A 41-year-old woman has progressive weight gain, proximal muscle weakness and easy bruising. Her 24-hour urinary free cortisol is 780 nmol/day (reference less than 165 nmol/day), plasma ACTH is 18 pmol/L (reference 2–11 pmol/L), and cortisol is not suppressed by low-dose dexamethasone. High-dose dexamethasone reduces cortisol by 60%. Which mechanism best accounts for the findings?
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