Which of the following is an exogenous cause of Cushing's syndrome?

Rationale: A. Lung tumor: A lung tumor, especially small-cell carcinoma, can secrete ectopic ACTH, stimulating the adrenal glands to produce excess cortisol. This is considered an endogenous cause of Cushing’s syndrome rather than an exogenous source. B. Adrenal carcinoma: Adrenal tumors can directly increase cortisol production without regulation by ACTH. This is another endogenous cause of Cushing’s syndrome and is not related to external factors. C. Long-term use of glucocorticoids: Prolonged administration of glucocorticoid medications, such as prednisone, is the most common exogenous cause of Cushing’s syndrome. These drugs mimic cortisol activity and suppress normal hypothalamic-pituitary-adrenal function, leading to iatrogenic Cushing’s. D. Pituitary tumor: A pituitary adenoma producing excess ACTH leads to overstimulation of the adrenal glands and high cortisol levels. This represents an endogenous cause of Cushing’s syndrome known as Cushing’s disease.