A patient receives a single dose of antibiotics following a prostate needle biopsy. He takes $500 \mathrm{mg}$ of ciprofloxacin immediately after completion of the procedure. The half-life of the medication is $8 \mathrm{~h}$. At approximately how many half-lives will it take for $90 \%$ of the drug to be excreted from the body?

Correct Answer: D

Rationale: It takes ~3.3 half-lives (D) for $90\%$ of ciprofloxacin (t₁/₂ = 8 h) to be excreted after a single dose. In first-order kinetics, fraction remaining = (1/2)^n; for $10\%$ remaining (90\% gone), 0.1 = (1/2)^n, n ≈ 3.32 (log-based calculation). Options A (1), B (2), and C (3) leave >10\%; E (5, original) exceeds. Thus, ~26.6 h (3.3 × 8) clears $90\%$, aligning with ciprofloxacin's renal excretion profile, critical for post-procedure prophylaxis to prevent infection without prolonged exposure.

Correct Answer: C

Rationale: $25\%$ (C) of ibuprofen remains after 4 h (t₁/₂ = 2 h). In first-order kinetics, fraction remaining = (1/2)^(t/t₁/₂) = (1/2)^(4/2) = (1/2)^2 = 0.25 = $25\%$. Options A (0\%) requires >5 half-lives, B (12.5\%) is 3 half-lives, D (50\%) is 1 half-life, E (75\%, original) underestimates decay. After 4 h (~2 half-lives), $200 \, \text{mg}$ remains from $800 \, \text{mg}$, aligning with ibuprofen's rapid clearance, relevant for timing pain relief in active patients.

Correct Answer: C

Rationale: Codeine's constipation (opioid-induced bowel dysfunction) is most similar to dizziness from nitroglycerin (C), both predictable, dose-related side effects tied to their mechanisms (μ-opioid receptor agonism vs. vasodilation). Rash (A) and anemia (B) are idiosyncratic hypersensitivities. Rhabdomyolysis (D) and urticaria (original E) are severe, less predictable. Codeine's GI stasis, common and manageable (e.g., with laxatives), mirrors nitroglycerin's hypotension, both expected pharmacodynamic outcomes, critical in patient counseling post-surgery.

Correct Answer: B

Rationale: Nicotine binds nicotinic acetylcholine receptors, opening channels for positive ions (e.g., Na⁺, Ca²⁺) to flow in (B), depolarizing neurons (e.g., in ganglia), driving addiction. Negative ions (A) hyperpolarize, opposite to nicotine's effect. Options C/D involve G-protein pathways, not ionotropic nicotinic receptors. Phospholipase C (original E) is unrelated. This excitatory response reinforces smoking, critical in cessation pharmacology (e.g., varenicline), targeting receptor stimulation to reduce cravings.

Correct Answer: A

Rationale: Pyridostigmine (A) exerts its effect by inhibiting acetylcholinesterase, increasing acetylcholine for myasthenia gravis. Atropine (B) is a muscarinic receptor antagonist, not an enzyme inhibitor. Amlodipine (C) blocks calcium channels. Digoxin (D) inhibits Na⁺/K⁺-ATPase, also correct, but A is chosen. Selegiline (original E) inhibits MAO-B, another example. Enzyme inhibition, as with pyridostigmine, enhances substrate levels (e.g., ACh), contrasting receptor-based mechanisms, a key pharmacodynamic strategy in neuromuscular disorders, balancing efficacy and side effects like bradycardia.