Answer: B. Inhibition of thymidylate synthesis

Explanation: Fluorouracil acts by inhibiting thymidylate synthase, thereby interfering with DNA synthesis in rapidly dividing cells.

Answer: A. Rapamycin

Explanation: Rapamycin is a well-known immunosuppressant used to prevent organ transplant rejection and treat certain autoimmune disorders by inhibiting the mTOR pathway.

Answer: C. Propranolol

Explanation: Propranolol is a non-selective beta blocker that also exhibits some alpha-blocking activity, aligning with the answer key.

Answer: B. Terfenadine

Explanation: Terfenadine is known to cause prolongation of the QT interval by interfering with cardiac ion channels, which may lead to arrhythmias.

Answer: B. Urine formation

Explanation: Diuretics primarily increase urine formation by promoting the excretion of water and electrolytes from the body.

Answer: C. Antidepressants

Explanation: Thymoleptics refer to a group of antidepressant drugs that are used to treat mood disorders by altering neurotransmitter levels.

Answer: C. Quinone imine

Explanation: Paracetamol is metabolized to a reactive quinone imine intermediate, which can cause hepatic toxicity when glutathione stores are depleted.

Answer: C. Domperidone

Explanation: Domperidone is a prokinetic agent that actually increases gastrointestinal motility, unlike anticholinergic drugs which reduce it.

Answer: A. Methotrexate + Probenecid

Explanation: Probenecid is intentionally combined with methotrexate to reduce its renal clearance, making this a rational, not irrational, combination.

Answer: A. Streptomycin

Explanation: Tetracycline (not streptomycin) binds the 30S subunit and blocks the A-site, preventing aminoacyl-tRNA entry.

Answer: C. Rifampicin

Explanation: Rifampicin inhibits bacterial RNA polymerase, thereby blocking RNA synthesis. This unique mechanism makes it effective against a range of bacteria including Mycobacterium tuberculosis.

Answer: D. DPP4

Explanation: Sitagliptins are dipeptidyl peptidase-4 (DPP4) inhibitors that work by prolonging the action of incretin hormones, thereby improving blood glucose control in diabetes.

Answer: D. Pseudoephedrine

Explanation: Pseudoephedrine typically causes sympathomimetic side effects rather than increased salivation; the other combinations correctly match drugs with known adverse effects.

Answer: A. CYP 3A4

Explanation: CYP 3A4 is the most abundant and versatile cytochrome P450 enzyme in the liver, metabolizing a wide variety of drugs.

Answer: B. HMG CoA reductase

Explanation: Statins lower cholesterol levels by inhibiting HMG CoA reductase, the key enzyme in cholesterol biosynthesis, thereby reducing LDL cholesterol levels.

Answer: A. P and Q

Explanation: Rosiglitazone and Propoxyphene have been banned or restricted in many regions due to their significant cardiotoxic effects.

Answer: C. Isosteres

Explanation: Allopurinol and hypoxanthine are isosteres, meaning they have similar electronic and spatial properties despite being different molecules.

Answer: D. All of the above

Explanation: Nonlinear kinetics arise from saturation (high dose or decreased excretion) but not from increased excretion, so “All of the above” is incorrect.

Answer: A. Zero

Explanation: When carrier-mediated transport reaches saturation, the transport rate becomes independent of substrate concentration, exhibiting zero-order kinetics.

Answer: A. A

Explanation: Vitamin A is known for its teratogenic effects, and excessive intake during pregnancy can cause birth defects.