FIRST MBBS Biochemistry - Paper I, Q.1 (MCQs 1-5) - Correct Answers with Explanations

Q.1 (1) - Vitamin Deficiency after Gastric Resection

• Gastric resection removes the parietal cells that produce Intrinsic Factor (IF), which is essential for Vitamin B12 absorption in the terminal ileum.
• Without IF, Vitamin B12 cannot be absorbed, leading to Vitamin B12 deficiency.
• Classic presentation: megaloblastic anemia + subacute combined degeneration of the cord (decreased positional and vibrational sense = posterior column involvement).
• This is a classic exam scenario distinguishing B12 from folate deficiency - neurological features (demyelination) only occur with B12 deficiency, not with folate alone.

• Vitamin C: scurvy (bleeding gums, poor wound healing) - not absorbed via IF
• Vitamin D: absorbed in jejunum - not affected by gastric resection
• Vitamin K: fat-soluble, not IF-dependent

"Nonspecific malabsorption syndromes or gastric resection can also cause vitamin B12 deficiency. In addition to general signs and symptoms of anemia, vitamin B12 deficiency anemia may cause tingling (pins and needles) in the hands and feet, difficulty walking, dementia..." - Lippincott Illustrated Reviews: Pharmacology

Q.1 (2) - Enzyme Deficiency in Newborn with Jaundice After Antimalarial

• G6PD is the rate-limiting enzyme of the Hexose Monophosphate (HMP) shunt / Pentose Phosphate Pathway.
• G6PD generates NADPH, which is essential to regenerate reduced glutathione (GSH), the main antioxidant protecting RBCs from oxidative damage.
• Oxidant drugs (e.g., primaquine, dapsone) trigger acute hemolytic anemia in G6PD-deficient patients → RBC destruction → jaundice + anemia.
• X-linked recessive; more common in males.

• Hexokinase deficiency: causes hemolytic anemia but NOT triggered by antimalarials/oxidants
• Pyruvate kinase deficiency: causes hemolytic anemia (non-spherocytic), not oxidant-triggered
• Succinate dehydrogenase: TCA cycle enzyme, not relevant here

"When treated with some antimalarials that induce an oxidant stress, some patients with G6PD deficiency develop a hemolytic anemia." - Lippincott Illustrated Reviews: Biochemistry, 8th ed.

Q.1 (3) - Antiphospholipid Antibody Syndrome - Target Antigen

• Antiphospholipid Antibody Syndrome (APS/APLS) is characterized by antibodies directed against phospholipid-binding proteins and phospholipids, most notably cardiolipin (a mitochondrial membrane phospholipid).
• The hallmark antibodies in APS are:
  1. Anti-cardiolipin antibodies (aCL)
  2. Lupus anticoagulant
  3. Anti-β2-glycoprotein I antibodies
• Clinically causes thrombosis (arterial + venous) and recurrent pregnancy loss.

• Sphingomyelin: target in Niemann-Pick disease, not APS
• Lecithin (phosphatidylcholine): component of lung surfactant; not the APS target
• Threonine: an amino acid, not relevant to APS

"Antiphospholipid antibodies represent a diverse family of immunoglobulins that bind to cardiolipin or to phospholipid-associated plasma proteins such as β2-glycoprotein I..." - Fuster and Hurst's The Heart, 15th Edition

"Antiphospholipid syndrome (APS) is characterized by thrombosis or pregnancy morbidity in the setting of persistent antiphospholipid antibodies (aPL), documented by anti-cardiolipin antibody [aCL] or anti-β2-glycoprotein I..." - Firestein & Kelley's Textbook of Rheumatology

Q.1 (4) - Drug of Choice for Osmotic Reduction of Intracranial Pressure

• Mannitol is a 6-carbon sugar alcohol (polyol) that is the gold standard osmotic diuretic for acute reduction of raised intracranial pressure (ICP).
• It works by creating an osmotic gradient across the blood-brain barrier, drawing water from the brain parenchyma into the bloodstream, thereby reducing cerebral edema and ICP.
• Given IV; freely filtered at the glomerulus but not reabsorbed → pulls water into the tubular lumen → osmotic diuresis.
• Onset: 15-30 minutes; effective for cerebral edema post-trauma.

• Sorbitol: used as a laxative, not as an IV osmotic agent for ICP
• Ribitol: a pentose alcohol found in riboflavin; not a diuretic
• Dulcitol (galactitol): accumulates in galactosemia; no therapeutic osmotic diuretic use

Mannitol's role as the osmotic diuretic of choice for cerebral edema and raised ICP is well-established across pharmacology texts including Harrison's Principles of Internal Medicine and standard biochemistry textbooks.

Q.1 (5) - Thrombolytic Enzyme Used in STEMI Treatment

• When PCI is not available, thrombolytic (fibrinolytic) therapy is used to dissolve the coronary thrombus in STEMI.
• Streptokinase (from bacteria) is an indirect plasminogen activator - it forms a complex with plasminogen, converting other plasminogen molecules to plasmin, which dissolves fibrin clots.
• Other agents: tPA (alteplase), tenecteplase, reteplase - all are plasminogen activators.
• These are enzymes (serine proteases) that act on the fibrinolytic cascade.

"Currently approved fibrinolytic agents include streptokinase; acylated plasminogen streptokinase activator complex (anistreplase); urokinase; recombinant tissue-type plasminogen activator (rtPA)..." - Harrison's Principles of Internal Medicine, 22nd Edition

"Streptokinase [from bacteria] activates both free and fibrin-bound plasminogen." - Lippincott Illustrated Reviews: Biochemistry, 8th ed.

