A 34-year-old female is referred by her primary care provider for evaluation of treatment-resistant depression. She has been trialed on two SSRIs and one SNRI over the past year with minimal improvement. She reports depressed mood, fatigue, difficulty concentrating, insomnia, and a 30-pound weight gain over 10 months despite no significant change in diet. On examination, she has central obesity with relatively thin extremities, a round plethoric face, and purple striae greater than 1 cm wide across her abdomen. She bruises easily, and her blood pressure is elevated at 152/96 mmHg. Laboratory findings reveal fasting glucose of 142 mg/dL, and a 24-hour urinary free cortisol level is 4 times the upper limit of normal. A low-dose dexamethasone suppression test fails to suppress morning cortisol. Which of the following is the most likely diagnosis?