The most common cause of Cushing's syndrome is:
Pituitary adenoma
Adrenal adenoma
Ectopic ACTH
Iatrogenic steroids
d. Iatrogenic steroids(Ref: Nelson's 20/e p 2227-2729, Ghai 8/e p 525)The most common cause of Cushing's syndrome is prolonged exogenous administration of glucocorticoid hormones, especially at the high doses used to treat lymphoproliferative disorders.
Endocrinology