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Cushing Syndrome — RACP Adult Medicine MCQ

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ModerateEndocrinology & DiabetesCushing SyndromeRACP Adult Medicine

A 40-year-old woman presents with weight gain, moon facies, purple striae, and proximal myopathy. Her 24-hour urinary free cortisol is 4 times the upper limit of normal. Low-dose dexamethasone suppression test fails to suppress cortisol. ACTH is suppressed (<5 pg/mL). What is the most likely source?

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Correct answer: D — Adrenal adenoma or carcinoma

ACTH-independent Cushing syndrome (suppressed ACTH with elevated cortisol) indicates an adrenal source – either adenoma (most common) or carcinoma. CT adrenals should be performed. Adrenal carcinoma is suspected if the lesion is >4 cm, has irregular margins, or shows rapid growth. Pituitary adenoma (Cushing disease) and ectopic ACTH both cause ACTH-dependent Cushing syndrome. Exogenous steroids suppress both cortisol and ACTH but the elevated UFC excludes this.

Reference: eTG – 2025 – Endocrinology; Endocrine Society – 2015 – Cushing Syndrome Guidelines