Which of the following is NOT a cause of Cushing's syndrome?

Cushing's syndrome is characterized by excessive cortisol levels, which can result from various causes. Among the options provided, an adrenal adenoma is actually a known cause of Cushing's syndrome because it represents a benign tumor of the adrenal gland that autonomously produces cortisol.

In contrast, corticosteroid therapy can lead to secondary adrenal insufficiency but is also a common cause of iatrogenic Cushing's syndrome, while excess ACTH secretion by the pituitary (as seen in Cushing's disease) directly stimulates the adrenal glands to produce more cortisol. Small cell lung carcinoma is known for paraneoplastic syndromes, where it can produce ACTH or ACTH-like substances, leading to increased cortisol production and ultimately causing Cushing's syndrome.

Thus, the question asks for a phenomenon that does not contribute to Cushing's syndrome, and the correct interpretation indicates that an adrenal adenoma is indeed a legitimate cause, making it incorrect to identify it as the answer in this context.