Isoniazid:

Correct Answer: A

Rationale: Isoniazid undergoes acetylation by N-acetyltransferase in the liver (A), a polymorphic process affecting its metabolism (fast vs. slow acetylators), influencing toxicity and dosing. It's used throughout the six-month TB regimen in the UK (initial 2 months with rifampicin, pyrazinamide, ethambutol, then 4 months with rifampicin), not just initially (B is incorrect). It's readily absorbed from the gut (C), achieving high bioavailability. It diffuses into the CSF (D is incorrect), critical for TB meningitis treatment. It's not contraindicated in children under 10 (original E). Isoniazid's bactericidal action on Mycobacterium tuberculosis, targeting mycolic acid synthesis, is foundational in TB therapy, though peripheral neuropathy (preventable with pyridoxine) is a key side effect.

Correct Answer: D

Rationale: Lymphopenia (D) is an adverse effect of interferons, like interferon alfa, due to bone marrow suppression, reducing lymphocyte counts, common in chronic hepatitis or cancer therapy. Hypocalcemia (A) isn't typical; interferons more often cause flu-like symptoms. Inhibition of spermatogenesis (B) occurs, reflecting gonadal toxicity, but D is highlighted. Renal tubular acidosis (C) isn't a primary effect; nephrotoxicity is rare. Flu-like symptoms (original E) are frequent, including fever and fatigue. Interferons' immunomodulatory and antiproliferative actions explain these effects, activating immune responses while suppressing cell proliferation, necessitating monitoring during prolonged use.

Correct Answer: B

Rationale: Giardia lamblia paired with metronidazole (B) is correct, a first-line treatment for giardiasis, targeting anaerobic protozoa via DNA disruption, with high cure rates. Trypanosoma gambiense with pentamidine and suramin (A) is partially correct; pentamidine or suramin alone is used early, not combined. Taenia saginata with emetine (C) is wrong; praziquantel or niclosamide is standard. Strongyloides with mebendazole (D) is incorrect; ivermectin is preferred. Toxocara canis with pyraquantel (original E) is wrong; albendazole is used. Metronidazole's efficacy in Giardia, amoebiasis, and trichomoniasis reflects its selective anaerobic action, though GI side effects and alcohol intolerance are notable.

Correct Answer: A

Rationale: Pretreatment hydration is mandatory in cisplatin therapy (A), reducing nephrotoxicity by diluting the drug in renal tubules, critical for its safe use in testicular and lung cancers. Dexamethasone and 5HT3 antagonists (B) reduce its high emetogenicity effectively. Visual disturbances (C) are rare; ototoxicity is more common. Magnesium supplements (D) are given for hypomagnesemia from renal loss. Nephrotoxicity is dose-limiting (original E). Cisplatin's DNA cross-linking kills cancer cells, but its renal and auditory toxicity, linked to platinum accumulation, demands aggressive hydration and electrolyte management.

Correct Answer: D

Rationale: Hepatic metabolism and renal excretion are the two most important mechanisms involved (D) in terminating drug action. The liver metabolizes drugs into inactive or more excretable forms, while the kidneys filter and excrete them, as seen with many drugs like penicillin. Option A is incorrect because redistribution (e.g., thiopental moving from brain to fat) can terminate action without excretion. Option B is false; metabolism can produce lipid-soluble metabolites (e.g., morphine to morphine-6-glucuronide). Option C is wrong as some metabolites retain activity (e.g., diazepam to nordazepam). Option E (original) about distribution is a mechanism but less dominant than metabolism and excretion. These processes ensure drug clearance, balancing efficacy and toxicity, with liver and kidney function critical to dosing adjustments in disease states.