Which finding indicates a negative response to a desmopressin challenge test?

The finding that indicates a negative response to a desmopressin challenge test is persistently low urine osmolality. Desmopressin is a synthetic analog of vasopressin (antidiuretic hormone), and it is often used in the diagnosis and management of conditions like central diabetes insipidus. When desmopressin is administered, a normal response in patients with central diabetes insipidus would typically be an increase in urine osmolality due to enhanced water reabsorption in the renal collecting ducts.

In a negative response, where there is no change in urine osmolality despite administration of desmopressin, one would expect the urine osmolality to remain persistently low. This suggests that the kidneys are not responding correctly to the antidiuretic hormone, which could indicate a diagnosis such as nephrogenic diabetes insipidus, where the kidneys cannot respond to vasopressin, or a lack of functional receptors.

Overall, persistently low urine osmolality clearly signifies that the kidneys are not able to concentrate urine, highlighting an insufficient response to the desmopressin challenge.