mrcem2-0505

You are following up on a 62 year old patient who had been referred to the medical team for investigation of electrolyte imbalances and unexplained symptoms. You observe that the patient has undergone a water deprivation test and the results are as follows:

Serum osmolality after fluid deprivation: 348 mOSm/Kg
Urine osmolality after fluid deprivation
but before desmopressin: 188 mOSm/Kg
Urine osmolality after desmopressin: 824 mOSm/Kg

What is the probable diagnosis?