A 42-year-old woman with a history of recurrent nephrolithiasis presents with polyuria (urine output 4–5 L/day), polydipsia, and serum sodium of 148 mEq/L. Serum osmolality is 305 mOsm/kg and urine osmolality is 150 mOsm/kg. Which investigation is most appropriate to differentiate between central diabetes insipidus (CDI) and nephrogenic diabetes insipidus (NDI)?