Diabetes Mellitus USMLE Step 1 Practice Question
A 41-year-old man with bipolar disorder on lithium presents with a 3-month history of polyuria and nocturia. Vital signs: BP 128/82, HR 88, RR 16, Temp 37°C, SpO2 98%. Labs show serum sodium 148 mEq/L, serum osmolality 305 mOsm/kg, and urine osmolality 180 mOsm/kg after water deprivation. Urine osmolality remains low even after desmopressin administration. Thyroid function tests are normal. Which mechanism best explains his clinical presentation?
Answer choices
- ADestruction of adrenal medulla chromaffin cells
- BAutoantibodies stimulating the TSH receptor
- CDeficiency of 21 hydroxylase
- DCollecting duct insensitivity to ADHCorrect answer
- EExcess pituitary ACTH production
- FHypothalamic osmoreceptor dysfunction with impaired ADH secretion
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