USMLE Step 1 Quiz
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PHARMACOLOGY
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A 58-year-old woman with epilepsy is stable on oral phenytoin with a total phenytoin concentration of 12 mcg/mL (therapeutic 10–20). She starts oral fluconazole for candidiasis. One week later she develops nystagmus and ataxia. Labs: total phenytoin 22 mcg/mL, albumin 4.0 g/dL, creatinine 0.9 mg/dL, AST 30 U/L. Phenytoin is hepatically metabolized and exhibits capacity-limited (zero-order) elimination at higher concentrations; small dose changes can cause disproportionate concentration increases. Which of the following best describes how the pharmacokinetics of phenytoin is altered in this patient?
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