Which condition is characterized by polyuria with low urine osmolality?

• A. Diabetes mellitus
• B. Syndrome of hyponatremia
• C. SIADH
• D. Diabetes insipidus (central or nephrogenic)

Answer: (D)

When urine is very dilute despite ongoing thirst and large volumes of urine, the problem is diabetes insipidus. In this condition the kidneys can’t concentrate urine because either there isn’t enough antidiuretic hormone acting properly (central diabetes insipidus) or the kidneys don’t respond to it (nephrogenic diabetes insipidus). The result is a lot of water staying in the urine, so the urine osmolality stays low (often well below 300 mOsm/kg) even as the body becomes dehydrated and serum osmolality rises.

This contrasts with diabetes mellitus, where polyuria comes from osmotic diuresis due to high glucose spilling into the urine, which tends to produce urine that is not preferentially dilute. SIADH features excess ADH leading to concentrated urine and hyponatremia, not dilute urine. Hyponatremia itself can have various causes, but it does not define the pattern of dilute urine with high-volume output seen in diabetes insipidus.

So the hallmark combination—polyuria with low urine osmolality—points to diabetes insipidus (central or nephrogenic).