An 18-year-old woman develops weakness, weight gain, amenorrhea, abdominal striae, and behavioral abnormalities. Physical examination reveals lateral visual field loss. Which of the following is the most likely diagnosis?

The patient has Cushing's syndrome secondary to an adrenocorticotropic hormone (ACTH)-secreting pituitary tumor. Relatively few of such patients have a large pituitary tumor that affects the visual pathways. Over 50% have a microadenoma, which is under 5 mm in diameter.