This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Medicine
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? 
Select and explain. 
A. a functional pituitary tumor, B. adrenal hyperplasia, C. anorexia nervosa with bulimia, D. glioblastoma multiforme
A: a functional pituitary tumor
Answer: A: a functional pituitary tumor 
Explanation: 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.