A 30-year-old woman presents with features of Cushing syndrome and the following lab pattern: urinary free cortisol increased, serum cortisol (8 A.M.) increased, plasma ACTH decreased, dexamethasone suppression test shows no suppression with overnight low dose and no suppression with high dose. What is the most probable diagnosis?

• A. Pituitary adenoma
• B. Ectopic ACTH lung cancer
• C. Adrenocortical carcinoma
• D. Addison disease

Answer: (C)

The key idea is ACTH-independent Cushing syndrome caused by an adrenal tumor. When cortisol is high but ACTH is suppressed, the source of excess cortisol is the adrenal gland itself, not the pituitary or an ectopic ACTH-producing tumor. The dexamethasone suppression tests reinforce this: cortisol fails to decrease with both low-dose and high-dose dexamethasone, which is typical for an autonomous adrenal source rather than pituitary (which might show suppression with high-dose dex) or ectopic ACTH (which usually does not suppress). Among adrenal causes, an adrenal cortical tumor that autonomously produces cortisol fits this pattern best, making adrenocortical carcinoma the most probable diagnosis in this scenario.