A 34-year-old male from Delhi presents to the nephrology clinic with a 6-month history of polyuria (urine output 4–5 L/day) and polydipsia. Serum sodium is 148 mEq/L, serum osmolality 310 mOsm/kg, and urine osmolality 150 mOsm/kg despite clinical dehydration. A water deprivation test shows no change in urine osmolality. The patient denies recent head trauma or medications. What is the most appropriate next step in management?