Summary Table

FIRST MBBS Biochemistry - Paper I, Q.1 (MCQs 5-10) - Correct Answers

Q.1 (5) - Thrombolytic Enzyme in STEMI Treatment (from previous page)

• Streptokinase is a thrombolytic enzyme (indirect plasminogen activator) derived from bacteria. It forms a complex with plasminogen, converting it to active plasmin, which then dissolves the fibrin clot blocking the coronary artery.
• When PCI is unavailable, streptokinase is the drug of choice for thrombolysis in STEMI.
• Other agents: alteplase (tPA), tenecteplase, reteplase - all are plasminogen activators.

• Creatine kinase: a diagnostic marker of myocardial damage, not a therapeutic enzyme
• Asparaginase: used in leukemia treatment
• Carnitine transferase: involved in fatty acid transport into mitochondria

Q.1 (6) - Bluish Discoloration Due to Excess Deoxygenated Hemoglobin

• Cyanosis is the bluish-purple discoloration of skin and mucous membranes (lips, fingertips) caused by excessive deoxygenated hemoglobin in capillary blood.
• It becomes clinically apparent when deoxygenated Hb exceeds 5 g/dL in capillary blood.
• In COPD, chronic airflow obstruction causes ventilation-perfusion mismatch → reduced O₂ saturation → increased deoxygenated Hb → central cyanosis.
• SpO₂ of 82% confirms severe hypoxemia.

• Anaemia: pallor (pale skin) due to low Hb, not blue discoloration
• Pallor: whitish appearance from reduced blood flow or low Hb
• Jaundice: yellowish discoloration from bilirubin accumulation

"Cyanosis is the bluish discoloration of the skin that occurs from the presence of deoxygenated hemoglobin (which is blue) in capillary beds. For cyanosis to be clinically apparent, 3 to 5 mg/dL of deoxygenated hemoglobin must be present..." - Tintinalli's Emergency Medicine

Q.1 (7) - Organelle Responsible for Protein Sorting, Modification & Packaging

• The Golgi apparatus is the central "post office" of the cell for intracellular protein trafficking.
• Functions of the Golgi:
  1. Post-translational modification of proteins (glycosylation, phosphorylation, sulfation)
  2. Sorting of proteins to their correct destinations (lysosomes, plasma membrane, secretion)
  3. Packaging into vesicles for transport
• Proteins arrive from the rough ER → Golgi (cis → medial → trans face) → dispatched to final destination.
• The clinical scenario describes a defect in protein trafficking after synthesis, which points precisely to Golgi dysfunction.

• Ribosomes: site of protein synthesis, not sorting/packaging
• Mitochondria: energy production (ATP via oxidative phosphorylation)
• Nuclear membrane: regulates gene expression / mRNA export

Q.1 (8) - Complete Loss of UDP-Glucuronyltransferase → Which Disorder?

• UDP-glucuronyltransferase (UGT1A1) conjugates unconjugated (indirect) bilirubin to form water-soluble conjugated (direct) bilirubin for excretion.
• Complete absence of this enzyme = Crigler-Najjar Syndrome Type I - the most severe form, presenting with extreme unconjugated hyperbilirubinemia (>20 mg/dL), severe neonatal jaundice, and risk of kernicterus (bilirubin encephalopathy).
• Treatment: phototherapy (10-12 hrs/day) and liver transplant (only cure).

"Crigler-Najjar syndrome type I - a rare disorder caused by complete absence of UDP-glucuronyltransferase A1, manifested by high concentrations of unconjugated bilirubin often exceeding 20 mg/dL..." - Tietz Textbook of Laboratory Medicine, 7th Edition

Q.1 (9) - Negative Nitrogen Balance with Increased Protein Catabolism

• Nitrogen balance = Nitrogen intake - Nitrogen output
• This patient: Intake = 100 g protein/day → N intake = 100/6.25 = 16 g N/day
• Output = 25 g N/day → Negative nitrogen balance (output > intake)
• This means the body is catabolizing more protein than it is receiving - a hallmark of severe metabolic stress (sepsis, trauma, burns, surgery).
• A critically ill patient in sepsis has massive cytokine-driven protein catabolism (via glucocorticoids, cytokines) causing negative nitrogen balance.

• Pregnant lady: positive nitrogen balance (protein retention for fetal growth)
• Nephrotic syndrome: protein loss in urine, but this would affect albumin specifically; nitrogen balance depends on diet
• Normal healthy individual: nitrogen equilibrium (intake = output)

"In response to trauma or infection... there is net loss of protein nitrogen from the body - negative nitrogen balance." - Harper's Illustrated Biochemistry, 32nd Ed.

Q.1 (10) - Diet with Highest Thermogenic Effect

• Diet-Induced Thermogenesis (DIT) = also called Specific Dynamic Action (SDA) or Thermic Effect of Food (TEF) - the energy required to digest, absorb, and metabolize nutrients.
• Thermogenic effect by macronutrient:
  • Protein: 20-30% of its caloric value is used for its own metabolism (highest TEF)
  • Carbohydrate: 5-10%
  • Fat: 0-3% (lowest TEF)
• Protein has the highest TEF because:
  1. Amino acid metabolism requires ATP-expensive urea cycle and gluconeogenesis
  2. Protein synthesis and catabolism are both ATP-costly
  3. Increased protein turnover generates more heat

"A high intake of protein... increases the rate of protein synthesis, it also increases the rate of protein catabolism... Both protein synthesis and catabolism are ATP expensive, and this increased rate of protein turnover explains the increased diet-induced thermogenesis seen in people consuming a high-protein diet." - Harper's Illustrated Biochemistry, 32nd Ed.

Summary Table (Q.5-10)

• Q9: Pregnant women are in positive nitrogen balance. Negative nitrogen balance with sepsis = Critically ill
• Q10: Protein has the highest thermic effect (20-30%), not fat (0-3%). High protein diet is the correct answer