Which statements correctly describe ACTH-dependent etiologies of Cushing syndrome?

• A. ACTH-dependent etiologies include adrenal adenoma and carcinoma
• B. ACTH-dependent etiologies include only ectopic ACTH production
• C. ACTH-dependent etiologies include pituitary adenoma and ectopic ACTH
• D. There are no ACTH-dependent etiologies

Answer: (C)

ACTH-dependent etiologies are those in which the excess cortisol is driven by adrenocorticotropic hormone rather than by an adrenal source. In these cases, ACTH levels are elevated or inappropriately normal because the source of ACTH—either a pituitary tumor or an ectopic source—stimulates the adrenal cortex to overproduce cortisol. The two main ACTH-dependent causes are pituitary adenoma (Cushing disease) and ectopic ACTH production from non-pituitary tumors such as certain lung cancers. By contrast, adrenal tumors (adenoma or carcinoma) cause cortisol overproduction independently of ACTH, which suppresses ACTH via negative feedback, leading to low ACTH levels. So the statement that ACTH-dependent etiologies include pituitary adenoma and ectopic ACTH correctly identifies the sources that drive cortisol production through ACTH.