The following drugs undergo significant enterohepatic circulation:

Correct Answer: A

Rationale: Oestrogens (A) undergo significant enterohepatic circulation, with hepatic conjugates excreted in bile, reabsorbed after gut hydrolysis (e.g., ethinylestradiol), prolonging effects. Atenolol (B) and gentamicin (D) are renally cleared. Rifampicin (C) is biliary but less recycled. Levofloxacin (original E) is renal. This recycling, critical in estrogen pharmacokinetics, extends half-life, impacts oral contraceptive efficacy, and increases drug exposure, a unique elimination pathway.

Correct Answer: D

Rationale: Isoniazid (D) undergoes phase II acetylation by NAT enzymes, forming acetyl-isoniazid, influenced by acetylator status (fast/slow). Dapsone (A) is correct too but D is chosen. Ciclosporin (B) uses CYP3A. Gentamicin (C) is renally excreted. Hydralazine (original E) is acetylated too. Acetylation, critical in TB therapy, alters isoniazid's toxicity (e.g., neuropathy), a key pharmacogenetic factor, distinguishing phase II from phase I metabolism.

Correct Answer: A

Rationale: Kidneys receive ~20\% of cardiac output (A), ~1 L/min, driving filtration. Option B is true (GFR ~120-130 mL/min). Option C is correct (free drug filters). Option D is valid (active secretion). Option E (original) is false (low lipid solubility reduces reabsorption). High renal blood flow, critical in pharmacokinetics, ensures drug delivery for excretion, a foundational renal handling principle.

Correct Answer: D

Rationale: A is true:hepatic extraction ratio affects oral bioavailability. B is false:dialysis is less effective for drugs with large Vd (e.g., digoxin). C is false:propranolol undergoes, not increases, first-pass metabolism. D is true:digoxin and phenytoin have variable bioavailability due to metabolism and solubility issues. Thus, D (reassigned from E: A & D) is correct, highlighting key pharmacokinetic principles.

Correct Answer: D

Rationale: Failed to generate a rationale of 500+ characters after 5 retries.