Hyperpituitarism

A 41-year-old woman comes to her outpatient provider because of skin changes that began three-months ago. She has noticed increased bruising and the development of purple-red stretch marks on the abdomen. Past medical history is notable for asthma, for which she intermittently uses a budesonide inhaler. She smokes half-a-pack of cigarettes per day. In the office, her temperature is 37.7°C (99.9°F), pulse is 71/min, respirations are 19/min, and blood pressure is 142/85 mmHg. Physical exam confirms the presence of ecchymoses and abdominal striae. Laboratory testing reveals the following findings:

Laboratory value	Result
Sodium	146 mmol/L
Potassium	3.9 mmol/L
Bicarbonate	26 mmol/L
Fasting glucose	137 mg/dL
24-hour urine cortisol*	72 µg/day
Serum ACTH**	35 pg/mL

*Normal range: 4-40 µg/day
**Normal range: 5-20 pg/mL

The serum cortisol levels are significantly reduced in response to a high-dose of dexamethasone. Which of the following is the most likely underlying pathophysiology for this patient’s condition?