A 45-year-old woman with a history of chronic diarrhoea and dehydration presents with polyuria (8 L/day) and polydipsia. Serum Na⁺ is 152 mEq/L, serum osmolality 310 mOsm/kg, and urine osmolality 200 mOsm/kg (inappropriately dilute). After a water deprivation test, urine osmolality does not increase above 210 mOsm/kg even after desmopressin administration. What is the most appropriate next step in management?