine reducing sugar is strongly positive but glucose oxidase test is negative. Which of the following is the name of this disorder?","o":["Classic galactosaemia","Essential fructosuria","Hereditary fructose intolerance","Lactose intolerance","Non-classic galactosaemia"],"a":2,"p":"HEREDITARY FRUCTOSE INTOLERANCE (HFI) = ALDOLASE B DEFICIENCY. Autosomal recessive. Onset when fructose-containing foods introduced (fruit, sucrose, honey). AVERSION TO SWEET FOODS = protective behavioural adaptation. Compare: ESSENTIAL FRUCTOSURIA = FRUCTOKINASE deficiency (benign, ASYMPTOMATIC — no vomiting, no hypoglycaemia, no aversion to sweets). Both present with fructose in urine (reducing sugar positive, glucose oxidase negative). DISTINGUISH by SYMPTOMS: HFI = severe (hypoglycaemia, hepatomegaly, liver failure if untreated). Essential fructosuria = no symptoms. Dietary treatment: eliminate fructose AND sucrose (sucrose = glucose + fructose) AND SORBITOL (converted to fructose).\\\n\\\n📹 [VIDEO LINK]"},{"id":1079,"s":"Carbohydrate Chemistry and Metabolism","t":"Fructose Metabolism","u":"Fructose Bypasses PFK-1 Obesity Soft Drinks Liver","q":"A teenage obese girl prefers soft drinks over water. Which of the following glycolysis enzyme is being bypassed by fructose in her liver, contributing towards obesity?","o":["Glucokinase","Hexokinase","Phosphofructokinase-1","Phosphofructokinase-2","Pyruvate kinase"],"a":2,"p":"FRUCTOSE bypasses PHOSPHOFRUCTOKINASE-1 (PFK-1) in the LIVER. Normal glucose pathway: glucose → (hexokinase/glucokinase) → G-6-P → (PFK-1, RATE-LIMITING STEP) → F-1,6-BP → glycolysis. Fructose pathway: fructose → (fructokinase) → fructose-1-P → (aldolase B) → DHAP + glyceraldehyde → enter GLYCOLYSIS BELOW PFK-1. PFK-1 = the main REGULATORY GATE of glycolysis — normally controls glucose flux. Fructose BYPASSES this gate → unregulated entry → excess acetyl-CoA → de novo LIPOGENESIS → HYPERTRIGLYCERIDAEMIA + OBESITY + NAFLD. High-fructose corn syrup in soft drinks = major contributor to metabolic syndrome.\\\n\\\n📹 [VIDEO LINK]"},{"id":1080,"s":"Carbohydrate Chemistry and Metabolism","t":"Fructose Metabolism","u":"Fructose in Muscle Enters as Fructose-6-Phosphate via Hexokinase","q":"In the liver, fructose metabolism produces triose phosphates entering glycolysis. In muscle, fructose metabolism produces which of the following glycolysis intermediate?","o":["Dihydroxyacetone phosphate","Fructose-1,6-bisphosphate","Fructose-1-phosphate","Fructose-6-phosphate","Glyceraldehyde-3-phosphate"],"a":3,"p":"TISSUE DIFFERENCES IN FRUCTOSE METABOLISM: LIVER (has FRUCTOKINASE + ALDOLASE B): fructose → fructose-1-P → DHAP + glyceraldehyde → enter glycolysis BELOW PFK-1. MUSCLE and ADIPOSE (have HEXOKINASE, no fructokinase): hexokinase (non-specific) converts FRUCTOSE → FRUCTOSE-6-PHOSPHATE → enters glycolysis AT PFK-1 step (does NOT bypass PFK-1). KEY DIFFERENCE: LIVER = bypasses PFK-1 (via F-1-P route, unregulated). MUSCLE = does NOT bypass PFK-1 (enters at F-6-P, regulated by PFK-1). This is why the liver bears the metabolic brunt of dietary fructose overload.\\\n\\\n📹 [VIDEO LINK]"},{"id":1081,"s":"Carbohydrate Chemistry and Metabolism","t":"Galactose Metabolism","u":"Classic Galactosaemia GALT Deficiency Galactose-1-Phosphate Accumulates CVS","q":"A 28-week fetus is found to have classic galactosaemia on chorionic villous sampling. Which of the following compound accumulates?","o":["Galactose-1-phosphate","Glucose-1-phosphate","Glucose-6-phosphate","UDP galactose","UDP glucose"],"a":0,"p":"CLASSIC GALACTOSAEMIA = GALT DEFICIENCY. GALT normally: galactose-1-P + UDP-glucose → glucose-1-P + UDP-galactose. Without GALT: GALACTOSE-1-PHOSPHATE ACCUMULATES → toxic to liver, brain, kidney, RBCs. GALACTITOL also accumulates (via aldose reductase) → CATARACTS. Neonatal features: JAUNDICE, HEPATOMEGALY, VOMITING (with breast milk/lactose), CATARACTS, E. coli SEPSIS (galactose-1-P impairs neutrophil function). Treatment: LACTOSE-FREE DIET immediately. Diagnosis confirmed: enzyme assay in RBCs (GALT activity absent) + elevated galactose-1-P in RBCs.\\\n\\\n📹 [VIDEO LINK]"},{"id":1082,"s":"Carbohydrate Chemistry and Metabolism","t":"Galactose Metabolism","u":"Galactosaemia Delayed Milestones Galactose-1-Phosphate Neurotoxic Accumulates","q":"A 6-month-old malnourished infant with persistent diarrhoea, failure to thrive, and delayed milestones has galactosaemia suspected. Accumulation of which of the following best explains the biochemical basis for delayed milestones?","o":["Galactitol","Galactose-1-phosphate","UDP galactose","UDP glucose","UDP hexoses"],"a":1,"p":"GALACTOSAEMIA → GALACTOSE-1-PHOSPHATE ACCUMULATES → NEUROTOXIC. Galactose-1-P inhibits multiple enzymes in brain: PHOSPHOGLUCOMUTASE and PHOSPHOGLUCOSE ISOMERASE → impairs glucose metabolism in neurons → neuronal dysfunction → INTELLECTUAL DISABILITY and DELAYED MILESTONES. Also: galactose-1-P directly inhibits Na⁺/K⁺-ATPase in brain. GALACTITOL causes CATARACTS (osmotic swelling of lens) — NOT the primary cause of delayed milestones. Even with early treatment: long-term neurological and OVARIAN complications persist in many patients (constitutional hypothesis — endogenous galactose production continues).\\\n\\\n📹 [VIDEO LINK]"},{"id":1083,"s":"Carbohydrate Chemistry and Metabolism","t":"Galactose Metabolism","u":"Classic Galactosaemia GALT Deficiency Infant Vomiting Jaundice Cataracts","q":"A 2-week-old breast-fed infant presents with vomiting and jaundice since birth. Examination reveals hepatomegaly and cataracts. Urine is positive for reducing sugar but negative for glucose. Which enzyme is deficient?","o":["Alanine aminotransferase","Bilirubin glucuronyltransferase","Carnitine palmitoyltransferase","Catechol-O-methyltransferase","Galactose-1-phosphate uridylyltransferase"],"a":4,"p":"CLASSIC GALACTOSAEMIA = GALACTOSE-1-PHOSPHATE URIDYLYLTRANSFERASE (GALT) DEFICIENCY. Breast milk contains LACTOSE → lactase → glucose + GALACTOSE → galactose-1-P accumulates (GALT absent) → HEPATOTOXICITY (jaundice, hepatomegaly, elevated transaminases) + CATARACTS (galactitol via aldose reductase) + NEUROTOXICITY. GALACTOSE in urine = REDUCING SUGAR (positive Clinitest) + NEGATIVE GLUCOSE OXIDASE. E. coli SEPSIS = classic complication. Treatment: LACTOSE-FREE formula immediately after birth. Newborn screening: elevated galactose or low GALT activity.\\\n\\\n📹 [VIDEO LINK]"},{"id":1084,"s":"Carbohydrate Chemistry and Metabolism","t":"Galactose Metabolism","u":"Galactose in Urine Classic Galactosaemia Breast Fed Infant","q":"A 2-week-old breast-fed infant presents with vomiting and jaundice since birth. Examination reveals hepatomegaly and cataracts. Urine is positive for reducing sugar but negative for glucose. Which sugar is most likely present in her urine?","o":["Fructose","Galactose","Mannose","Ribose","Sucrose"],"a":1,"p":"GALACTOSAEMIA → GALACTOSE in urine. Breast milk → lactose → (lactase) → GLUCOSE + GALACTOSE. GALT deficiency → galactose-1-P accumulates → EXCESS GALACTOSE in blood → exceeds renal threshold → GALACTOSE excreted in urine. GALACTOSE = REDUCING SUGAR (free anomeric C1 in open chain) → POSITIVE CLINITEST → NEGATIVE GLUCOSE OXIDASE. GALACTOSURIA + reducing sugar positive + glucose oxidase negative + infant on BREAST MILK or lactose formula = GALACTOSAEMIA until proven otherwise. Compare: FRUCTOSURIA = onset with FRUIT JUICE or sucrose (not breast milk).\\\n\\\n📹 [VIDEO LINK]"},{"id":1085,"s":"Carbohydrate Chemistry and Metabolism","t":"Galactose Metabolism","u":"Galactokinase Deficiency Cataracts Only No Systemic Features","q":"A 3-month-old infant is evaluated for cataracts. Urine is positive for reducing sugar but negative for glucose. No other abnormalities are present. Which enzyme is deficient?","o":["Fructokinase","Galactokinase","Glucokinase","Hexokinase","Triose kinase"],"a":1,"p":"GALACTOKINASE DEFICIENCY: galactokinase converts galactose → galactose-1-phosphate. Deficiency → GALACTOSE ACCUMULATES → ALDOSE REDUCTASE converts galactose → GALACTITOL → accumulates in LENS → osmotic swelling → CATARACTS ONLY. NO systemic toxicity (galactose-1-P NOT formed → no hepatic or neurological damage). MILDER than classic galactosaemia. GALACTOSE in urine (reducing sugar positive, glucose oxidase negative). KEY DISTINGUISHING FEATURE: CATARACTS ONLY + no hepatomegaly + no jaundice + no intellectual disability. Treatment: remove galactose from diet → cataracts stop progressing (may partially reverse if early).\\\n\\\n📹 [VIDEO LINK]"},{"id":1086,"s":"Carbohydrate Chemistry and Metabolism","t":"Galactose Metabolism","u":"Galactitol Causes Cataracts Galactokinase Deficiency Polyol Pathway","q":"A 3-month-old infant has cataracts with reducing sugar in urine but negative glucose oxidase. Accumulation of which of the following compound is responsible for cataract formation?","o":["Galactitol","Glycerol","Mannitol","Sorbitol","Xylitol"],"a":0,"p":"GALACTITOL = sugar alcohol formed from GALACTOSE by ALDOSE REDUCTASE (polyol pathway: galactose + NADPH → galactitol + NADP⁺). In galactokinase deficiency AND classic galactosaemia: galactose accumulates → aldose reductase converts to GALACTITOL → ACCUMULATES IN LENS (lens membrane impermeable to galactitol, cannot be further metabolised by SDH) → OSMOTIC SWELLING → protein precipitation → CATARACTS. SORBITOL = polyol from GLUCOSE (causes DIABETIC cataracts — different substrate, same enzyme). GALACTITOL = polyol from GALACTOSE (causes GALACTOSAEMIA/GALACTOKINASE DEFICIENCY cataracts).