Electrolyte Disorders USMLE Step 1 Practice Question
A 30-year-old man presents to the emergency department with a 3-week history of polyuria and polydipsia. He reports drinking approximately 4-5 liters of water daily and urinating frequently throughout the day and night. Vital signs: BP 138/88 mmHg, HR 98/min, RR 16/min, Temp 37°C, SpO2 98% on room air. Physical examination is unremarkable. Laboratory studies reveal: Serum sodium: 152 mEq/L (normal 136-145)
Serum osmolality: 310 mOsm/kg (normal 280-295)
Urine osmolality: 150 mOsm/kg (normal 300-900)
Urine sodium: 45 mEq/L
Creatinine: 1.0 mg/dL
TSH: 2.1 mIU/L The patient denies head trauma, medications, or recent illness. A water deprivation test is performed, which does not increase urine osmolality above 200 mOsm/kg. Administration of desmopressin (synthetic vasopressin) does not further increase urine osmolality. Which of the following is the most likely diagnosis?
Answer choices
- ACentral diabetes insipidus with incomplete response to desmopressin
- BNephrogenic diabetes insipidusCorrect answer
- CPrimary polydipsia (psychogenic)
- DHyperthyroidism with increased insensible losses
- EPrimary hyperaldosteronism
- FChronic lithium use causing nephrogenic damage
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