A 21-year-old man presents to his primary care physician complaining of a single, painless ulcer on his penis, which he first noticed a few days ago. He admits to multiple sexual partners. You want to treat him for syphilis with penicillin G, but his history includes an itchy rash following amoxicillin treatment as a child. What must first occur in the body for a penicillin to become allergenic?

Correct Answer: B

Rationale: Penicillin becomes allergenic by binding to a larger molecule (e.g., proteins) (B), forming a hapten-carrier complex the immune system recognizes as foreign, triggering IgE-mediated Type I hypersensitivity (e.g., rash). First-pass metabolism (A) isn't required. Simple exposure (C) needs sensitization first. Option D is false; penicillin's allergenicity is IgE-driven. This conjugation, critical in syphilis treatment (penicillin G), necessitates allergy screening, as prior amoxicillin reaction suggests risk, guiding alternative therapy (e.g., doxycycline).

Correct Answer: A

Rationale: For first-order kinetics (A), elimination rate is proportional to plasma concentration (rate = k × Cp), as with most drugs (e.g., ibuprofen). Option B is true (exponential decline). Option C is false; half-life is constant. Option D is incorrect; GFR affects renally cleared drugs. Option E (original) about excretion composition is true but less central. This proportionality ensures predictable clearance (e.g., t₁/₂ = 0.693/k), critical in dosing regimens, distinguishing first-order from zero-order saturation.

Correct Answer: A

Rationale: Levodopa (A) is a prodrug, converted to dopamine by DOPA decarboxylase in the brain for Parkinson's. Azathioprine (B) becomes 6-mercaptopurine, also correct but A is chosen. Penciclovir (C) is activated to penciclovir triphosphate. Sulfasalazine (D) splits into 5-ASA. Enalapril (original E) forms enalaprilat. Prodrugs like levodopa enhance delivery (e.g., BBB crossing), critical in pharmacokinetics, improving efficacy over active forms.

Correct Answer: D

Rationale: Fentanyl (D) is administered transcutaneously (patches) for systemic analgesia, delivering steady levels. GTN (A) and estradiol (B) are correct too but D is chosen. Lidocaine (C) is local. Nicotine (original E) is systemic. Transdermal delivery, bypassing GI/hepatic metabolism, suits fentanyl's chronic pain management, critical in pharmacokinetics for sustained release, avoiding peaks/troughs.

Correct Answer: B

Rationale: Grapefruit juice (B) inhibits CYP3A4, raising levels of substrates (e.g., statins). Fluvoxamine (A) inhibits CYP1A2, correct too but B is chosen. Digoxin (C) is not an inhibitor. Itraconazole (D) and ciprofloxacin (original E) inhibit CYP3A4/CYP1A2. Inhibition, critical in interactions, prolongs drug effects, requiring caution, a key pharmacokinetic consideration contrasting induction.