\\\n\\\n📹 [VIDEO LINK]"},{"id":1087,"s":"Carbohydrate Chemistry and Metabolism","t":"Galactose Metabolism","u":"GALT Enzyme Classic Galactosaemia PCR Sequencing Diagnosis","q":"A 3-month-old infant fed on standard formula since birth presents with jaundice, poor feeding, and a reducing substance in urine that is negative on glucose oxidase. Which enzyme of carbohydrate metabolism is likely to be deficient?","o":["Galactokinase","Galactose-1-phosphate uridylyltransferase","Galactose epimerase","Lactase","UDP-glucose pyrophosphorylase"],"a":1,"p":"CLASSIC GALACTOSAEMIA = GALACTOSE-1-PHOSPHATE URIDYLYLTRANSFERASE (GALT) DEFICIENCY. GALT converts: Galactose-1-P + UDP-glucose → Glucose-1-P + UDP-galactose. Without GALT: galactose-1-P ACCUMULATES → damages liver, brain, kidneys. DIAGNOSIS: (1) ENZYME ASSAY in erythrocytes (GALT activity markedly reduced/absent). (2) Elevated galactose-1-P in RBCs. (3) MOLECULAR DIAGNOSIS by PCR and DNA SEQUENCING of GALT gene (most common mutation: Q188R in Caucasians). Treatment: LACTOSE-FREE DIET lifelong (lactose = glucose + galactose; galactose must be eliminated completely).\\\n\\\n📹 [VIDEO LINK]"},{"id":1088,"s":"Carbohydrate Chemistry and Metabolism","t":"Galactose Metabolism","u":"GALT Required to Convert Galactose-1-P to Glucose-1-P Leloir Pathway","q":"A deficiency of which of the following enzymes prevents the conversion of galactose-1-phosphate to glucose-1-phosphate in the Leloir pathway?","o":["Gal-1-P uridyl transferase","Galactokinase","UDP-Gal 4-epimerase","UDP-Gal 6-epimerase","UDP-Gal pyrophosphorylase"],"a":0,"p":"HUMANS require GALT (Galactose-1-P Uridylyltransferase) for the LELOIR PATHWAY: galactose-1-P + UDP-glucose → glucose-1-P + UDP-galactose. This two-step process uses UDP as an obligate intermediate carrier. There is NO DIRECT PHOSPHOMUTASE or ISOMERASE that can convert galactose-1-P → glucose-1-P without UDP. UDP-GALACTOSE-4-EPIMERASE interconverts UDP-galactose ↔ UDP-glucose (separate reaction). GALACTOKINASE = phosphorylates galactose → galactose-1-P (first step, BEFORE GALT). In GALT DEFICIENCY: galactose-1-P accumulates because it CANNOT PROCEED to glucose-1-P.\\\n\\\n📹 [VIDEO LINK]"},{"id":1089,"s":"Carbohydrate Chemistry and Metabolism","t":"Galactose Metabolism","u":"Lactase Deficiency Neonate Galactose Source from UDP-Galactose Epimerase","q":"A neonate is diagnosed with lactase enzyme deficiency. Which of the following will be the source of galactose in him?","o":["Lactose","UDP-galactose","UDP-glucuronic acid","UDP-N-acetylgalactosamine","UDP-N-acetylglucosamine"],"a":1,"p":"When LACTASE is deficient: cannot digest lactose → no DIETARY GALACTOSE absorbed. However, galactose is STILL NEEDED for synthesis of glycolipids, glycoproteins, lactose (in nursing mothers), and brain development. Source of galactose: DE NOVO SYNTHESIS from UDP-GALACTOSE via UDP-GALACTOSE-4-EPIMERASE, which interconverts UDP-GLUCOSE ↔ UDP-GALACTOSE. So: glucose-1-P → (UTP) → UDP-glucose → (epimerase) → UDP-GALACTOSE → galactose-1-P → galactose. This SALVAGE PATHWAY ensures galactose availability even without dietary intake. This is also why galactosaemia patients on lactose-free diet still have some galactose-1-P (endogenous production).\\\n\\\n📹 [VIDEO LINK]"},{"id":1090,"s":"Carbohydrate Chemistry and Metabolism","t":"Galactose Metabolism","u":"Galactosaemia Dietary Modification Avoid Lactose Containing Foods","q":"A newborn is diagnosed with galactosaemia. Foods containing which of the following should be advised to avoid?","o":["Fructose","Glucose","Lactose","Maltose","Xylitol"],"a":2,"p":"GALACTOSAEMIA dietary management: AVOID ALL LACTOSE-CONTAINING FOODS (dairy products, breast milk, standard infant formula). Replace with LACTOSE-FREE or SOY-BASED formula. Rationale: lactose (glucose + galactose) → galactose → galactose-1-P accumulates (GALT deficiency) → organ damage. HIDDEN SOURCES of galactose to avoid: some medications, processed foods with whey, casein, lactalbumin, and even some non-dairy foods. Despite strict diet: neurological and OVARIAN complications (premature ovarian failure in females) may still develop (endogenous galactose production and constitutional factors).\\\n\\\n📹 [VIDEO LINK]"},{"id":1091,"s":"Carbohydrate Chemistry and Metabolism","t":"Polyol Pathway","u":"Aldose Reductase Produces Sorbitol Diabetic Retina Lens Complications","q":"In uncontrolled diabetes, which enzyme in the polyol pathway produces large amounts of sorbitol in the retina and lens, leading to long-term microvascular complications?","o":["Aldolase A","Aldolase B","Aldolase C","Aldose reductase","Sorbitol dehydrogenase"],"a":3,"p":"POLYOL PATHWAY: ALDOSE REDUCTASE: glucose + NADPH → SORBITOL (+ NADP⁺). In UNCONTROLLED DIABETES: high glucose → ALDOSE REDUCTASE overwhelmed → excess SORBITOL in lens, retina, peripheral nerves, kidney. These tissues LACK INSULIN for GLUT4 → glucose enters based on blood concentration → overwhelms normal pathway. SORBITOL cannot exit cells → OSMOTIC STRESS → cell swelling → CATARACTS (lens), RETINOPATHY, NEUROPATHY, NEPHROPATHY. Also: NADPH depletion → ↓ glutathione → ↑ oxidative stress. Second step: SORBITOL DEHYDROGENASE: sorbitol + NAD⁺ → FRUCTOSE + NADH. Low SDH activity in lens/nerve/retina → sorbitol TRAPPED.\\\n\\\n📹 [VIDEO LINK]"},{"id":1092,"s":"Carbohydrate Chemistry and Metabolism","t":"Polyol Pathway","u":"Sorbitol Dehydrogenase Deficiency Diabetic Complications Lens Nerve Kidney","q":"A 55-year-old man with type 2 diabetes develops cataracts, peripheral neuropathy, and early nephropathy despite moderate glycaemic control. Long-term complications in the lens, nerves, and kidneys are primarily due to a deficiency of which enzyme in these tissues?","o":["Aldolase","Aldose dehydrogenase","Aldose reductase","Sorbitol dehydrogenase","Sorbitol reductase"],"a":3,"p":"SORBITOL DEHYDROGENASE (SDH) is DEFICIENT or has LOW ACTIVITY in lens, peripheral nerves, and kidneys → sorbitol CANNOT be converted to fructose → SORBITOL ACCUMULATES → osmotic damage. Mechanism: sorbitol accumulation → ↑ intracellular osmolality → water influx → cell swelling → disrupted cellular function. Tissues with HIGH SDH (liver, ovaries, seminal vesicles): sorbitol quickly converted to fructose → less accumulation. Tissues with LOW SDH (lens, nerve, kidney, retina): sorbitol TRAPPED → DIABETIC COMPLICATIONS. ALDOSE REDUCTASE = the enzyme that MAKES sorbitol (it is not deficient; it is overactive in diabetes). SDH DEFICIENCY in target tissues = WHY sorbitol accumulates there specifically.\\\n\\\n📹 [VIDEO LINK]"},{"id":1093,"s":"Carbohydrate Chemistry and Metabolism","t":"Polyol Pathway","u":"GALT Deficiency Galactitol via Polyol Pathway Cataracts Lens","q":"A deficiency in the enzyme Gal-1-P uridyl transferase can lead to osmotic stress and cataracts in the lens due to increased production of which substance via the polyol pathway?","o":["Galactitol","Maltitol","Mannitol","Sorbitol","Xylitol"],"a":0,"p":"CLASSIC GALACTOSAEMIA (GALT deficiency): galactose ACCUMULATES → ALDOSE REDUCTASE converts galactose → GALACTITOL (via polyol pathway). GALACTITOL accumulates in LENS → osmotic swelling → protein precipitation → CATARACTS. Key: ALDOSE REDUCTASE is the SAME ENZYME that produces sorbitol from glucose in diabetes. In galactosaemia: acts on GALACTOSE substrate instead. Galactitol CANNOT be metabolised further by SDH (SDH acts on sorbitol → fructose; does NOT act on galactitol) → PERMANENTLY TRAPPED in lens. Unlike sorbitol (which has some SDH pathway to reduce accumulation), galactitol accumulates more severely in lens → cataracts are often more rapidly progressive in galactosaemia than in diabetes.\\\n\\\n📹 [VIDEO LINK]"},{"id":1094,"s":"Carbohydrate Chemistry and Metabolism","t":"Polyol Pathway","u":"Sorbitol Dehydrogenase Deficiency Male Infertility Seminal Vesicle","q":"A 32-year-old man presents to a fertility clinic with primary infertility. Semen analysis reveals absent fructose and severely reduced sperm motility. A defect in the polyol pathway is suspected. Which enzyme of the polyol pathway is deficient in him?","o":["Aldol reductase","Aldose reductase","Glucose-6-phosphatase","Hexokinase","Sorbitol dehydrogenase"],"a":4,"p":"SEMINAL VESICLES use the POLYOL PATHWAY to produce FRUCTOSE: glucose → (aldose reductase) → sorbitol → (SORBITOL DEHYDROGENASE) → FRUCTOSE. SORBITOL DEHYDROGENASE DEFICIENCY in seminal vesicles → sorbitol accumulates but FRUCTOSE NOT PRODUCED → sperm cannot generate ATP → ASTHENOZOOSPERMIA (reduced motility) + ABSENT SEMEN FRUCTOSE → MALE INFERTILITY. Clinical test: SEMEN FRUCTOSE MEASUREMENT = test of seminal vesicle function (absent fructose = seminal vesicle agenesis, obstruction, or SDH defect). Normal semen fructose = 1.3-7.8 mg/mL. ALDOSE REDUCTASE = makes sorbitol FROM glucose (upstream of SDH — not the deficient enzyme here).\n\n📹 [VIDEO LINK]"},{"id":10941,"s":"Carbohydrate Chemistry and Metabolism","t":"Polyol Pathway","u":"Fructose Primary Energy Source Spermatozoa Seminal Fluid","q":"A 28-year-old man is investigated for infertility. Semen analysis reveals markedly reduced sperm motility and absent fructose in seminal fluid. Which of the following is the primary energy source for spermatozoa in seminal fluid?","o":["Fructose","Galactose","Glucose","Sorbitol","Sucrose"],"a":0,"p":"FRUCTOSE = PRIMARY ENERGY SOURCE for SPERMATOZOA. Sperm oxidise FRUCTOSE via GLYCOLYSIS in the sperm midpiece → ATP → flagellar movement → MOTILITY. Fructose is produced in SEMINAL VESICLES via the POLYOL PATHWAY: glucose → (aldose reductase) → sorbitol → (sorbitol dehydrogenase) → FRUCTOSE. ABSENT SEMEN FRUCTOSE = either seminal vesicle agenesis (congenital absence) OR sorbitol dehydrogenase deficiency. DIAGNOSTIC TEST: semen fructose measurement differentiates obstructive from non-obstructive causes of azoospermia/asthenozoospermia. Sperm also use OXIDATIVE PHOSPHORYLATION (in mitochondrial sheath of midpiece) for additional ATP when O₂ available.