CNS Pharmacology USMLE Step 1 Practice Question
A 34-year-old woman with bipolar I disorder has been stable on lithium 900 mg daily for 3 years. She presents with tremor, confusion, and ataxia. Temperature is 37.1°C, blood pressure is 104/68 mmHg, and heart rate is 96/min. Laboratory studies show a serum lithium level of 2.8 mEq/L (normal: 0.6–1.2) and serum creatinine of 0.9 mg/dL, indicating normal renal function. She reports recent gastroenteritis with significant diarrhea, for which she was prescribed a thiazide diuretic five days ago. Which of the following best explains the elevated lithium level?
Answer choices
- AThiazide diuretics decrease glomerular filtration and lithium renal clearanceCorrect answer
- BLithium protein binding is decreased due to hypoalbuminemia
- CLithium metabolism via hepatic glucuronidation is inhibited
- DDiarrhea increases lithium absorption in the small intestine
- EGastroenteritis directly inhibits the Na-K-ATPase pump
- FThiazide diuretics increase proximal tubule reabsorption of lithium through enhanced sodium reabsorption
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