MTM Pharmacotherapy Review & Drug Interactions 2 — Questions and Answers
Question 1: A patient on warfarin begins fluconazole therapy. What is the expected effect on INR?
- INR will decrease due to CYP2C9 induction
- INR will increase due to CYP2C9 inhibition (Correct answer)
- No change; fluconazole does not affect warfarin
- INR will decrease due to increased warfarin clearance
Correct answer: INR will increase due to CYP2C9 inhibition
Fluconazole inhibits CYP2C9, the primary enzyme responsible for warfarin (S-enantiomer) metabolism, leading to elevated warfarin levels and increased INR.
Question 2: Which combination most significantly increases the risk of QT prolongation?
- Metformin + lisinopril
- Azithromycin + haloperidol (Correct answer)
- Atorvastatin + amlodipine
- Metoprolol + furosemide
Correct answer: Azithromycin + haloperidol
Both azithromycin and haloperidol independently prolong the QT interval, and their combination significantly raises the risk of torsades de pointes.
Question 3: A patient takes lithium and is started on ibuprofen for pain. What monitoring is most critical?
- Blood glucose
- Serum lithium levels (Correct answer)
- Thyroid function
- Hepatic enzymes
Correct answer: Serum lithium levels
NSAIDs reduce renal prostaglandin synthesis, decreasing renal blood flow and lithium clearance, which can lead to lithium toxicity.
Question 4: Which pharmacokinetic parameter best describes the fraction of a drug that reaches systemic circulation unchanged?
- Volume of distribution
- Bioavailability (Correct answer)
- Half-life
- Clearance
Correct answer: Bioavailability
Bioavailability (F) represents the fraction of an administered dose that reaches the systemic circulation in active form.
Question 5: Clopidogrel requires activation by which enzyme, and what common drug inhibits this conversion?
- CYP3A4; clarithromycin
- CYP2C19; omeprazole (Correct answer)
- CYP2D6; fluoxetine
- CYP1A2; ciprofloxacin
Correct answer: CYP2C19; omeprazole
Clopidogrel is a prodrug activated by CYP2C19; omeprazole inhibits CYP2C19 and can reduce active metabolite formation, diminishing antiplatelet effect.
Question 6: A patient on methotrexate is prescribed trimethoprim-sulfamethoxazole. What serious interaction must be anticipated?
- Reduced renal elimination of TMP-SMX
- Additive bone marrow suppression and increased methotrexate toxicity (Correct answer)
- Decreased methotrexate absorption
- Hepatotoxicity from sulfonamide accumulation
Correct answer: Additive bone marrow suppression and increased methotrexate toxicity
TMP-SMX inhibits dihydrofolate reductase (same mechanism as methotrexate) and displaces methotrexate from plasma proteins, causing additive myelosuppression and increased toxicity.
Question 7: Which statement best describes a Type B (idiosyncratic) adverse drug reaction?
- It is dose-dependent and predictable from the drug's pharmacology
- It is unpredictable, not dose-dependent, and unrelated to the drug's mechanism (Correct answer)
- It results from drug accumulation due to renal impairment
- It occurs only in patients with known drug allergies
Correct answer: It is unpredictable, not dose-dependent, and unrelated to the drug's mechanism
Type B (bizarre) ADRs are unpredictable, not dose-dependent, and cannot be anticipated from the drug's pharmacological actions; they include hypersensitivity and idiosyncratic reactions.
MTM Pharmacotherapy Review & Drug Interactions 2
A patient on warfarin begins fluconazole therapy.
What is the expected effect on INR?