Review Question - QID 216320

A 42-year-old woman presents to the clinic with a 6-month history of weight gain and fatigue. She notes amenorrhea for the past 3 months, which she attributes to menopause, as well as some difficulty climbing stairs. She has a history of cholecystitis for which she underwent an uncomplicated cholecystectomy 5 years ago. She also has seasonal allergies for which she takes loratadine. She has no other significant medical or surgical history and takes no other medications. She reports eating a healthy diet mainly consisting of lean meats, vegetables, and rice. She does not smoke tobacco, drink alcohol, or use recreational drugs. Her temperature is 96.8°F (37°C), blood pressure is 144/90 mmHg, pulse is 76/min, and respirations are 16/min. Physical exam is notable for a round face. The patient has a protuberant abdomen with striae. Thinning hair is noted when inspecting the scalp as well as a posterior cervical fat pad. There is also muscle wasting in the thighs bilaterally. Laboratory values are obtained as below.

24-hour free cortisol (urine): 1984 nmol/d (normal 120–384 nmol/d)
Morning adrenocorticotropic hormone (ACTH) concentration (serum): 2 pg/mL (normal 10-60 pg/mL)

Which of the following is most likely implicated as the source of this patient's symptoms?