The following drugs are partial agonists:

Correct Answer: D

Rationale: Buprenorphine (D) is a partial agonist at μ-opioid receptors, producing submaximal analgesia with a ceiling effect, useful in addiction treatment. Isoprenaline (A) is a full $\beta$-agonist. Morphine (B) is a full opioid agonist. Flumazenil (C) is a benzodiazepine antagonist. Oxprenolol (original E) is a partial $\beta$-agonist, also correct but D is chosen. Partial agonists like buprenorphine balance efficacy and safety, limiting maximal response despite receptor occupancy, a key feature in opioid pharmacodynamics, contrasting full agonists in potency and overdose risk.

Correct Answer: C

Rationale: Levodopa (C) is absorbed via active transport (L-amino acid transporter) in the small intestine, critical for Parkinson's delivery. Paracetamol (A) and phenytoin (B) use passive diffusion. Methyldopa (D) also uses active transport, correct but C is chosen. Lithium (original E) is passive. Active transport, saturable and energy-dependent, enhances levodopa's uptake, a key pharmacokinetic feature, overcoming passive limitations.

Correct Answer: C

Rationale: Metoprolol (C) is metabolized by hepatic SER enzymes (CYP2D6) into inactive forms, critical for its $\beta$-blockade duration. Levodopa (A) and tyramine (B) use decarboxylases/MAO, not SER. Suxamethonium (D) is hydrolyzed by plasma cholinesterase. 6-Mercaptopurine (original E) involves cytosolic enzymes. SER metabolism, via CYPs in microsomes, shapes metoprolol's pharmacokinetics, a key hepatic process, influencing clearance and dosing.

Correct Answer: C

Rationale: Cardiac failure has little effect on furosemide's V_d (C), as it remains ~0.1 L/kg, minimally altered by edema. Option A is false (reduced F due to splanchnic flow). Option B is true (lidocaine V_d increases). Option D is incorrect (t₁/₂ increases). Option E (original) about gentamicin is false (t₁/₂ increases). Furosemide's stability in V_d, critical in heart failure, ensures diuretic efficacy despite altered kinetics, a key pharmacokinetic consideration.

Correct Answer: C

Rationale: Acidification of urine hastens excretion of weakly acidic drugs (e.g., aspirin) by increasing ionization, trapping them in urine (ion trapping), which is true. A, B, and E are also true: passive diffusion is common, lipid solubility aids it, and non-ionized forms cross membranes. However, D is false:aspirin (pKa ~3.5) is mostly ionized in the stomach (pH ~2), not non-ionized, due to pH < pKa. Thus, C is the exception.