A 60-year-old man presents with polyuria and polydipsia. He has no history of diabetes. His serum sodium is 148 mmol/L (135-145) and serum osmolality is 305 mOsm/kg (275-295). His urine osmolality is 150 mOsm/kg (300-900). A water deprivation test is performed. After 8 hours, his urine osmolality remains low (160 mOsm/kg). Administration of Desmopressin (DDAVP) increases urine osmolality to 600 mOsm/kg. What is the most likely diagnosis?