Cushing Syndrome — Clinical Features

A 45-year-old woman presents with progressive weight gain over 2 years, with fat redistribution to the face and upper back. On examination, she demonstrates the classic features marked **A** in the diagram: moon facies, buffalo hump, and broad violaceous abdominal striae. Laboratory investigation reveals elevated 24-hour urinary free cortisol (4.2× upper limit of normal) and failure to suppress cortisol below 1.8 μg/dL on overnight 1 mg dexamethasone suppression test. Subsequent ACTH measurement is 8 pg/mL. Which of the following is the most appropriate next diagnostic step?

A 45-year-old woman presents with the clinical features marked **A** in the diagram—moon face with central obesity and purple striae. She has been on oral corticosteroids for 8 years for management of systemic lupus erythematosus. Her 24-hour urinary free cortisol is elevated, and overnight 1-mg dexamethasone suppression test shows a morning cortisol of 2.5 μg/dL. Which of the following is the most likely etiology of her Cushing syndrome?