Electrolyte Disorders USMLE Step 1 Practice Question
A 56-year-old woman with metastatic breast cancer presents with polyuria and polydipsia. She takes lithium for mood stabilization. Vital signs: BP 136/82, HR 92, RR 16, Temp 37.2°C, SpO2 98%. Labs show Na+ 148 mEq/L, serum osmolality 298 mOsm/kg, and urine osmolality 185 mOsm/kg despite desmopressin administration. Urine specific gravity remains inappropriately low. Which mechanism explains lithium-induced nephrogenic diabetes insipidus?
Answer choices
- ALithium increases GFR, causing osmotic diuresis and polyuria
- BLithium competitively blocks ADH receptors on the collecting duct
- CLithium inhibits adenylyl cyclase, reducing cAMP in collecting duct cellsCorrect answer
- DLithium depletes cellular magnesium in the loop of Henle
- ELithium induces hypothyroidism, reducing metabolic water reabsorption
- FLithium directly damages the osmotic gradient in the medullary interstitium by inhibiting the Na-K-ATPase in the thick ascending limb
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