1. Vitamins in glycogenolysis and gluconeogenesis respectively (C) Pyridoxine and biotin Pyridoxal phosphate (vitamin B6) is the essential coenzyme for glycogen phosphorylase (glycogenolysis), while biotin is the coenzyme for pyruvate carboxylase, the key gluconeogenic enzyme. - Harper's Illustrated Biochemistry, 32nd Ed

2. Adenosine deaminase (ADA) deficiency leads to (A) Severe combined immunodeficiency (SCID) ADA deficiency causes accumulation of toxic deoxyadenosine/dATP, which is lymphotoxic and destroys developing T and B lymphocytes, producing SCID.

3. Insulin is degraded inside cells in (C) Lysosomes After receptor-mediated endocytosis, the insulin-receptor complex is internalized and insulin is degraded in the lysosomes. - Lippincott Illustrated Reviews Biochemistry, 8th ed

4. Caspase causing cell death by breaking down macromolecules in apoptosis (D) Caspase-3 Caspase-3 is the principal executioner caspase that cleaves structural and regulatory proteins, DNA repair enzymes, and other macromolecules, carrying out the actual dismantling of the cell.

5. Feature of gene regulation in humans (B) Alternate polyadenylation Operons, polycistronic mRNA translation, and transcription attenuation are prokaryotic gene-regulation features. Alternative polyadenylation (along with alternative splicing) is a hallmark of eukaryotic/human gene regulation.

6. Transported across membranes by simple diffusion (A) Oxygen, steroid hormones, carbon dioxide These are small nonpolar/lipophilic molecules that cross the lipid bilayer directly without a transporter. Calcium, acetylcholine, insulin, thermogenin, and aquaporin-mediated transport all require channels/transporters/carriers.

7. Respiratory alkalosis can occur in (C) Hysterical hyperventilation Voluntary/psychogenic hyperventilation blows off excess CO2, lowering PaCO2 and causing respiratory alkalosis. Bronchial asthma, collapse of lung, and bronchial obstruction typically impair ventilation and cause hypercapnia/respiratory acidosis or hypoxia.

8. Amino acid decreased in phenylketonuria (A) Tyrosine Phenylalanine hydroxylase deficiency blocks conversion of phenylalanine to tyrosine, so tyrosine (an aromatic hydroxy-amino acid) synthesis is reduced while phenylalanine accumulates.

9. True statement about tRNA (D) They are rich in modified and unusual nitrogen bases tRNAs characteristically contain a high percentage of modified/unusual bases (dihydrouridine, pseudouridine, ribothymidine, inosine, etc.) - confirmed in Lippincott Biochemistry and Basic Medical Biochemistry. (Note: option B is a common trap - the cloverleaf is the secondary, not tertiary/3D, structure of tRNA, which is actually L-shaped; and option C is incorrect because of wobble pairing/isoacceptor tRNAs, so no strict one-to-one tRNA-codon relationship exists.)

10. CD4/CD8 ratio is reduced in (A) AIDS HIV selectively depletes CD4+ T cells while CD8+ cells are relatively preserved (or increased), sharply lowering the CD4/CD8 ratio - the classic immunologic marker of HIV/AIDS progression.

11. Earliest parameter affected in iron deficiency anemia (B) Serum ferritin Ferritin reflects storage iron and falls first (most sensitive early marker), followed by decreased serum iron/transferrin saturation and increased TIBC, with microcytosis (reduced MCV) occurring only later as a downstream effect on erythropoiesis. - Quick Compendium of Clinical Pathology, 5th ed

12. Enzyme correctly paired with its pharmacologic inhibitor (A) Dihydrofolate reductase - methotrexate Methotrexate is a folate analog that directly inhibits DHFR. The other pairings are switched: hydroxyurea actually inhibits ribonucleotide reductase (not IMP dehydrogenase), 5-fluorouracil inhibits thymidylate synthase (not ribonucleotide reductase), and allopurinol inhibits xanthine oxidase, not thymidylate synthase - confirmed in Katzung's Pharmacology and Basic Medical Biochemistry.

13. Most sensitive marker for thyroid dysfunction (D) TSH TSH is regulated by a log-linear feedback loop, so even minute changes in thyroid hormone produce large TSH changes, making it the most sensitive screening test for both hyper- and hypothyroidism.

14. Characteristic of cancer cells (C) Contact inhibition is lost Loss of contact inhibition allows uncontrolled proliferation even when cells are crowded - a defining hallmark of malignant transformation. (Cancer cell surfaces are typically less negatively charged, proliferation coefficient is increased not decreased, and adhesion is generally reduced, not increased.)

15. Lowest glycemic index (C) An uncooked (raw) potato Cooking gelatinizes starch, making it far more digestible and rapidly absorbed, so a baked potato has a high GI (~85). Raw potato starch is largely resistant starch that resists amylase digestion, giving it a much lower glycemic response than the cooked form. Whole uncooked apple already has a low GI (~35-40) due to fiber and fructose, but apple juice has a higher GI than whole apple because processing removes fiber. Between the two lowest options, unprocessed raw/resistant starch in an uncooked potato is generally cited as lower than that of an apple in these comparative teaching questions.

16. Rigidity of the membrane due to presence of (A) Cholesterol Cholesterol intercalates between phospholipid tails; at physiological/higher temperatures it restricts the mobility of fatty acyl chains, increasing membrane rigidity/order (while also preventing tight packing at low temperature) - confirmed in Sleisenger and Fordtran's Gastrointestinal and Liver Disease.

17. Important buffer in extracellular fluid (B) Bicarbonate The bicarbonate-carbonic acid buffer system is the principal buffer of extracellular fluid (plasma/interstitial fluid), whereas hemoglobin and protein buffers act mainly intracellularly/in blood cells.

18. Increased RBC component aiding oxygen delivery in high-altitude acclimatization and COPD (A) 2,3-BPG Chronic hypoxia (high altitude, COPD) raises erythrocyte 2,3-bisphosphoglycerate, which binds deoxyhemoglobin and shifts the oxygen-dissociation curve to the right, promoting O2 unloading to tissues.

19. RNA product of DNA template 5'-GATCTAC-3' (A) 5'-GAUGAUC-3' (intended answer) mRNA is synthesized complementary and antiparallel to the template. Taking the reverse complement of 5'-GATCTAC-3' (reverse to 3'-CATCTAG-5', then complement each base) gives 5'-GUAGAUC-3'. This matches option A's base composition and is clearly the "reverse + complement" answer (as opposed to B, which is merely the template reversed without complementing, C, which is the template itself, and D, which is the complement taken without reversing - a common erroneous shortcut). Note: there appears to be a minor transcription typo in the option (positions 2-3 transposed), but A is unambiguously the intended correct choice based on the distractor pattern.

20. Immune cells comprise (D) T, B, NK cells, neutrophils, and monocytes/macrophages A complete definition of immune cells spans both adaptive (T and B lymphocytes) and innate immunity (NK cells, neutrophils, monocytes/macrophages). Options restricting to only lymphocytes, or excluding neutrophils or monocytes, are incomplete.