\n\n📹 [VIDEO LINK]"},{"id":1095,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycolysis Regulation","u":"Fructose-2,6-Bisphosphate Allosterically Activates PFK-1 After Meal","q":"A person eats biryani with cold drink at dinner. Which of the following compound will be increased in his hepatocytes that allosterically activates the rate-limiting enzyme of glycolysis?","o":["Adenosine diphosphate","Adenosine triphosphate","Fructose-1,6-bisphosphate","Fructose-1-phosphate","Fructose-2,6-bisphosphate"],"a":4,"p":"After HIGH CARBOHYDRATE MEAL: ↑ glucose → ↑ insulin → PFK-2 (kinase domain active) → FRUCTOSE-2,6-BISPHOSPHATE (F2,6BP) INCREASES in hepatocytes. F2,6BP = MOST POTENT ALLOSTERIC ACTIVATOR of PFK-1 (rate-limiting enzyme of glycolysis). F2,6BP: (1) Binds allosteric site on PFK-1 → stabilises active conformation → ↑ glycolytic flux. (2) ALSO INHIBITS FBPase-1 (gluconeogenesis enzyme) → dual regulation. FASTING: glucagon → cAMP → PKA → phosphorylates PFK-2/FBPase-2 → PFK-2 INACTIVATED → F2,6BP FALLS → PFK-1 less active → glycolysis slows. Note: FRUCTOSE-1,6-BISPHOSPHATE is a product of PFK-1, not the activator.\\\n\\\n📹 [VIDEO LINK]"},{"id":1096,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycolysis Regulation","u":"GLUT1 Deficiency Ketogenic Diet Seizures Microcephaly Brain","q":"An infant presents with delayed milestones, seizures, and microcephaly. His mother is advised to give a ketogenic diet. Which of the following glucose transporter is deficient in him?","o":["GLUT1","GLUT2","GLUT3","GLUT4","GLUT5"],"a":0,"p":"GLUT1 DEFICIENCY SYNDROME (De Vivo disease): GLUT1 = glucose transporter in BLOOD-BRAIN BARRIER (BBB) endothelial cells. Deficiency → BRAIN CANNOT IMPORT GLUCOSE → low CSF glucose despite normal blood glucose (HYPOGLYCORRHACHIA) → seizures, intellectual disability, movement disorder, microcephaly. KETOGENIC DIET treatment: HIGH FAT, LOW CARBOHYDRATE → liver → KETONE BODIES (acetoacetate, beta-hydroxybutyrate) → cross BBB via MCT (monocarboxylate transporters, NOT GLUT1) → fuel for brain. Ketogenic diet bypasses the GLUT1 defect. GLUT1 also in: RBCs, eye, kidney tubules. GLUT3 (high-affinity neuronal transporter) is different from GLUT1 at BBB.\\\n\\\n📹 [VIDEO LINK]"},{"id":1097,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycolysis Regulation","u":"PFK-2 Inactivated Phosphorylated Form During Ramadan Fast","q":"A person fasting for religious reasons has been fasting in Ramadan for 15 hours. He is waiting for azan maghrib to break his fast. At this point in time, which of the following enzyme will be inactivated in his liver in its phosphorylated form?","o":["Fructose-1,6-bisphosphatase","Glucose-6-phosphatase","PEP-carboxykinase","Phosphofructokinase-2","Pyruvate carboxylase"],"a":3,"p":"FASTING (waiting for iftari): GLUCAGON raised → cAMP → PKA → PHOSPHORYLATES the bifunctional enzyme PFK-2/FBPase-2 bifunctional protein. In PHOSPHORYLATED FORM: PFK-2 (kinase domain) is INACTIVATED, FBPase-2 (phosphatase domain) is ACTIVATED → fructose-2,6-bisphosphate (F2,6BP) DECREASES → PFK-1 less activated → glycolysis slows → gluconeogenesis promoted (PEPCK and FBPase-1 active). FED STATE (insulin at iftari): protein phosphatase dephosphorylates PFK-2/FBPase-2 → PFK-2 ACTIVE, FBPase-2 inactive → F2,6BP increases → PFK-1 activated → glycolysis resumes. Key: PHOSPHOFRUCTOKINASE-2 is the enzyme INACTIVATED (not activated) when phosphorylated.\\\n\\\n📹 [VIDEO LINK]"},{"id":1098,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycolysis Regulation","u":"AMP Activates PFK-1 Myocardial Infarction Anaerobic Glycolysis","q":"A 45-year-old man with chest pain is diagnosed with myocardial infarction. Reduced blood flow leads to increased AMP levels and anaerobic glycolysis. Which enzyme is allosterically activated by AMP in his myocytes?","o":["Hexokinase","Phosphofructokinase-1","Phosphofructokinase-2","Phosphoglycerate kinase","Pyruvate kinase"],"a":1,"p":"PHOSPHOFRUCTOKINASE-1 (PFK-1) = ALLOSTERICALLY ACTIVATED BY AMP (and ADP, Pi). In MI: ↓ O₂ → ↓ oxidative phosphorylation → ↓ ATP + ↑ ADP + ↑ AMP → AMP ACTIVATES PFK-1 → ↑ glycolytic flux → ↑ anaerobic ATP production (2 ATP/glucose via substrate-level phosphorylation). AMP is the most sensitive ENERGY SENSOR (a small fall in ATP creates a proportionally much larger rise in AMP due to adenylate kinase: 2 ADP → ATP + AMP). AMPK also activated by AMP → further promotes catabolic pathways. PFK-1 INHIBITORS: ATP (high energy), CITRATE (TCA full, H⁺). PFK-1 ACTIVATORS: AMP, ADP, F2,6BP, Pi.\\\n\\\n📹 [VIDEO LINK]"},{"id":1099,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycolysis Regulation","u":"Alcohol Increased NADH NAD Ratio Lactic Acidosis","q":"A 45-year-old with chronic alcohol abuse presents with elevated lactate levels and lactic acidosis. Increased ratio of which of the following coenzymes in his cells is responsible?","o":["FAD plus/FADH2","FADH2/FAD plus","NAD plus/NADH","NADH/NAD plus","NADPH/NADP plus"],"a":3,"p":"ALCOHOL METABOLISM → INCREASED NADH/NAD⁺ RATIO. Mechanism: (1) Alcohol dehydrogenase: ethanol + NAD⁺ → acetaldehyde + NADH. (2) Aldehyde dehydrogenase: acetaldehyde + NAD⁺ → acetate + NADH. BOTH reactions consume NAD⁺ → NADH → HIGH NADH/NAD⁺ RATIO. Consequences: (1) MALATE DH: OAA + NADH → malate (depletes OAA → cannot run gluconeogenesis → HYPOGLYCAEMIA). (2) LDH: pyruvate + NADH → LACTATE (diverts pyruvate → LACTIC ACIDOSIS). (3) TCA CYCLE IMPAIRED (needs NAD⁺ for isocitrate DH, alpha-KG DH, malate DH). Result: LACTIC ACIDOSIS + HYPOGLYCAEMIA + KETOSIS (acetyl-CoA accumulates → ketogenesis).\\\n\\\n📹 [VIDEO LINK]"},{"id":1100,"s":"Carbohydrate Chemistry and Metabolism","t":"Glycolysis Regulation","u":"GLUT4 Responsible for Insulin Induced Glucose Uptake Hypoglycaemia","q":"An old lady took her morning insulin but did not eat and was found unresponsive with blood glucose below 45mg/dL. Which of the following glucose transporters was responsible for hypoglycaemia?","o":["GLUT1","GLUT2","GLUT3","GLUT4","GLUT5"],"a":3,"p":"GLUT4 = INSULIN-SENSITIVE glucose transporter in SKELETAL MUSCLE and ADIPOSE TISSUE. Normally sequestered in intracellular vesicles → insulin → PI3K → AKT → GLUT4 vesicle TRANSLOCATION to plasma membrane → ↑ glucose uptake → blood glucose falls. In this case: INSULIN GIVEN + NO FOOD → GLUT4 activated → muscle and adipose take up glucose → blood glucose falls → HYPOGLYCAEMIA → coma. GLUT1 (RBCs, BBB — constitutive), GLUT2 (liver, pancreatic beta cells — glucose SENSOR, high Km), GLUT3 (neurons — high affinity, low Km), GLUT5 (fructose absorption — intestine, no glucose transport). Only GLUT4 = insulin-regulated.\\\n\\\n📹 [VIDEO LINK]"},{"id":1101,"s":"Carbohydrate Metabolism","t":"Gluconeogenesis","u":"PEPCK Inhibition Gluconeogenesis Blocked Endotoxin Sepsis","q":"A 60-year-old male with severe sepsis is admitted to ICU. Laboratory tests reveal endotoxin-induced inhibition of phosphoenolpyruvate carboxykinase (PEPCK). Which of the following processes will be affected in him?","o":["Gluconeogenesis","Glycogenesis","Glycogenolysis","Glycolysis","Ketogenesis"],"a":0,"p":"PEPCK (phosphoenolpyruvate carboxykinase) catalyses: OAA → PEP + CO₂ (KEY GLUCONEOGENIC STEP bypassing the irreversible pyruvate kinase reaction). PEPCK INHIBITION → GLUCONEOGENESIS BLOCKED → cannot produce glucose from lactate, amino acids, or glycerol (via OAA) → HYPOGLYCAEMIA in sepsis. GLYCOLYSIS = not affected (uses entirely different enzymes: hexokinase, PFK-1, pyruvate kinase). GLYCOGENOLYSIS = not affected (glycogen phosphorylase, debranching enzyme — completely separate pathway). Endotoxin inhibition of PEPCK = one mechanism of HYPOGLYCAEMIA in severe sepsis, alongside increased glucose utilisation and adrenal insufficiency.\\\n\\\n📹 [VIDEO LINK]"},{"id":1102,"s":"Carbohydrate Metabolism","t":"Gluconeogenesis","u":"PEPCK Inhibition Glycerol Still Used for Gluconeogenesis","q":"A 45-year-old female with cholangitis and endotoxaemia has inhibition of phosphoenolpyruvate carboxykinase (PEPCK). Which of the following substrates will still be able to participate in gluconeogenesis?","o":["Amino acids","Glycerol","Lactate","Odd-chain fatty acids","Pyruvate"],"a":1,"p":"PEPCK INHIBITION: substrates requiring OAA → PEP step CANNOT make glucose. LACTATE → pyruvate → OAA → BLOCKED AT PEPCK → cannot make glucose. AMINO ACIDS giving pyruvate (alanine, serine, glycine, cysteine, threonine) → pyruvate → OAA → BLOCKED AT PEPCK. ODD-CHAIN FATTY ACIDS → propionyl-CoA → succinyl-CoA → OAA → BLOCKED AT PEPCK. GLYCEROL BYPASSES PEPCK: glycerol → (glycerol kinase) → glycerol-3-phosphate → (G3PDH) → DHAP → enters gluconeogenesis AT THE TRIOSE PHOSPHATE LEVEL (below PEP step) → can still form fructose-1,6-bisphosphate → glucose-6-phosphate → glucose via FBPase-1 and G6Pase. GLYCEROL = the ONLY substrate listed that bypasses PEPCK.