A 40-year-old woman presents with central obesity, proximal muscle weakness, facial plethora, and wide violaceous striae on the abdomen. Midnight salivary cortisol is elevated on two occasions. Plasma ACTH is 85 pg/mL (normal below 10). High-dose dexamethasone suppression test shows suppression of 24-hour urinary cortisol by 60 percent. What is the most likely cause?

• A Adrenal adenoma
• B Pituitary microadenoma ✓
• C Small cell lung carcinoma
• D Adrenal carcinoma

Explanation

Elevated ACTH confirms ACTH-dependent Cushing syndrome. High-dose dexamethasone suppression exceeding 50 percent strongly suggests pituitary Cushing disease, as pituitary adenomas retain partial feedback sensitivity. Ectopic ACTH secretion from small cell lung carcinoma typically does not suppress. Adrenal adenoma and carcinoma are ACTH-independent causes with suppressed ACTH levels.

Reference: Williams Textbook of Endocrinology, 14th ed.

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