USMLE step 2 Mcq 133: A 57 year old man has a 30 year old history of manic depressive episodes

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Question:
A 57 year old man has a 30 year old history of manic depressive episodes. He has been successfully treated with lithium. He begins drinking massive amounts of water. On some days, he drinks over 10 liters of water. His urine output is also very high. He sees his doctor and several tests are performed. His urine specific gravity is very low. His urine is negative for glucose and negative for ketones. His blood glucose is 100 mg/dl (normal 65-110 mg/dl) and his serum osmolality is 305 mosm/kg (normal 275-300 msOsm/l). His serum level of ADH is found to be normal. What is the most likely diagnosis?

a) diabetes mellitus
b) diabetes insipidus
c) psychogenic polydipsia
d) syndrome of inappropriate ADH secretion
e) oliguria


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