\\\n\\\n📹 [VIDEO LINK]"},{"id":1103,"s":"Carbohydrate Metabolism","t":"Gluconeogenesis","u":"PEPCK Inhibition Pyruvate-Yielding Amino Acids Cannot Make Glucose","q":"A patient with a bacterial infection producing an endotoxin that inhibits PEPCK. Amino acids that produce which of the following will be unable to produce glucose?","o":["Glycerol","Leucine","Lysine","Ornithine","Pyruvate"],"a":4,"p":"PEPCK INHIBITION → PYRUVATE CANNOT → PEP → glucose. PYRUVATE-yielding substrates BLOCKED: amino acids giving PYRUVATE (alanine, serine, glycine etc). Among options: PYRUVATE directly blocked. LEUCINE = ketogenic only (acetyl-CoA → cannot make glucose even without PEPCK inhibition). LYSINE = ketogenic only (same — only two purely ketogenic amino acids). ORNITHINE → enters urea cycle → not a gluconeogenic precursor. GLYCEROL → DHAP → BYPASSES PEPCK ENTIRELY (enters gluconeogenesis below the PEP step) → CAN STILL MAKE GLUCOSE. ANSWER = PYRUVATE (the substrate that directly requires PEPCK to proceed to glucose).\n\n📹 [VIDEO LINK]"},{"id":1104,"s":"Carbohydrate Metabolism","t":"Gluconeogenesis","u":"Glycerol Lactate Common Intermediates DKA Gluconeogenesis F16BP G6P","q":"A 12-year-old student presents with diabetic ketoacidosis. Raised lactate is being channelled into gluconeogenesis and glycerol from lipolysis is also entering gluconeogenesis. Which of the following are the two common intermediates formed during conversion of both glycerol and lactate to glucose?","o":["Fructose-1,6-bisphosphate and glucose-6-phosphate","Fructose-1-phosphate and glucose-6-phosphate","Fructose-2,6-bisphosphate and fructose-1-phosphate","Fructose-2,6-bisphosphate and fructose-6-phosphate","Pyruvate and oxaloacetate"],"a":0,"p":"CONVERGENCE OF LACTATE AND GLYCEROL IN GLUCONEOGENESIS: LACTATE → (LDH) → pyruvate → (pyruvate carboxylase) → OAA → (PEPCK) → PEP → → → FRUCTOSE-1,6-BISPHOSPHATE → (FBPase-1) → fructose-6-P → (PGI) → GLUCOSE-6-PHOSPHATE → (G6Pase) → GLUCOSE. GLYCEROL → (glycerol kinase) → glycerol-3-P → (G3PDH) → DHAP → (aldolase in reverse) → enters at FRUCTOSE-1,6-BISPHOSPHATE level. BOTH CONVERGE at FRUCTOSE-1,6-BISPHOSPHATE and GLUCOSE-6-PHOSPHATE. In DKA: excess lipolysis → glycerol + fatty acids. Glycerol → gluconeogenesis. Fatty acids → beta-oxidation → acetyl-CoA → KETONE BODIES (cannot make glucose from even-chain fatty acids).\\\n\\\n📹 [VIDEO LINK]"},{"id":1105,"s":"Carbohydrate Metabolism","t":"Gluconeogenesis","u":"Preterm Infant Hypoglycaemia Limited Gluconeogenesis Immature Enzymes","q":"A baby girl is born via caesarean section at 34 weeks of gestation. Blood glucose levels at birth were low and she was given dextrose. What is the likely cause of hypoglycaemia in preterm infants?","o":["Excessive glycogen storage due to rapid fetal growth","Limited ability for gluconeogenesis","Overproduction of insulin","Small size of head with respect to body","Very low fat stores only"],"a":1,"p":"PRETERM INFANT HYPOGLYCAEMIA: primary cause = LIMITED ABILITY FOR GLUCONEOGENESIS. Preterm infants have: (1) IMMATURE GLUCONEOGENIC ENZYME SYSTEMS — PEPCK and glucose-6-phosphatase expressed predominantly in LATE third trimester (34-40 weeks). (2) LIMITED GLYCOGEN STORES — hepatic glycogen deposited mainly in last 4-6 weeks of gestation. (3) Inadequate gluconeogenic substrates (insufficient alanine, lactate, glycerol from limited muscle/fat). Treatment: IV 10% DEXTROSE bolus → maintenance infusion. Full-term infants: adequate hepatic glycogen + mature gluconeogenic enzymes → can maintain blood glucose during first hours of life even without feeding.\\\n\\\n📹 [VIDEO LINK]"},{"id":1106,"s":"Carbohydrate Metabolism","t":"Gluconeogenesis","u":"Aspartate Immediate Precursor for Oxaloacetate in Gluconeogenesis","q":"In the gluconeogenesis pathway, aspartate plays a crucial role as an immediate precursor for which of the following compounds?","o":["Fumarate","Lactate","Malate","Oxaloacetate","Pyruvate"],"a":3,"p":"ASPARTATE → (ASPARTATE TRANSAMINASE, AST/GOT) → OXALOACETATE + GLUTAMATE. Reaction: aspartate + alpha-KG → OAA + glutamate. OAA = KEY GLUCONEOGENIC INTERMEDIATE → (PEPCK) → PEP → upstream gluconeogenesis. ASPARTATE provides a DIRECT route to OAA: bypasses the pyruvate carboxylase step (unlike alanine which must go pyruvate → OAA via pyruvate carboxylase). ASPARTATE ALSO functions in: UREA CYCLE (carries nitrogen as aspartate → argininosuccinate), MALATE-ASPARTATE SHUTTLE (transfers reducing equivalents), PURINE SYNTHESIS. Clinically: ELEVATED SERUM AST = hepatocyte damage (AST abundant in hepatocyte mitochondria and cytoplasm).\\\n\\\n📹 [VIDEO LINK]"},{"id":1107,"s":"Carbohydrate Metabolism","t":"Gluconeogenesis","u":"Aspartate Transaminase Converts Aspartate to Oxaloacetate","q":"In the gluconeogenesis pathway, which enzyme catalyses the conversion of aspartate to oxaloacetate?","o":["Aspartate transaminase","Malate dehydrogenase","Phosphoenolpyruvate carboxykinase","Pyruvate carboxylase","Pyruvate kinase"],"a":0,"p":"ASPARTATE TRANSAMINASE (AST, GOT): aspartate + alpha-ketoglutarate ⇌ OAA + GLUTAMATE. This REVERSIBLE TRANSAMINATION converts aspartate → OAA (gluconeogenic precursor). Requires PLP (vitamin B6). CLINICAL SIGNIFICANCE: ELEVATED SERUM AST in: MI (cardiac mitochondrial AST released), LIVER DISEASE (hepatocyte damage), HAEMOLYSIS. AST:ALT > 2 = ALCOHOLIC HEPATITIS (mitochondrial damage preferentially releases mitochondrial AST). Compare: PYRUVATE CARBOXYLASE = converts pyruvate → OAA (different reaction, requires biotin + ATP, mitochondrial). Both produce OAA for gluconeogenesis but from DIFFERENT SUBSTRATES (aspartate vs pyruvate).\\\n\\\n📹 [VIDEO LINK]"},{"id":1108,"s":"Carbohydrate Metabolism","t":"Gluconeogenesis","u":"Glucagon Treats Insulin-Induced Hypoglycaemia Alzheimer Double Dose Coma","q":"An elderly man with Alzheimer's disease accidentally administers insulin twice and is found comatose. Which of the following medications may be life-saving for him?","o":["Dopamine","Epinephrine","Glucagon","Insulin","Metformin"],"a":2,"p":"INSULIN-INDUCED HYPOGLYCAEMIA → GLUCAGON (IM or SC) = first-line treatment in an UNCONSCIOUS patient who cannot receive oral glucose and may not have IV access. GLUCAGON mechanism: (1) Activates GLYCOGEN PHOSPHORYLASE in LIVER → glycogenolysis → glucose-1-P → glucose-6-P → (G6Pase) → FREE GLUCOSE released into blood. (2) Activates GLUCONEOGENESIS via cAMP/PKA pathway. Result: RAPID BLOOD GLUCOSE ELEVATION within 10-15 minutes. Glucagon kits available for home use. Also: IV DEXTROSE (D50W) if IV access available. CAUTION: glucagon is ineffective if GLYCOGEN STORES ARE DEPLETED (prolonged fasting, chronic alcoholism, starvation) → IV dextrose essential in those cases.\\\n\\\n📹 [VIDEO LINK]"},{"id":1109,"s":"Carbohydrate Metabolism","t":"Gluconeogenesis","u":"Exogenous Insulin No C-Peptide Fainting Hypoglycaemia Factitious","q":"A patient is rushed to the emergency department after a fainting episode. Blood glucose is extremely low, insulin levels are normal, but there is no detectable C-peptide. Which of the following might be the trigger for his hypoglycaemia?","o":["Endogenous insulin","Exogenous insulin","Glucagonoma","Insulinoma","Munchausen syndrome"],"a":1,"p":"NO C-PEPTIDE + ELEVATED INSULIN = EXOGENOUS INSULIN. PROINSULIN processing: proinsulin → C-PEPTIDE + INSULIN (equimolar amounts, released together from beta cell granules). ENDOGENOUS insulin → C-PEPTIDE ALWAYS PRESENT. EXOGENOUS injected insulin → NO C-PEPTIDE. INSULINOMA = beta cell tumour → BOTH insulin AND C-peptide ELEVATED. GLUCAGONOMA = glucagon-secreting tumour → causes HYPERGLYCAEMIA (opposite of hypoglycaemia — wrong answer here). C-peptide measurement is the KEY TEST: absent C-peptide + high insulin = factitious hypoglycaemia from EXOGENOUS INSULIN injection. May represent Munchausen syndrome, suicide attempt, or accidental double dose.\n\n📹 [VIDEO LINK]"},{"id":1110,"s":"Carbohydrate Metabolism","t":"Gluconeogenesis","u":"Ketogenesis Supplies Brain Prolonged Starvation No Hypoglycaemic Symptoms","q":"An underweight medical student with severe dietary restriction has fasting blood glucose of 55-70mg/dL but denies having any hypoglycaemic symptoms. Which of the following is the likely biochemical mechanism?","o":["Gluconeogenesis","Glycogenesis","Glycogenolysis","Glycolysis","Ketogenesis"],"a":4,"p":"PROLONGED STARVATION + LOW BLOOD GLUCOSE WITHOUT SYMPTOMS = BRAIN ADAPTED TO USE KETONE BODIES. KETOGENESIS: liver oxidises fatty acids (from adipose lipolysis) → acetyl-CoA → HMG-CoA → ACETOACETATE + BETA-HYDROXYBUTYRATE (ketone bodies) → released into blood → cross BBB via MCT transporters. Brain: ketone bodies → acetoacetyl-CoA → acetyl-CoA → TCA cycle → ATP. After 3-4 days of starvation: brain uses ketones for up to 75% of its fuel → REDUCED GLUCOSE REQUIREMENT → blood glucose of 55-70 mg/dL is SUFFICIENT → no hypoglycaemic symptoms. Normal glycaemic symptoms occur when glucose falls below ~50mg/dL AND ketones are not yet elevated.\\\n\\\n📹 [VIDEO LINK]"},{"id":1111,"s":"Carbohydrate Metabolism","t":"Gluconeogenesis","u":"Gluconeogenesis Supplies Brain K2 Mountaineer 3 Days No Food","q":"A mountaineer is trapped on K2 bottleneck for 65 hours without any food supplies. Which of the following processes is ensuring glucose supply to his brain?","o":["Gluconeogenesis","Glycogenesis","Glycogenolysis","Glycolysis","Ketosis"],"a":0,"p":"After 3 DAYS OF FASTING: GLUCONEOGENESIS (in LIVER and KIDNEY) = MAIN SOURCE of blood glucose. Timeline: Day 1: liver GLYCOGENOLYSIS (liver glycogen ~100g depleted in ~24 hours). Days 2-3+: GLUCONEOGENESIS takes over completely. Gluconeogenic PRECURSORS: (1) AMINO ACIDS from muscle proteolysis → alanine (glucose-alanine cycle) → liver → pyruvate → OAA → PEP → glucose. (2) GLYCEROL from adipose lipolysis → DHAP → gluconeogenesis. (3) LACTATE from RBCs (always glycolysing anaerobically, lack mitochondria) → liver → Cori cycle. The BRAIN continues to use GLUCOSE EXCLUSIVELY in the first few days before adapting to ketone bodies.