Repeated USMLE Questions Step 1- 181

A 22 year-old man presents to the clinic due to excessive urination. He goes to the restroom almost every 15 minutes. His physical examination is otherwise normal.

Laboratory tests show:

Serum Na: 140 mEq/L

Urine output: 4 L/24 hours

Urine is negative for glucose and protein and no pus cells

Urine Na: 18 mEq/L

Urine osmolality: 260 mOsm/kg after 4h fluid deprivation.

What is the most likely diagnosis?

A-Diabetes mellitus

B-Diabetes insipidus

C-Psychogenic polydipsia

D-Urinary tract infection

E-Solute diuresis

Answer:







Urine deprivation test shows urine osmolality lower than 400 mOsm/kg, which is diagnostic for diabetes insipidus. It is >800 in normal person and >500 in polydipsia. Absence of glucose and pus in urine excludes DM and UTI respectively.

The correct answer is B