MRCP2-0600

MRCP2-0600

A 42-year-old man suffered a mild head injury, was seen in the emergency department with a headache, and was discharged after a normal computed tomography scan of the head. He returned two months later with excessive thirst and frequent urination. He has a history of recurrent kidney infections for the past three years and is currently being evaluated by urologists. A water deprivation test was conducted, with the following results:

Water deprivation phase:
Plasma osmolality 335 mOsmol/kg (290–300 mOsmol/kg)
Urine osmolality 225 mOsmol/kg
Desmopressin (DDAVP) phase (after administration of 2 μg desmopressin):
Plasma osmolality 305 mOsmol/kg
Urine osmolality 260 mOsmol/kg

What is the diagnosis?