\\\n\\\n📹 [VIDEO LINK]"},{"id":1112,"s":"Carbohydrate Metabolism","t":"Gluconeogenesis","u":"Gluconeogenesis Supplies Brain After 12 Hour Overnight Fast","q":"A tired medical student falls asleep at 8pm and wakes at 8am. Which of the following processes is the source of glucose for his brain at this point in time?","o":["Gluconeogenesis","Glycogenesis","Glycogenolysis","Glycolysis","Ketogenesis"],"a":0,"p":"AFTER 12-HOUR OVERNIGHT FAST: GLUCONEOGENESIS is the PRIMARY source of blood glucose at this point. Per Lippincott Biochemistry: LIVER GLYCOGEN is DEPLETED in 8-10 HOURS of fasting. Timeline: 0-4 hours: dietary glucose absorbed. 4-8 hours: glycogenolysis primary. 8-10 hours: liver glycogen EXHAUSTED. 10 hours onwards: GLUCONEOGENESIS takes over and maintains blood glucose (can sustain blood glucose for up to 72 hours of fasting). >72 hours: fatty acid oxidation and ketogenesis become dominant (lipolysis-driven). At 8am (12 hours fast): glycogen is GONE → GLUCONEOGENESIS = primary glucose source for brain. Precursors: alanine (from muscle via glucose-alanine cycle), lactate (from RBCs via Cori cycle), glycerol (from adipose lipolysis).\n\n📹 [VIDEO LINK]"},{"id":1113,"s":"Carbohydrate Metabolism","t":"Gluconeogenesis","u":"Liver Main Organ 90% Blood Glucose During Prolonged Ramadan Fast","q":"A man is fasting from sehri at 3am to iftari at 6pm. As he waits for iftari, which of the following is the main organ providing him 90% blood glucose?","o":["Brain","Intestine","Kidney","Liver","Muscles"],"a":3,"p":"LIVER = MAIN SOURCE of blood glucose during fasting (provides ~90% of blood glucose). HEPATIC MECHANISMS: (1) GLYCOGENOLYSIS (dominant in early fasting, first 12-24 hours): liver glycogen → glucose-1-P → glucose-6-P → (G6Pase) → free GLUCOSE released. (2) GLUCONEOGENESIS (dominant after ~4-6 hours and increasingly): from lactate, alanine, glycerol → glucose. KIDNEY contributes ~10% via gluconeogenesis (increases in prolonged fasting → up to 20-25% in prolonged starvation). MUSCLE = CANNOT release free glucose (lacks G6Pase) → provides LACTATE and ALANINE (gluconeogenic PRECURSORS) to liver. BRAIN = CONSUMER (not producer) → uses ~120g glucose/day.\\\n\\\n📹 [VIDEO LINK]"},{"id":1114,"s":"Carbohydrate Metabolism","t":"TCA Cycle","u":"ATP Yield from One Acetyl-CoA TCA Cycle is 10 ATP","q":"Which of the following best describes the total ATP yield generated from complete oxidation of one molecule of acetyl-CoA in the TCA cycle?","o":["6 ATP","10 ATP","16 ATP","20 ATP","24 ATP"],"a":1,"p":"ONE ACETYL-CoA THROUGH TCA CYCLE GENERATES 10 ATP: ISOCITRATE DH → NADH (2.5 ATP). ALPHA-KG DH → NADH (2.5 ATP). SUCCINYL-CoA SYNTHETASE → GTP (1 ATP). SUCCINATE DH → FADH₂ (1.5 ATP). MALATE DH → NADH (2.5 ATP). TOTAL = 2.5 + 2.5 + 1 + 1.5 + 2.5 = 10 ATP. Per GLUCOSE: 2 pyruvate → 2 NADH (PDH step, 2 × 2.5 = 5 ATP) + 2 acetyl-CoA → 2 × 10 = 20 ATP from TCA + glycolysis substrate-level = 2 ATP + 2 NADH (glycolysis, 5 ATP via malate-aspartate shuttle) = total ~32 ATP per glucose (modern P:O values).\\\n\\\n📹 [VIDEO LINK]"},{"id":1115,"s":"Carbohydrate Metabolism","t":"TCA Cycle","u":"FADH2 Generated from Succinate Oxidation by Succinate Dehydrogenase","q":"Which of the following reactions of the TCA cycle generates FADH₂?","o":["Alpha-ketoglutarate dehydrogenation","Citrate isomerisation","Citrate synthesis","Fumarate hydration","Succinate oxidation"],"a":4,"p":"SUCCINATE DEHYDROGENASE (Complex II of ETC): SUCCINATE → FUMARATE + FADH₂. ONLY TCA cycle enzyme that is ALSO an INTEGRAL PROTEIN of the INNER MITOCHONDRIAL MEMBRANE (directly feeds electrons into CoQ). FAD used (not NAD⁺) because the succinate-fumarate oxidation yields insufficient energy to reduce NAD⁺ (higher standard reduction potential). FADH₂ → CoQ (Complex II) → Complex III → Complex IV → O₂ → 1.5 ATP per FADH₂. Compare: all other TCA dehydrogenases use NAD⁺ → NADH → Complex I → 2.5 ATP per NADH. Succinate DH INHIBITOR: malonate (competitive, structure analogous to succinate), used historically to demonstrate TCA cycle cycling.\\\n\\\n📹 [VIDEO LINK]"},{"id":1116,"s":"Carbohydrate Metabolism","t":"TCA Cycle","u":"Arsenic Inhibits Alpha-KG Dehydrogenase Army Chief Hair Arsenic","q":"A 56-year-old man collapses suddenly. Forensic analysis of his hair reveals chronically elevated arsenic levels. His family later reports that their dinner plates had been polished with arsenic-containing compound for years. Inhibition of which enzyme explains his toxicity?","o":["Alpha-ketoglutarate dehydrogenase","Citrate synthase","Isocitrate dehydrogenase","Succinate dehydrogenase","Succinate thiokinase"],"a":0,"p":"ARSENIC (arsenite, As(III)) INHIBITS LIPOATE-CONTAINING ENZYMES by covalently binding LIPOIC ACID (vicinal dithiol groups). LIPOATE-CONTAINING ENZYMES: (1) ALPHA-KETOGLUTARATE DEHYDROGENASE (TCA cycle). (2) PYRUVATE DEHYDROGENASE (PDH). (3) BRANCHED-CHAIN KETOACID DEHYDROGENASE (BCKAD). ALPHA-KG DH inhibition → TCA CYCLE BLOCKED at alpha-KG → citrate and alpha-KG accumulate → severe ATP reduction → CARDIAC ARREST. Hair arsenic = chronic exposure marker (hair grows ~1cm/month → timeline of exposure). ARSENATE (As(V), pentavalent) acts differently: mimics PHOSPHATE → substitutes for Pi in GAPDH reaction → unstable arsenate ester → uncouples glycolysis.\\\n\\\n📹 [VIDEO LINK]"},{"id":1117,"s":"Carbohydrate Metabolism","t":"TCA Cycle","u":"Arsenic Alpha-KG Dehydrogenase Contaminated Groundwater Punjab","q":"A 30-year-old man from southern Punjab develops diarrhoea, abdominal pain, and darkened skin over the years from drinking arsenic-contaminated groundwater. Inhibition of which enzyme is responsible for his symptoms?","o":["Alpha-ketoglutarate dehydrogenase","Isocitrate dehydrogenase","Malate dehydrogenase","NADH dehydrogenase","Succinate dehydrogenase"],"a":0,"p":"CHRONIC ARSENICOSIS (arsenite, As(III)): ALPHA-KETOGLUTARATE DEHYDROGENASE inhibited (+ PDH). SYMPTOMS of chronic arsenicosis: SKIN DARKENING (hyperpigmentation + arsenical keratosis — hallmark, especially on palms and soles), GI symptoms (diarrhoea, abdominal pain), peripheral neuropathy, Mees' lines on nails, cancer risk (skin, bladder, lung). MECHANISM: TCA cycle inhibited at alpha-KG DH → reduced ATP production → multi-organ dysfunction. Pakistan (southern Punjab, Sindh) = endemic for naturally occurring arsenic in groundwater from geological leaching. Treatment: remove from source + BAL (dimercaprol) chelation for acute toxicity.\\\n\\\n📹 [VIDEO LINK]"},{"id":1118,"s":"Carbohydrate Metabolism","t":"TCA Cycle","u":"PDHC Deficiency Lactic Acid Elevated Neurological Neonate","q":"A 1-month-old male shows neurological problems. Enzyme activity assay for pyruvate dehydrogenase complex shows decreased activity. Which compound is likely to be elevated in this patient?","o":["Acetyl CoA","Citrate","Lactic acid","Oxaloacetate","Thiamine"],"a":2,"p":"PDH COMPLEX DEFICIENCY → pyruvate CANNOT → acetyl-CoA → TCA cycle. PYRUVATE ACCUMULATES → (LDH) → LACTATE → LACTIC ACIDOSIS. Also: pyruvate → (ALT) → ALANINE elevated. Elevated LACTATE:PYRUVATE RATIO (>20, vs normal ~10). Symptoms: LACTIC ACIDOSIS (Kussmaul breathing, low pH), NEUROLOGICAL DETERIORATION (brain deprived of acetyl-CoA → impaired energy metabolism → cerebral lesions, basal ganglia damage). PDH complex components: E1 (pyruvate decarboxylase, TPP required), E2 (dihydrolipoyl transacetylase, lipoate + CoA), E3 (dihydrolipoyl dehydrogenase, FAD + NAD⁺). Treatment: KETOGENIC DIET (bypasses PDH).\\\n\\\n📹 [VIDEO LINK]"},{"id":1119,"s":"Carbohydrate Metabolism","t":"TCA Cycle","u":"Fumarase Mutation Uterine Leiomyoma HLRCC Fumarate Accumulation","q":"A 28-year-old female presents with heavy menstrual bleeding. Ultrasound shows solid hypoechoic masses in the uterus. Probable diagnosis is leiomyoma. Which of the following enzyme is most likely mutated in her?","o":["Aconitase","Fumarase","Isocitrate dehydrogenase","Succinate dehydrogenase","Succinate thiokinase"],"a":1,"p":"FUMARASE (FUMARATE HYDRATASE, FH) GENE MUTATION → HEREDITARY LEIOMYOMATOSIS AND RENAL CELL CARCINOMA (HLRCC). Fumarase: fumarate + H₂O → malate. Mutation → FUMARATE ACCUMULATES → (1) Inhibits PROLYL HYDROXYLASE → HIF-1alpha NOT degraded → stabilised → pseudohypoxic tumour gene expression. (2) Inhibits ALPHA-KG-DEPENDENT DIOXYGENASES (TET enzymes, JHDM demethylases) → EPIGENETIC changes → oncogenic reprogramming. UTERINE LEIOMYOMAS (fibroids) = benign smooth muscle tumours. HLRCC also causes: CUTANEOUS leiomyomas (painful skin lesions) and TYPE 2 PAPILLARY RENAL CELL CARCINOMA (aggressive). SDH mutations → paraganglioma and pheochromocytoma (different tumour spectrum).\\\n\\\n📹 [VIDEO LINK]"},{"id":1120,"s":"Carbohydrate Metabolism","t":"TCA Cycle","u":"PDH Deficiency Alanine Elevated Pyruvate Accumulates Transamination ALT","q":"A patient is diagnosed with PDHC deficiency. Which of the following amino acid levels will be raised in his blood?","o":["Alanine","Arginine","Asparagine","Histidine","Valine"],"a":0,"p":"PDH DEFICIENCY → PYRUVATE ACCUMULATES. Two major fates of accumulated pyruvate: (1) LDH: pyruvate + NADH → LACTATE → LACTIC ACIDOSIS. (2) ALT/GPT: pyruvate + GLUTAMATE → ALANINE + ALPHA-KETOGLUTARATE. Excess pyruvate DRIVES this transamination reaction → ALANINE ELEVATED in blood. DIAGNOSTIC CLUE: elevated ALANINE alongside elevated LACTATE and PYRUVATE in blood = PDH deficiency until proven otherwise. ALANINE is the CARRIER of pyruvate from peripheral tissues to liver (glucose-alanine cycle). In PDH deficiency: alanine accumulates because pyruvate (its carbon skeleton) cannot be processed. Compare: MSUD = BRANCHED-CHAIN AMINO ACIDS elevated (leucine, isoleucine, valine) — NOT alanine.