A 44-year-old black male is brought to the emergency department with 6 h of worsening lethargy and confusion. Past medical history is significant for easy bruising, 3 months of bone pain, and frequent pneumococcal infections. Labs were ordered, revealing serum calcium of $17 \mathrm{mg} / \mathrm{dL}$ (normal: 9.0 to $10.5 \mathrm{mg} / \mathrm{dL}$ ). To rapidly lower his serum calcium, you administer calcitonin. However, calcitonin alone is insufficient because it is known to rapidly and suddenly lose its effectiveness within 2 to 3 days of repeated dosing. For this reason, a bisphosphonate, which take 2 to 3 days to become effective, is added simultaneously. What is the term for the rapid decrease in response to calcitonin?

Correct Answer: C

Rationale: Tachyphylaxis (C) is the rapid decrease in response to calcitonin after repeated dosing (2-3 days), likely from receptor downregulation or desensitization, common with peptide hormones. Anaphylaxis (A) is allergic. Prophylaxis (B) is prevention. Tolerance (D) is slower, chronic. Calcitonin's short-term calcium drop (hypercalcemia, 17 mg/dL) pairs with bisphosphonates' delayed onset, a strategic combo in multiple myeloma (suggested by history), highlighting tachyphylaxis's impact on acute management.

Correct Answer: B

Rationale: Mannitol (B) acts via osmotic diuresis, increasing urine volume without receptor binding, used in cerebral edema. Magnesium trisilicate (A) neutralizes gastric acid chemically, also correct but B is selected. Methotrexate (C) inhibits DHFR, an enzyme. Dimercaprol (D) chelates metals, non-receptor mediated too. Sumatriptan (original E) is a 5-HT₁ agonist. Non-receptor mechanisms, like mannitol's osmotic effect, bypass traditional signaling, critical in emergencies for rapid, physical action, contrasting receptor-based drugs in pharmacology.

Correct Answer: C

Rationale: Phenytoin (C) exhibits non-linear kinetics, with saturable hepatic metabolism (CYP2C9) causing half-life to increase at high doses (e.g., >300 mg/day). Aspirin overdose (A) and ethanol (D) are zero-order at high levels, also correct but C is chosen. Heparin (B) is complex, not purely non-linear. Ceftazidime (original E) is first-order. Non-linear kinetics, as with phenytoin, complicate dosing (e.g., toxicity risk), requiring monitoring, unlike linear drugs with predictable clearance.

Correct Answer: A

Rationale: Oxytetracycline (A) has increased bioavailability fasting, as food (e.g., calcium) chelates it, reducing absorption. Amoxicillin (B) is unaffected. Levodopa (C) competes with dietary amino acids, correct but A is chosen. Acetylsalicylic acid (D) and fluconazole (original E) are minimally impacted. Fasting enhances tetracycline uptake, critical in antibiotic therapy, avoiding food interactions that lower efficacy, a practical dosing consideration.

Correct Answer: A

Rationale: Ciclosporin (A) is a CYP3A substrate, metabolized in the liver, affecting its immunosuppressive levels (e.g., transplant therapy). Clarithromycin (B) is too, also correct but A is chosen. Phenytoin (C) uses CYP2C9. Adrenaline (D) is COMT/MAO-metabolized. Warfarin (original E) is CYP2C9. CYP3A's broad substrate range, critical in drug interactions (e.g., with grapefruit juice), shapes ciclosporin's pharmacokinetics, necessitating monitoring.