A 52-year-old woman with a 6-month history of progressive proximal muscle weakness, hypertension, and hirsutism is found to have elevated 24-hour urinary free cortisol and suppressed ACTH. Imaging reveals a 2 cm left adrenal mass. Which single feature best distinguishes adrenocorticotropic hormone (ACTH)-independent Cushing syndrome from ACTH-dependent Cushing syndrome in this patient?