A 45-year-old woman presents with progressive weight gain over 2 years, predominantly in the face and upper back. On examination, the structure marked **A** in the diagram is prominent. She also has proximal muscle weakness, purple abdominal striae, and hypertension. Her 24-hour urinary free cortisol is elevated at 180 μg/24 h (normal < 50), and her ACTH level is 65 pg/mL (normal 10–50). Which of the following is the most likely diagnosis?