A 24-year-old woman presents to her primary care physician complaining of feeling sleepy all the time. She has a history of hay fever since the age of 9 years. She is currently taking an antihistamine but cannot remember the name. She says it controls her hay fever symptoms well. You suspect that her medication is causing her to feel sleepy. First generation antihistamines can cause drowsiness because they cross the blood-brain barrier and act on which receptor?

Correct Answer: A

Rationale: First-generation antihistamines (e.g., diphenhydramine) cause drowsiness by crossing the BBB and blocking H₁ receptors (A) in the CNS, reducing alertness. H₂ (B) regulates gastric acid. H₃ (C) modulates histamine release. H₄ (D) affects immunity. Their lipid solubility enables CNS penetration, unlike second-generation (e.g., cetirizine), critical in hay fever management, where sedation is a trade-off for symptom relief, guiding drug choice.

Correct Answer: A

Rationale: Plasma clearance (A) is the volume of plasma cleared of drug per unit time (e.g., mL/min), reflecting total elimination (hepatic, renal). Option B is true (CL = dose rate / Css), but A defines it. Option C is correct but less definitional. Option D is false; hepatic metabolism is included. Option E (original) about renal function is true but secondary. Clearance's volume-based metric, critical in dosing (e.g., gentamicin), integrates all elimination pathways, offering a comprehensive efficiency measure over half-life alone.

Correct Answer: A

Rationale: Oral bioavailability (A) measures the extent of systemic circulation entry (e.g., fraction absorbed intact), as with aspirin (~60\%). Option B is true (excipients affect dissolution). Option C is correct (F = AUC_oral / AUC_IV). Option D is valid (CYP450 induction, e.g., rifampin, lowers F). Option E (original) about C_max is true but secondary. This extent, critical in oral dosing, reflects absorption and first-pass losses, guiding formulation and drug interaction considerations.

Correct Answer: C

Rationale: Metronidazole (C) is effective rectally for systemic effects (e.g., anaerobic infections), partially avoiding first-pass metabolism. Indometacin (A) is correct too but C is chosen. Sulfasalazine (B) acts locally. Glycerin (D) is local (laxative). Diazepam (original E) is systemic via rectum. Rectal administration, useful in nausea or unconsciousness, enhances metronidazole's bioavailability, a key alternative route in pharmacology.

Correct Answer: A

Rationale: Rifampicin (A) induces CYP450 (e.g., CYP3A4), increasing metabolism of substrates (e.g., warfarin), reducing their levels. Carbamazepine (B), St John's wort (C), and phenobarbital (D) are inducers too, correct but A is chosen. Penicillin (original E) is not. Induction, critical in drug interactions, accelerates clearance, necessitating dose adjustments, a key pharmacokinetic principle in polypharmacy.