A 23-year-old woman comes to the physician with the chief complaint of a depressed mood for 6 months. She states that she has felt lethargic, does not sleep well, and has decreased energy and difficulty concentrating. She notes that she has gained over 15 lb without attempting to do so, and seems to bruise much more easily than previously. On physical examination, she is noted to have numerous purple striae on her abdomen, proximal muscle weakness, and a loss of peripheral vision. A brain tumor is found on MRI. In which of the following areas of the brain was this tumor most likely found?

a. Frontal lobe

b. Cerebellum

c. Thalamus

d. Pituitary

e. Brainstem