American Board of Internal Medicine (ABIM) Certification Practice Exam

Which laboratory finding would suggest nephrogenic diabetes insipidus in a water restriction test?

1. Decreased serum osmolality

2. Increased urine concentration

3. Inadequate rise in urine osmolality

4. Hypotonic urine

The correct answer is: Inadequate rise in urine osmolality

The laboratory finding that suggests nephrogenic diabetes insipidus during a water restriction test is an inadequate rise in urine osmolality. In nephrogenic diabetes insipidus, the kidneys are unable to respond properly to the antidiuretic hormone (ADH), which plays a critical role in concentrating urine. During a water restriction test, the expectation in normal physiology would be that with increased plasma osmolality due to dehydration, the kidneys would concentrate urine in response to the stimulation from ADH. However, in nephrogenic diabetes insipidus, despite the presence of ADH, the kidneys fail to concentrate the urine adequately. As a result, there will be little to no rise in urine osmolality, indicating the kidneys are not able to respond appropriately. This phenomenon is a hallmark of nephrogenic diabetes insipidus and differentiates it from other forms of diabetes insipidus, such as central diabetes insipidus, which would typically show an appropriate response with a rise in urine osmolality after ADH administration. The other laboratory findings provided do not accurately reflect the pathophysiology of nephrogenic diabetes insipidus in this context. Decreased serum osmolality, increased urine concentration, and hypot