Cushing Syndrome — Moon Facies, Buffalo Hump, Purple Striae

A 36-year-old woman presents with a 12-month history of progressive weight gain in the face, neck, and abdomen with paradoxical thinning of her limbs. She reports easy bruising, proximal muscle weakness, amenorrhea, and emotional lability. On examination, she has a rounded plethoric moon facies, a dorsocervical fat pad, central obesity with thin limbs, and the feature marked **D** in the diagram. Laboratory investigations confirm elevated late-night salivary cortisol and failure of suppression on 1-mg overnight dexamethasone test. Plasma ACTH is elevated. Which of the following best explains the pathophysiology of the feature marked **D**?