On checkup, a 55-year-old Caucasian male with a past medical history significant for hypertension, presents with a constellation of signs and symptoms characteristic of Cushing syndrome. These...

On checkup, a 55-year-old Caucasian male with a past medical history significant for hypertension, presents with a constellation of signs and symptoms characteristic of Cushing syndrome. These symptoms include weight gain with truncal obesity, a buffalo hump, and moon facies as well as atrophic limbs and thin friable skin. A dexamethasone suppression test reveals failure of low and high doses of dexamethasone to suppress ACTH. What is the underlying etiology?

 A. ACTH secreting nonpituitary tumor

 B. ACTH secreting adrenal tumor

 C. Exogenous steroids

 D. Pituitary Cushing syndrome