\\\n\\\n📹 [VIDEO LINK]"},{"id":1121,"s":"Carbohydrate Metabolism","t":"TCA Cycle","u":"Thiamine Deficiency Pyruvic Acid Accumulates Muscle Cramps Fatigue","q":"A patient diagnosed with thiamine deficiency exhibits fatigue and muscle cramps. The muscle cramps are related to accumulation of metabolic acids. Which of the following acid is most likely to accumulate?","o":["Isocitric acid","Malic acid","Oxaloacetic acid","Pyruvic acid","Succinic acid"],"a":3,"p":"THIAMINE (VITAMIN B1) DEFICIENCY → TPP UNAVAILABLE → inhibits: (1) PDH → PYRUVATE ACCUMULATES (primary upstream metabolite — PYRUVIC ACID is the direct answer). (2) Alpha-KG DH → TCA cycle slowed. (3) Transketolase → pentose phosphate pathway impaired. PYRUVIC ACID accumulates in blood and muscle → drives LDH reaction → LACTIC ACID also accumulates. MUSCLE CRAMPS and FATIGUE = result of accumulated pyruvic and lactic acids impairing muscle metabolism. LACTIC ACIDOSIS contributes to fatigue but PYRUVIC ACID is the PRIMARY and IMMEDIATE accumulation upstream. Clinical presentations: WET BERIBERI (high-output cardiac failure), DRY BERIBERI (peripheral neuropathy), WERNICKE'S ENCEPHALOPATHY (confusion + ataxia + ophthalmoplegia), KORSAKOFF'S PSYCHOSIS (anterograde amnesia + confabulation).\n\n📹 [VIDEO LINK]"},{"id":1122,"s":"Carbohydrate Metabolism","t":"PDH Complex","u":"PDH Deficiency Oxidative Decarboxylation Impaired Ketogenic Diet Treatment","q":"A 3-year-old boy with generalised hypotonia, severe psychomotor delay, and refractory seizures is diagnosed with PDH deficiency and placed on a ketogenic diet with reduced seizure frequency. Which of the following reactions is impaired in this boy?","o":["Oxidative decarboxylation","Oxidative deamination","Oxidative phosphorylation","Oxidative transamination","Substrate-level phosphorylation"],"a":0,"p":"PDH COMPLEX catalyses OXIDATIVE DECARBOXYLATION of pyruvate: pyruvate + NAD⁺ + CoA → acetyl-CoA + NADH + CO₂. OXIDATIVE = NAD⁺ is REDUCED to NADH (oxidation of pyruvate). DECARBOXYLATION = CO₂ removed from pyruvate (3C → 2C). PDH deficiency → PYRUVATE CANNOT enter TCA → energy crisis in brain. KETOGENIC DIET treatment: HIGH FAT, LOW CARBOHYDRATE → fatty acids → beta-oxidation → ACETYL-CoA (BYPASSES PDH) → TCA cycle → ATP for brain (via MCT transporters at BBB). PDH reaction is IRREVERSIBLE under physiological conditions — cannot convert acetyl-CoA back to pyruvate (unlike plants/bacteria).\\\n\\\n📹 [VIDEO LINK]"},{"id":1123,"s":"Carbohydrate Metabolism","t":"PDH Complex","u":"PDH Deficiency Acetyl CoA Formation Impaired Pyruvate Accumulates","q":"A 2-year-old boy presents with delayed milestones, lethargy, and irritability. Workup reveals PDH deficiency with increased pyruvate. The root cause of his problems is impaired conversion of pyruvate to which of the following?","o":["Acetyl CoA","Crotonyl CoA","Malonyl CoA","Propionyl CoA","Succinyl CoA"],"a":0,"p":"PDH COMPLEX: pyruvate + NAD⁺ + CoA → ACETYL-CoA + NADH + CO₂. ACETYL-CoA = ENTRY POINT for glucose-derived carbon into TCA CYCLE. PDH deficiency → acetyl-CoA CANNOT be formed from pyruvate → TCA cycle deprived of glucose-derived fuel → ATP deficiency → NEUROLOGICAL SYMPTOMS (brain particularly vulnerable). KETOGENIC DIET: fatty acids → beta-oxidation → ACETYL-CoA (via thiolase, not PDH) → TCA cycle → ATP for brain. Note: other CoA compounds in options: MALONYL-CoA = fatty acid synthesis (ACC). SUCCINYL-CoA = from propionyl-CoA (odd-chain FA) or from TCA cycle. CROTONYL-CoA and PROPIONYL-CoA = beta-oxidation intermediates. None are products of PDH.\\\n\\\n📹 [VIDEO LINK]"},{"id":1124,"s":"Carbohydrate Metabolism","t":"PDH Complex","u":"PDH Deficiency 2 ATP from Glucose Only Glycolysis No TCA","q":"A 3-year-old girl with pyruvate dehydrogenase deficiency — how much ATP does she get from one molecule of glucose?","o":["2 ATP","4 ATP","6 ATP","8 ATP","10 ATP"],"a":0,"p":"PDH DEFICIENCY → PYRUVATE CANNOT → ACETYL-CoA → TCA CYCLE BLOCKED (from glucose). ONLY GLYCOLYSIS can produce ATP: NET GLYCOLYSIS = 2 ATP per glucose (substrate-level phosphorylation: 4 ATP generated at PGK and PK − 2 ATP invested at hexokinase and PFK-1 = net 2 ATP). The 30 ATP from oxidative phosphorylation (via NADH and FADH₂ from TCA cycle) CANNOT be produced because acetyl-CoA from glucose is blocked. TOTAL = 2 ATP (net glycolysis only). Normal = ~32 ATP per glucose. PDH deficiency = 94% reduction in ATP yield from glucose → SEVERE ENERGY DEFICIT → especially devastating in high-energy-demand brain tissue → SEIZURES, NEURODEGENERATION.\\\n\\\n📹 [VIDEO LINK]"},{"id":1125,"s":"Carbohydrate Metabolism","t":"PDH Complex","u":"Pyruvate Inhibits PDH Kinase Activates PDH Complex Feed-Forward","q":"Elevated concentration of which of the following substrate will result in activation of PDH complex through inhibition of PDH kinase?","o":["Glutamate","Glycerate","Lactate","Phosphate","Pyruvate"],"a":4,"p":"PDH REGULATION MECHANISM: PDH KINASE phosphorylates PDH E1 → INACTIVATES PDH (phosphoserine). PDH KINASE INHIBITORS (allow PDH to stay ACTIVE): HIGH PYRUVATE (feed-forward activation — substrate keeps enzyme active), HIGH ADP, HIGH CoA, HIGH NAD⁺ → signal that PDH should run → inhibit kinase → PDH UNPHOSPHORYLATED → ACTIVE. PDH KINASE ACTIVATORS (cause PDH inactivation): HIGH ACETYL-CoA (product inhibition), HIGH NADH (product), HIGH ATP → signal TCA full/energy sufficient → activate kinase → PDH PHOSPHORYLATED → INACTIVE. PYRUVATE = substrate of PDH → ALSO inhibits PDH kinase = FEED-FORWARD ACTIVATION (when substrate is high, enzyme is kept active to process it).\\\n\\\n📹 [VIDEO LINK]"},{"id":1126,"s":"Carbohydrate Metabolism","t":"PDH Complex","u":"Thiamine Deficiency Inhibits E1 Pyruvate Decarboxylase PDH Component","q":"In a chronic alcoholic with thiamine deficiency, which of the following component of PDH complex is most likely inhibited?","o":["Dihydrolipoyl dehydrogenase","Dihydrolipoyl transacetylase","PDH kinase","PDH phosphatase","Pyruvate decarboxylase"],"a":4,"p":"PDH COMPLEX THREE ENZYMES: E1 = PYRUVATE DECARBOXYLASE (requires THIAMINE PYROPHOSPHATE/TPP as prosthetic group). E2 = DIHYDROLIPOYL TRANSACETYLASE (requires LIPOIC ACID and CoA). E3 = DIHYDROLIPOYL DEHYDROGENASE (requires FAD and NAD⁺). THIAMINE (VITAMIN B1) DEFICIENCY → TPP UNAVAILABLE → E1 (PYRUVATE DECARBOXYLASE) CANNOT FUNCTION → PDH complex inactive → PYRUVATE and LACTATE ACCUMULATE. CHRONIC ALCOHOLICS = most common cause (poor nutritional intake + alcohol impairs thiamine absorption + increased thiamine demand for ethanol metabolism). TREATMENT: IMMEDIATE IV THIAMINE (given BEFORE glucose — glucose infusion without thiamine can precipitate or worsen Wernicke's encephalopathy by rapidly depleting remaining TPP).\\\n\\\n📹 [VIDEO LINK]"},{"id":1127,"s":"Carbohydrate Metabolism","t":"PDH Complex","u":"PDH E1 Alpha Deficiency Pyruvate Decarboxylase Lactic Acidosis X-Linked","q":"A 5-month baby boy presents with signs of lactic acidosis. PDH E1-alpha deficiency (PDHAD) is suspected. Which of the following component of PDH complex is most likely deficient?","o":["Dihydrolipoamide dehydrogenase","Dihydrolipoamide transacetylase","PDH kinase","PDH phosphatase","Pyruvate decarboxylase"],"a":4,"p":"PDH E1-ALPHA DEFICIENCY = PYRUVATE DECARBOXYLASE (E1) DEFICIENCY. E1 = alpha₂beta₂ heterotetramer. E1-ALPHA SUBUNIT = encoded by X-linked PDHA1 gene → X-LINKED INHERITANCE (males more severely affected, but females can also be affected due to skewed X-inactivation). E1-alpha is the CATALYTIC SUBUNIT that decarboxylates pyruvate (first step, requires TPP). Deficiency → PYRUVATE accumulates → LACTIC ACIDOSIS + PYRUVIC ACIDAEMIA. DIAGNOSIS: elevated blood lactate + pyruvate + lactate:pyruvate ratio (>20) + elevated alanine. TREATMENT: KETOGENIC DIET + thiamine supplementation (some patients respond partially). Early diagnosis critical (neurological damage can begin in utero).\\\n\\\n📹 [VIDEO LINK]"},{"id":1128,"s":"Carbohydrate Metabolism","t":"PDH Complex","u":"PDH Deficiency Ketogenic Diet High Fat Low Carb Treatment Seizures","q":"A 2-year-old boy presents with developmental delay, seizures, lethargy, and poor feeding. His parents are advised a high fat, low carbohydrate diet which improves his symptoms. Which enzyme is likely deficient?","o":["Alpha-ketoglutarate dehydrogenase","Branched-chain amino acid dehydrogenase","Glucose-6-phosphate dehydrogenase","Lactate dehydrogenase","Pyruvate dehydrogenase"],"a":4,"p":"PDH DEFICIENCY: KETOGENIC DIET (high fat, low carbohydrate) is the TREATMENT because: fatty acids → beta-oxidation → ACETYL-CoA (BYPASSES PDH entirely) → TCA cycle → ATP for brain. WITHOUT DIETARY CARBOHYDRATES: less pyruvate produced → less substrate accumulation → less lactic acidosis. IMPROVEMENT WITH KETOGENIC DIET = DIAGNOSTIC CLUE for PDH deficiency. Compare other options: MSUD (BCAA dehydrogenase deficiency) = low-protein diet (restrict BCAAs) NOT ketogenic diet. G6PD deficiency = no specific diet. LACTATE DH deficiency = very rare. ALPHA-KG DH deficiency = associated with arsenic toxicity. PDH DEFICIENCY + KETOGENIC DIET IMPROVEMENT = classic pairing in paediatric metabolic disease.