A 54-year-old male presents with facial rounding ('moon facies'), proximal muscle weakness, purple abdominal striae, and newly diagnosed hypertension. A 24-hour urinary free cortisol test confirms hypercortisolism. High-dose (8 mg) dexamethasone suppression testing demonstrates an 80% reduction in plasma cortisol levels, and plasma ACTH is elevated. What is the specific underlying etiology?

Answer and explanation

Correct answer: A. ACTH-secreting pituitary microadenoma (Cushing Disease)

  1. Step 1: Cushing disease refers specifically to hypercortisolism driven by an ACTH-secreting pituitary adenoma.
  2. Step 2: Because pituitary adenomas retain partial negative-feedback responsiveness, high-dose dexamethasone suppresses ACTH and cortisol secretion by > 50%, distinguishing it from ectopic ACTH tumors (which do not suppress) and adrenal tumors (where ACTH is suppressed at baseline).
Answer
ACTH-secreting pituitary microadenoma (Cushing Disease).
Why not the others: 'Primary adrenal adenoma' produces autonomous cortisol with suppressed ACTH and will not suppress with high-dose dexamethasone; 'Ectopic ACTH secretion' (e.g., lung neuroendocrine tumors) is autonomous and resistant to high-dose dexamethasone suppression; 'Exogenous corticosteroid' causes Cushingoid features with suppressed ACTH and low endogenous cortisol.

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