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Difficulty: Very hardGeneral Internal Medicine

Match each clinical vignette describing an adverse pharmacologic event to the most accurate underlying pharmacokinetic or pharmacodynamic mechanism of drug interaction.

  • A 71-year-old man with a mechanical mitral valve on chronic anticoagulation develops gross hematuria and an INR of 7.2 three days after starting empiric oral therapy for an uncomplicated urinary tract infection.Decreased cytochrome P450 2C9-mediated hepatic clearance
  • A 52-year-old woman with stable bipolar disorder and essential hypertension presents with new-onset ataxia, coarse tremors, and confusion three weeks after a first-line diuretic is added for blood pressure control.Enhanced proximal tubular reabsorption coupled with sodium
  • A 77-year-old man taking a lipid-lowering agent for coronary artery disease develops profound muscle weakness and an acute rise in serum creatinine after being treated for a severe chronic obstructive pulmonary disease exacerbation.Inhibition of cytochrome P450 3A4-mediated hepatic metabolism
  • A 49-year-old man taking an immunosuppressant for severe Crohn's disease develops acute, severe pancytopenia one month after starting a new daily medication to prevent recurrent attacks of podagra.Inhibition of xanthine oxidase-mediated drug degradation

Answer

The correct matches require identifying the specific drug-drug interaction mechanisms: TMP-SMX inhibits CYP2C9 (elevating warfarin), thiazides increase proximal reabsorption of sodium and lithium, clarithromycin inhibits CYP3A4 (elevating statins), and allopurinol inhibits xanthine oxidase (preventing azathioprine/6-MP degradation).
Each clinical scenario requires a two-step deduction: first, identifying the unstated offending drug, and second, identifying the specific pharmacokinetic mechanism. Warfarin is metabolized by CYP2C9, which is inhibited by TMP-SMX. Lithium is reabsorbed in the proximal tubule alongside sodium, a process enhanced when thiazides cause downstream volume depletion. Statins are metabolized by CYP3A4, which is potently inhibited by macrolides like clarithromycin. Azathioprine is converted to 6-mercaptopurine, which requires xanthine oxidase for degradation; allopurinol inhibits this enzyme, causing toxic accumulation and pancytopenia.

Step-by-Step Solution

1
Analyze the vignette of the patient with the mechanical valve and UTI.
The patient is on warfarin and was likely given trimethoprim-sulfamethoxazole (TMP-SMX) for the UTI.
Identifying the baseline high-risk drug and the likely new empiric antibiotic is required to deduce the mechanism.
2
Analyze the vignette of the bipolar patient with new hypertension.
The patient is on lithium and was likely given a thiazide diuretic.
Thiazides are common first-line antihypertensives that have a classic, severe interaction with renal lithium handling.
3
Analyze the vignette of the COPD patient on a lipid-lowering agent.
The patient is on a statin and was likely treated with a macrolide (e.g., clarithromycin) for the COPD exacerbation.
Macrolides are frequently used for respiratory tract infections and are notorious inhibitors of hepatic enzymes.
4
Analyze the vignette of the Crohn's disease patient with gout.
The patient is on azathioprine (or 6-mercaptopurine) and was started on allopurinol for gout prevention.
Gout prevention involves urate-lowering therapy, which can catastrophically interfere with purine analog metabolism.

Key Concept

Polypharmacy drug interactions in internal medicine often involve highly specific alterations in cytochrome P450 metabolism, renal tubular handling, or shared enzymatic degradation pathways.
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General Internal Medicine

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