\\\n\\\n📹 [VIDEO LINK]"},{"id":1129,"s":"Carbohydrate Metabolism","t":"Renal Threshold","u":"Renal Threshold Glucose 10mmol/L 180mg/dL Glycosuria SGLT2","q":"A 55-year-old diabetic woman with blood glucose 200mg/dL is started on a new medication that blocks glucose reabsorption in the proximal tubule, causing her to spill glucose in her urine. This effect occurs because her blood glucose exceeds which of the following renal threshold values?","o":["10mmol/L","20mmol/L","30mmol/L","40mmol/L","50mmol/L"],"a":0,"p":"RENAL THRESHOLD FOR GLUCOSE = 180 mg/dL = 10 mmol/L. When blood glucose EXCEEDS 180 mg/dL: SGLT2 (sodium-glucose cotransporter 2) in PROXIMAL TUBULE becomes SATURATED → glucose CANNOT be completely reabsorbed → GLYCOSURIA. Below 180 mg/dL: ALL filtered glucose is reabsorbed (primarily SGLT2 = 90%, SGLT1 = 10% in S3 segment). SGLT2 INHIBITORS (dapagliflozin, empagliflozin, canagliflozin = FLOZINS): BLOCK SGLT2 → glucose excreted in urine → lowers blood glucose independently of insulin → used in TYPE 2 DM treatment (also have cardiovascular and renal protective benefits). RENAL GLYCOSURIA (normal blood glucose but low renal threshold) = SGLT2 mutation → benign.\\\n\\\n📹 [VIDEO LINK]"},{"id":1130,"s":"Carbohydrate Metabolism","t":"Renal Threshold","u":"Glucose in Urine After Sweet Chocolates Exceeds Renal Threshold","q":"A man consumes a box of sweet chocolates. Shortly after, his urine is likely to contain which of the following?","o":["Creatinine","Glucose","Ketones","Proteins","Urea"],"a":1,"p":"LARGE AMOUNT OF CHOCOLATE (rapidly absorbed sucrose → glucose) → BLOOD GLUCOSE TRANSIENTLY EXCEEDS 180 mg/dL (RENAL THRESHOLD) → GLYCOSURIA (glucose in urine). GLUCOSE = positive glucose oxidase dipstick. This is TRANSIENT ALIMENTARY GLYCOSURIA — blood glucose briefly exceeds renal threshold then returns to normal. NOT diabetes. DISTINGUISH from DIABETES: in diabetics, blood glucose chronically elevated → persistent glycosuria + polyuria + polydipsia. KETONES: appear only in prolonged fasting, DKA, or very low-carb diet (NOT after eating chocolates). PROTEINS: in nephrotic syndrome or heavy exercise. CREATININE and UREA: always present in urine at baseline (not the answer here).\\\n\\\n📹 [VIDEO LINK]"},{"id":1131,"s":"Carbohydrate Metabolism","t":"Cori Cycle","u":"Muscle Lacks Glucose-6-Phosphatase Cannot Release Free Glucose Cori Cycle","q":"A 22-year-old healthy student goes for a morning jog after an overnight fast. His muscles degrade glycogen but cannot release free glucose into the blood, instead sending lactate to the liver. This occurs due to absence of which enzyme?","o":["Glucokinase","Glucose-6-phosphatase","Glucose-6-phosphate dehydrogenase","Glycogen phosphorylase","Glycogen synthase"],"a":1,"p":"MUSCLE LACKS GLUCOSE-6-PHOSPHATASE → cannot dephosphorylate glucose-6-phosphate → CANNOT release free glucose into blood. Instead: muscle glycogen → glucose-6-P → GLYCOLYSIS → PYRUVATE → LACTATE (LDH, regenerates NAD⁺) → LACTATE released into blood. LIVER (has G6Pase): lactate → (LDH) → pyruvate → GLUCONEOGENESIS → GLUCOSE → released into blood → back to muscle = CORI CYCLE. LIVER and KIDNEY = ONLY tissues that can release free glucose (have G6Pase). MUSCLE uses its glycogen exclusively for its OWN ATP needs (cannot share with other tissues directly). Net energy cost of Cori cycle to WHOLE BODY: 2 ATP (glycolysis) − 6 ATP (gluconeogenesis) = NET LOSS of 4 ATP (paid by liver using amino acid oxidation).\\\n\\\n📹 [VIDEO LINK]"},{"id":1132,"s":"Carbohydrate Metabolism","t":"Cori Cycle","u":"Cori Cycle Lactate from Muscle to Liver Back as Glucose Sprinting","q":"A 35-year-old man experiences muscle fatigue and shortness of breath while sprinting to catch a bus. His actively contracting muscles produce lactate that is transported to the liver, where it is converted back to glucose and returned to the muscles. This process is characteristic of which cycle?","o":["Amino acid cycle","Cori cycle","Glucose-alanine cycle","Glutathione cycle","Urea cycle"],"a":1,"p":"CORI CYCLE (LACTIC ACID CYCLE): (1) MUSCLE: anaerobic glycolysis → LACTATE (regenerates NAD⁺ for continued glycolysis, produces 2 ATP). (2) Lactate released → blood → LIVER. (3) LIVER: lactate → (LDH) → pyruvate → GLUCONEOGENESIS → GLUCOSE (consumes 6 ATP). (4) Glucose released → blood → back to MUSCLE. PHYSIOLOGICAL IMPORTANCE: (1) Prevents LACTIC ACIDOSIS during exercise. (2) Recycles carbon skeleton for re-use. (3) Links ANAEROBIC MUSCLE glycolysis with HEPATIC GLUCONEOGENESIS. (4) NET: carbon recycled but NET ENERGY TRANSFERRED from liver (aerobic) to muscle (anaerobic). Compare GLUCOSE-ALANINE CYCLE: transfers ALANINE (not lactate) + carries NITROGEN for urea disposal.\\\n\\\n📹 [VIDEO LINK]"},{"id":1133,"s":"Carbohydrate Metabolism","t":"Glucose Alanine Cycle","u":"Transamination Hallmark Glucose Alanine Cycle Generation Conversion","q":"Generation of amino acid through transamination and conversion of amino acid to keto acid through transamination is the hallmark of which of the following cycle?","o":["Amino acid cycle","Cori cycle","Glucose-alanine cycle","Glutathione cycle","Urea cycle"],"a":2,"p":"GLUCOSE-ALANINE CYCLE: HALLMARK = TRANSAMINATION AT BOTH ENDS. (1) MUSCLE: pyruvate + GLUTAMATE → (ALT) → ALANINE + alpha-KG. Alanine released → blood → LIVER. (2) LIVER: alanine + alpha-KG → (ALT) → pyruvate + GLUTAMATE. Pyruvate → GLUCONEOGENESIS → GLUCOSE → back to muscle. NITROGEN disposal: glutamate → urea cycle → UREA excreted. NET FUNCTION: (1) Transfers CARBON (pyruvate/glucose) between muscle and liver. (2) Transfers NITROGEN (amino group) from muscle to liver for UREA SYNTHESIS. Compare CORI CYCLE: transfers LACTATE (not alanine) — carbon transfer only, NO nitrogen transfer. GLUCOSE-ALANINE CYCLE = carbon + NITROGEN transfer (dual function).\\\n\\\n📹 [VIDEO LINK]"},{"id":1134,"s":"Carbohydrate Metabolism","t":"Glucose Alanine Cycle","u":"Glutamate Facilitates Pyruvate to Alanine Conversion Transamination ALT","q":"Which of the following facilitates conversion of pyruvate to alanine in the glucose-alanine cycle?","o":["Alpha-ketoglutarate","Fumarate","Glutamate","Oxaloacetate","Succinate"],"a":2,"p":"ALT (ALANINE AMINOTRANSFERASE, GPT): PYRUVATE + GLUTAMATE → ALANINE + ALPHA-KETOGLUTARATE. GLUTAMATE = the AMINO GROUP DONOR that converts pyruvate (a keto acid) → alanine (an amino acid). Where does glutamate come from in muscle? From TRANSAMINATION of ALPHA-KETOGLUTARATE with amino acids derived from MUSCLE PROTEIN CATABOLISM during fasting and exercise. The amino groups from muscle proteins → transferred to alpha-KG → GLUTAMATE → then ALT transfers the amino group to pyruvate → ALANINE released into blood → liver → (ALT) → pyruvate + glutamate. Pyruvate → GLUCONEOGENESIS → glucose. Amino group → UREA CYCLE → urea. ALT requires PLP (vitamin B6). Clinically: ELEVATED ALT = hepatocyte damage (most specific marker of liver cell injury).\n\n📹 [VIDEO LINK]"},{"id":1135,"s":"Carbohydrate Metabolism","t":"Diabetic Ketoacidosis","u":"DKA Coma Hyperosmolality Cerebral Cellular Dehydration","q":"A 24-year-old woman is brought to casualty in coma. Her plasma glucose is 35mmol/L, sodium 130mmol/L, potassium 5.9mmol/L, bicarbonate 10mmol/L, pH 7.1, pCO₂ 30, and urine is positive for ketones. Which of the following best explains the biochemical basis of her coma?","o":["Acidosis directly depresses CNS","Cerebral cellular dehydration from hyperosmolality","Hyperkalaemia depresses cardiac output","Hypoglycaemia from insulin excess","Uraemia from renal failure"],"a":1,"p":"DKA COMA MECHANISM: This patient has classic DKA (ketones positive, severe acidosis pH 7.1, HCO₃ 10). COMA in DKA is primarily due to: (1) SEVERE METABOLIC ACIDOSIS (pH 7.1) → direct CNS depression — brain function impaired by profound acidosis. (2) DEHYDRATION from osmotic diuresis + vomiting → reduced cerebral perfusion. (3) CEREBRAL OEDEMA (particularly in children receiving rapid IV fluids during treatment). Compare: HYPEROSMOLAR HYPERGLYCAEMIC STATE (HHS) = much higher glucose (>30-40mmol/L), NO significant ketosis, severe hyperosmolality → coma from OSMOTIC DEHYDRATION of brain cells. DKA and HHS have DIFFERENT coma mechanisms. In this DKA case: acidosis + dehydration = primary cause. Treatment: gradual rehydration + insulin infusion + electrolyte replacement.\n\n📹 [VIDEO LINK]"},{"id":1136,"s":"Carbohydrate Metabolism","t":"Diabetic Ketoacidosis","u":"DKA Metabolic Acidosis Low Bicarbonate Ketone Buffering","q":"A 16-year-old girl with type 1 diabetes presents in DKA with pH 7.1 and bicarbonate 10mmol/L. Which of the following best explains the biochemical basis of her acidosis?","o":["Bicarbonate loss","CO₂ retention","Ketone-generated H plus exceeding HCO₃ minus buffering","Lactic acidosis","Uraemic acid retention"],"a":2,"p":"DKA ACIDOSIS MECHANISM: insulin deficiency → uncontrolled LIPOLYSIS → fatty acids → BETA-OXIDATION → excess ACETYL-CoA → KETOGENESIS: ACETOACETATE + BETA-HYDROXYBUTYRATE (STRONG ACIDS). HYDROGEN IONS produced → buffered initially by HCO₃⁻. When RATE OF KETONE PRODUCTION EXCEEDS RATE OF BICARBONATE REGENERATION → HCO₃⁻ FALLS (10mmol/L) → pH FALLS (7.1) → METABOLIC ACIDOSIS. HIGH ANION GAP = Na⁺ − (Cl⁻ + HCO₃⁻) = 130 − (92 + 10) = 28 mEq/L (ELEVATED, normal <12) = ketoacid anions accumulate. KUSSMAUL RESPIRATION (deep sighing) = compensatory respiratory alkalosis (↓ pCO₂ to 30 confirms compensation). FRUITY BREATH = ACETONE (volatile ketone, exhaled).