NAPLEX QBANK Drug Interactions and Adverse Effects 2 — Questions and Answers
Question 1: A patient on metformin is scheduled for a CT scan requiring iodinated contrast. What is the primary pharmacokinetic concern?
- Contrast-induced nephropathy impairs metformin excretion, raising lactic acidosis risk (Correct answer)
- Allergic cross-reaction between metformin and iodine
- Contrast media reduces CT image quality in patients on metformin
- Iodinated contrast increases hypoglycemia risk
Correct answer: Contrast-induced nephropathy impairs metformin excretion, raising lactic acidosis risk
Iodinated contrast can precipitate acute kidney injury, which impairs metformin's renal elimination; the resulting metformin accumulation shifts glucose metabolism toward anaerobic pathways and risks potentially fatal lactic acidosis.
Question 2: Which serious musculoskeletal adverse effect is most strongly associated with fluoroquinolone antibiotics, particularly in elderly patients?
- Avascular necrosis of the femoral head
- Osteonecrosis of the jaw
- Tendinopathy and tendon rupture (Correct answer)
- Stress fractures
Correct answer: Tendinopathy and tendon rupture
Fluoroquinolones disrupt tenocyte metabolism and matrix metalloproteinase activity, predisposing tendons — most commonly the Achilles tendon — to degeneration and rupture, especially in older patients and those on corticosteroids.
Question 3: Rifampin is a potent inducer of which CYP450 isoenzyme, accounting for the majority of its clinically significant drug interactions?
- CYP2D6
- CYP3A4 (Correct answer)
- CYP2C19
- CYP1A2
Correct answer: CYP3A4
Rifampin is among the most potent inducers of CYP3A4, the most abundant hepatic CYP enzyme, dramatically increasing the metabolism of drugs such as oral contraceptives, warfarin, antiretrovirals, and cyclosporine.
Question 4: A post-MI patient is taking both warfarin and clopidogrel. What is the primary clinical concern of this combination?
- Reduced efficacy of both anticoagulant agents
- Substantially increased risk of major bleeding (Correct answer)
- Additive QT interval prolongation
- Synergistic nephrotoxicity
Correct answer: Substantially increased risk of major bleeding
Warfarin reduces clotting factor synthesis while clopidogrel inhibits platelet aggregation; their combination produces additive antithrombotic effects and significantly elevates the risk of major hemorrhagic events.
Question 5: Why does hypokalemia potentiate digoxin toxicity?
- Hypokalemia increases digoxin renal clearance
- Potassium and digoxin compete for the same binding site on Na+/K+-ATPase (Correct answer)
- Hypokalemia decreases digoxin plasma protein binding
- Low potassium independently suppresses cardiac conduction
Correct answer: Potassium and digoxin compete for the same binding site on Na+/K+-ATPase
Potassium competes with digoxin for binding to the cardiac Na+/K+-ATPase; when serum potassium is low, digoxin binds more avidly to the pump, amplifying both therapeutic and toxic effects at normal drug concentrations.
Question 6: Which antibiotic is contraindicated with tizanidine due to inhibition of CYP1A2, causing potentially dangerous elevations in tizanidine levels?
- Amoxicillin
- Ciprofloxacin (Correct answer)
- Clindamycin
- Azithromycin
Correct answer: Ciprofloxacin
Ciprofloxacin is a potent CYP1A2 inhibitor; since tizanidine is primarily metabolized by CYP1A2, this combination can cause a 10-fold increase in tizanidine exposure, leading to profound hypotension and sedation.
Question 7: Which metabolic adverse effect most commonly results from long-term systemic corticosteroid therapy?
- Primary hypothyroidism
- Hyperglycemia and steroid-induced diabetes (Correct answer)
- Dilutional hyponatremia
- Hypercalcemia
Correct answer: Hyperglycemia and steroid-induced diabetes
Corticosteroids stimulate hepatic gluconeogenesis, impair insulin signaling at peripheral tissues, and promote glycogenolysis, collectively causing hyperglycemia that can progress to steroid-induced diabetes mellitus.
NAPLEX QBANK Drug Interactions and Adverse Effects 2
A patient on metformin is scheduled for a CT scan requiring iodinated contrast.
What is the primary pharmacokinetic concern?