A 35-year-old woman experiences marked thirst and excessive urination after brain tumor surgery, with very low urine specific gravity. What is the most likely diagnosis?

• A. Diabetes insipidus
• B. Urinary incontinence
• C. Acute renal failure
• D. Chronic renal failure

Answer: (A)

Central diabetes insipidus is the most likely diagnosis. Brain tumor surgery can injure the hypothalamus or posterior pituitary, reducing or eliminating ADH release. Without ADH, the kidneys cannot concentrate urine, so large volumes are produced as very dilute urine, which drives marked thirst. This explains the combination of polyuria, polydipsia, and a very low urine specific gravity.

Urinary incontinence isn’t driven by a problem of urine concentration. Acute or chronic renal failure would alter kidney function and urine output differently, often with reduced or abnormal urine production and signs of azotemia or edema rather than dilute, copious urine. In DI, the key feature is dilute urine from lack of ADH-mediated water reabsorption.