A patient is given 1 mg of dexamethasone at 11:00 PM; a plasma cortisol level recorded at 8:00 PM ...

ANS: C
The overnight dexamethasone suppression test, which is performed by administering dexamethasone as described, is used to diagnose Cushing's syndrome. In normal individuals, dexamethasone suppresses the release of adrenocorticotropic hormone (ACTH), thereby suppressing the synthesis and release of cortisol, which results in a low cortisol level. In patients with Cushing's syndrome, the cortisol level is normal or only mildly low. This is not an indication of Addison's disease, which is diagnosed by administering cosyntropin. Neither test is useful for diagnosing congenital adrenal hyperplasia or secondary adrenal insufficiency.