29th September 2021

A 32-year-old female presents to her General Practitioner (GP) with a 2-month history of polyuria and polydipsia

A 32-year-old female presents to her General Practitioner (GP) with a 2-month history of polyuria and polydipsia. She excretes upwards of 3L of urine a day despite trying to limit her liquid intake. She has a BMI of 23 and is otherwise healthy. Her standing vs sitting blood pressure has a marked difference of 30mmHg. The GP carries out a urine dipstick which comes back negative.

A series of blood tests are sent off, which come back showing:

• Ca2+: 2.3 (2.1-2.6)
• Na+: 150 (133-146)
• K+: 4 (3.5-5.3)
• Serum osmolality: 300 mOsm/kg (>290)

The patient undergoes a water deprivation test which shows:

• Urine osmolality (mOsm/kg):
• Before fluid deprivation: <300
• After fluid deprivation: <300.
• An AVP (desmopressin) stimulation test shows:
• Urine osmolality: 800 mOsm/kg

Given the above information, what is the most likely diagnosis?

Select the SINGLE most appropriate option from the list below. Select ONE option only.