CPJE CPJE Drug Interactions and Therapeutics 2 — Questions and Answers
Question 1: Which class of antibiotics is avoided in patients with myasthenia gravis due to neuromuscular blockade?
- Fluoroquinolones (Correct answer)
- Penicillins
- Sulfonamides
- Tetracyclines
Correct answer: Fluoroquinolones
Fluoroquinolones can block neuromuscular transmission and exacerbate weakness in patients with myasthenia gravis.
Question 2: A patient on methotrexate is prescribed trimethoprim. What serious toxicity can result?
- Severe bone marrow suppression (Correct answer)
- Nephrotoxicity
- Hepatotoxicity only
- Pulmonary fibrosis
Correct answer: Severe bone marrow suppression
Both methotrexate and trimethoprim inhibit dihydrofolate reductase, and their combination causes additive antifolate toxicity including bone marrow suppression.
Question 3: Which substrate is most affected by rifampin co-administration due to strong CYP3A4 induction?
- Oral contraceptives (Correct answer)
- Aspirin
- Heparin
- Insulin
Correct answer: Oral contraceptives
Rifampin is a potent CYP3A4 inducer that significantly reduces plasma levels of oral contraceptives, risking contraceptive failure.
Question 4: Combining ACE inhibitors with potassium-sparing diuretics poses a risk of which electrolyte abnormality?
- Hyperkalemia (Correct answer)
- Hyponatremia
- Hypokalemia
- Hypercalcemia
Correct answer: Hyperkalemia
Both ACE inhibitors and potassium-sparing diuretics reduce potassium excretion, and their combination can cause dangerous hyperkalemia.
Question 5: A patient on phenytoin is started on fluconazole. What is the expected outcome?
- Phenytoin toxicity due to elevated levels (Correct answer)
- Reduced antifungal efficacy
- Subtherapeutic phenytoin levels
- No clinically significant interaction
Correct answer: Phenytoin toxicity due to elevated levels
Fluconazole inhibits CYP2C9, which metabolizes phenytoin, leading to accumulation and potential toxicity including nystagmus and ataxia.
Question 6: Which combination is most likely to cause prolonged QT interval and torsades de pointes?
- Azithromycin and haloperidol (Correct answer)
- Amoxicillin and metformin
- Lisinopril and losartan
- Prednisone and ibuprofen
Correct answer: Azithromycin and haloperidol
Both azithromycin and haloperidol independently prolong the QT interval, and their combination significantly increases the risk of torsades de pointes.
CPJE CPJE Drug Interactions and Therapeutics 2
Which class of antibiotics is avoided in patients with myasthenia gravis due to neuromuscular blockade?