\\\n\\\n📹 [VIDEO LINK]"},{"id":1137,"s":"Carbohydrate Metabolism","t":"Diabetic Ketoacidosis","u":"DKA Hyperkalaemia Insulin Deficiency Potassium Not Entering Cells","q":"A 17-year-old girl with type 1 diabetes presents in DKA with potassium 5.9mmol/L despite total body potassium deficit. Deficiency of which of the following is the likely cause of hyperkalaemia in her?","o":["Cortisol","Ghrelin","Glucagon","Insulin","Leptin"],"a":3,"p":"DKA HYPERKALAEMIA PARADOX: APPARENT hyperkalaemia despite TOTAL BODY K⁺ DEFICIT. PRIMARY CAUSE = INSULIN DEFICIENCY. INSULIN normally: (1) Stimulates Na⁺/K⁺-ATPase → K⁺ ENTERS cells. (2) Promotes glucose uptake → K⁺ co-transported intracellularly. Without INSULIN: K⁺ STAYS in ECF → HYPERKALEMIA. ADDITIONAL FACTOR: ACIDOSIS → H⁺ enters cells → K⁺ exits in exchange → worsens hyperkalaemia. TOTAL BODY K⁺ IS DEPLETED (urinary K⁺ loss from osmotic diuresis) despite elevated plasma K⁺. DURING DKA TREATMENT with insulin: K⁺ rapidly re-enters cells → HYPOKALAEMIA (life-threatening arrhythmia risk). CRITICAL: start K⁺ REPLACEMENT when K⁺ < 5.0mmol/L before or as insulin infusion begins.\n\n📹 [VIDEO LINK]"},{"id":1138,"s":"Carbohydrate Metabolism","t":"Diabetic Ketoacidosis","u":"DKA Hyponatraemia Osmotic Effect Glucose Dilutes Sodium","q":"A 16-year-old girl with type 1 diabetes presents in DKA with blood glucose 35mmol/L and sodium 130mmol/L. Which of the following best explains the biochemical basis of her hyponatraemia?","o":["ADH secretion","Glucose osmotically diluting plasma sodium","Renal sodium wasting","Salt loss from vomiting","Total body sodium depletion"],"a":1,"p":"DKA HYPONATRAEMIA MECHANISM: HYPERGLYCAEMIA (35mmol/L) is osmotically active in ECF → draws WATER OUT OF CELLS into ECF → DILUTES PLASMA SODIUM → DILUTIONAL (PSEUDOHYPONATRAEMIA). CORRECTION FACTOR: for every 5.6mmol/L rise in glucose above normal → plasma Na falls by ~1.6mmol/L. Corrected Na ≈ 138mmol/L (actually normal). TRUE BODY Na is adequate but diluted by water shift. NOTE: This is NOT the same hyponatraemia seen in vomiting (salt loss) or SIADH (water retention) — this is GLUCOSE-DRIVEN water shift. MANAGEMENT: rehydration with isotonic saline → insulin lowers glucose → water re-enters cells → Na rises automatically.\n\n📹 [VIDEO LINK]"},{"id":1139,"s":"Carbohydrate Metabolism","t":"Diabetic Ketoacidosis","u":"DKA Low pCO2 Kussmaul Respiration Compensatory for Metabolic Acidosis","q":"A 16-year-old girl with type 1 diabetes presents in DKA with pH 7.1, HCO₃ 10mmol/L, and pCO₂ 30mmHg (normal 35-45). Which of the following best explains the biochemical basis of her low pCO₂?","o":["CNS depression","Compensation for metabolic alkalosis","Kussmaul respiration blowing off CO₂","Pulmonary oedema","TCA cycle inhibition"],"a":2,"p":"DKA LOW pCO₂ MECHANISM: METABOLIC ACIDOSIS (pH 7.1, HCO₃⁻ 10) → CHEMORECEPTORS stimulated by low pH → KUSSMAUL RESPIRATION (deep, sighing, laboured breathing) → INCREASED CO₂ EXCRETION → pCO₂ FALLS TO 30 (NORMAL 35-45 mmHg) → RESPIRATORY COMPENSATION for metabolic acidosis. ACETONE (volatile ketone) = exhaled → FRUITY ODOUR on breath. WINTER'S FORMULA (expected pCO₂ compensation): = 1.5 × HCO₃⁻ + 8 ± 2 = 1.5 × 10 + 8 = 23 ± 2 mmHg. pCO₂ of 30 = slightly above expected → adequate compensation. Low pCO₂ = body attempting to correct pH by blowing off CO₂ (CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ — removing CO₂ pulls equation LEFT → reduces H⁺ → raises pH).\n\n📹 [VIDEO LINK]"},{"id":1140,"s":"Carbohydrate Metabolism","t":"Diabetic Ketoacidosis","u":"Insulin Decreases Blood Glucose GLUT4 Peripheral Tissue Uptake DKA","q":"A 10-year-old girl with DKA is started on insulin infusion therapy. Which of the following is the mechanism by which insulin decreases blood sugar levels?","o":["Glucagon secretion by pancreas","Gluconeogenesis in liver","Glucose uptake in peripheral tissues","Glycolysis in skeletal muscles","Ketone metabolism in adipose tissue"],"a":2,"p":"INSULIN DECREASES BLOOD GLUCOSE BY: (1) PRIMARY MECHANISM — GLUT4 TRANSLOCATION → ↑ GLUCOSE UPTAKE in SKELETAL MUSCLE and ADIPOSE TISSUE. (2) INHIBITING HEPATIC GLUCONEOGENESIS (↓ PEPCK, pyruvate carboxylase) → ↓ glucose production. (3) INHIBITING GLYCOGENOLYSIS → promotes glycogen synthesis. (4) ACTIVATING GLYCOGEN SYNTHASE → stores glucose as glycogen. (5) INHIBITING KETOGENESIS (↓ lipolysis → ↓ fatty acid supply to liver → ↓ acetyl-CoA → ↓ ketone bodies). In DKA: insulin infusion → GLUT4 translocation → glucose removed from blood → blood glucose falls. CAUTION: ensure K⁺ replacement given with insulin (glucose + K⁺ re-enter cells → risk of rapid hypokalaemia).\\\n\\\n📹 [VIDEO LINK]"},{"id":1141,"s":"Carbohydrate Metabolism","t":"Diabetic Ketoacidosis","u":"Insulin Therapy DKA Inhibits Ketogenesis in Liver","q":"A 7-year-old boy with type 1 DM in DKA is started on insulin infusion. Which of the following was inhibited in his liver as a result of this therapy?","o":["Cholesterol synthesis","Fatty acid synthesis","Gluconeogenesis","Ketogenesis","Protein synthesis"],"a":3,"p":"INSULIN INFUSION in DKA → INHIBITS KETOGENESIS: Mechanism: (1) Insulin → inhibits HORMONE-SENSITIVE LIPASE (HSL) in adipose tissue → ↓ LIPOLYSIS → ↓ FREE FATTY ACIDS delivered to liver. (2) Insulin → activates ACETYL-CoA CARBOXYLASE → ↑ MALONYL-CoA → INHIBITS CPT-1 (carnitine palmitoyltransferase-1) → fatty acids CANNOT enter mitochondria → CANNOT be oxidised → CANNOT form ketone bodies. (3) Insulin → activates PYRUVATE DEHYDROGENASE → pyruvate → acetyl-CoA → OXALOACETATE replenished → acetyl-CoA directed to TCA, NOT ketogenesis. Result: KETOGENESIS INHIBITED → ketoacidosis resolves. PRIMARY GOAL of insulin in DKA: STOP KETOGENESIS (not just lower blood glucose — hyperglycaemia is secondary).\\\n\\\n📹 [VIDEO LINK]"},{"id":1142,"s":"Carbohydrate Metabolism","t":"Somogyi Effect","u":"Somogyi Effect Glucagon Counter-Regulatory Hormone Nocturnal Hypoglycaemia Rebound","q":"A known type 1 diabetic is advised adjustment of his night-time insulin dose due to frequent high blood sugar readings in the morning. This phenomenon known as the Somogyi effect is due to release of which of the following counter-regulatory hormones?","o":["Gastrin","Glucagon","Insulin","Thyroid hormone","Vasopressin"],"a":1,"p":"SOMOGYI EFFECT MECHANISM: (1) Excessive night-time insulin → NOCTURNAL HYPOGLYCAEMIA (typically 2-3am). (2) Hypoglycaemia → COUNTER-REGULATORY HORMONE SURGE: GLUCAGON (primary — stimulates glycogenolysis + gluconeogenesis), EPINEPHRINE (stimulates glycogenolysis + lipolysis), CORTISOL (gluconeogenesis), GROWTH HORMONE (anti-insulin). (3) Counter-regulatory hormones → GLYCOGENOLYSIS + GLUCONEOGENESIS → REBOUND HYPERGLYCAEMIA by morning. GLUCAGON = PRIMARY counter-regulatory hormone. MANAGEMENT: REDUCE night-time insulin dose (NOT increase). DIAGNOSE: check blood glucose at 3am (if low at 3am + high at 7am = Somogyi). COMPARE DAWN PHENOMENON: cortisol/GH surge in early morning → hyperglycaemia WITHOUT preceding nocturnal hypoglycaemia.\\\n\\\n📹 [VIDEO LINK]"},{"id":1143,"s":"Carbohydrate Metabolism","t":"Somogyi Effect","u":"Nocturnal Hypoglycaemia Somogyi Rebound Morning Hyperglycaemia Insulin Adjustment","q":"A 42-year-old woman with type 2 diabetes on insulin presents with night sweats and nightmares after midnight, followed by high blood glucose readings in the morning. This is due to which of the following?","o":["Dawn phenomenon","Pasteur effect","Somogyi effect","Waning insulin effect","Warburg effect"],"a":2,"p":"SOMOGYI EFFECT: (1) Excessive INSULIN → blood glucose falls after midnight → NOCTURNAL HYPOGLYCAEMIA (night sweats, nightmares after midnight). (2) Hypoglycaemia → COUNTER-REGULATORY HORMONES (glucagon, epinephrine, cortisol, GH) released. (3) Counter-regulatory hormones → GLYCOGENOLYSIS + GLUCONEOGENESIS → blood glucose REBOUNDS → HIGH FASTING GLUCOSE in morning. MANAGEMENT: REDUCE night-time insulin. KEY DIAGNOSTIC PATTERN: glucose LOW at 2-3am → HIGH at 7am. Other options: DAWN PHENOMENON = GH/cortisol surge in early morning → hyperglycaemia WITHOUT preceding nocturnal hypoglycaemia. WANING INSULIN EFFECT = insulin wearing off → consistently elevated glucose from midnight (no nadir-rebound pattern). WARBURG EFFECT = cancer cells preferentially use aerobic glycolysis (not relevant here). PASTEUR EFFECT = O₂ inhibits fermentation in yeast (not relevant to insulin management).\n\n📹 [VIDEO LINK]"},{"id":1144,"s":"Carbohydrate Metabolism","t":"Diabetic Ketoacidosis","u":"Insulin Surge Slight Glucagon Dip After High Carbohydrate Meal","q":"After consumption of pasta and sweetened tea, which of the following best describes the hormonal levels of a healthy medical student?","o":["Glucagon dip and unchanged insulin","Glucagon surge and slight insulin dip","Insulin surge and glucagon sharp dip","Insulin surge and slight glucagon dip","Insulin surge and unchanged glucagon"],"a":3,"p":"POSTPRANDIAL STATE (after high carbohydrate meal): ↑ BLOOD GLUCOSE → INSULIN SURGE from pancreatic beta cells. Glucagon from alpha cells: SLIGHTLY DECREASES (glucose directly suppresses glucagon secretion from alpha cells) but does NOT sharply dip — some basal glucagon remains (maintains hepatic glucose output partially). NET: INSULIN SURGE + SLIGHT GLUCAGON DIP → INSULIN:GLUCAGON RATIO INCREASES → ANABOLIC STATE (glycogen synthesis, lipogenesis, protein synthesis). Compare: FASTING = insulin low, glucagon high → CATABOLIC STATE (glycogenolysis, gluconeogenesis, lipolysis). Why not sharp glucagon dip? Glucagon has protective function — a total glucagon suppression would risk overshoot hypoglycaemia.\\\n\\\n📹 [VIDEO LINK]"}]

MBBS Biochemistry Exam Pearls

Applied & Clinical Biochemistry — Prof. Dr. Jariya

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