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What is the most common endogenous cause of Cushing's syndrome?
		A.	An adrenal tumor.
		B.	A liver tumor.
		C.	A lung tumor.
		D.	A pituitary tumor. Cushing's syndrome results from sustained hypercortisolemia. The most common cause is administration of exogenous glucocorticoids. Secretion of corticotropin from the pituitary (Cushing's disease) accounts for approximately 70% of endogenous cases; adrenal tumors and the ectopic production of corticotropin each account for approximately 15% of cases. The clinical and laboratory features of Cushing's syndrome overlap with many other medical conditions, and very few patients fulfill the classic presentation of facial rounding, weight gain, striae, hirsutism, hypertension, and muscle weakness. The majority of patients have abnormal glucose tolerance, but edema and hypokalemic alkalosis, as seen in this patient, occur in only a minority. The diagnosis of Cushing's syndrome requires the confirmation of hypercortisolism, generally with the measurement of 24-hour urinary cortisol excretion, measurement of midnight salivary cortisol levels, or both. Autonomous production of cortisol can be demonstrated with the use of a dexamethasone (1-mg) suppression test. Once hypercortisolism is established, a corticotropin level of more than 20 pg per milliliter (4.4 pmol per liter) suggests corticotropin dependency; a level below 5 pg per milliliter (1 pmol per liter) suggests an adrenal source. When corticotropin dependency is established, magnetic resonance imaging can identify pituitary tumors approximately 60% of the time. Answer: D