**Question:** Most common cause of Cushing's syndrome is -

A. Adrenal carcinoma
B. Adrenal adenoma
C. Pituitary adenoma
D. Exogenous corticosteroid use

**Core Concept:**
Cushing's syndrome is a condition characterized by hypercortisolism due to prolonged exposure to cortisol or its precursors. It is classified into three main types: ectopic ACTH syndrome, primary adrenal hypercortisolism, and secondary adrenal hypercortisolism.

**Why the Correct Answer is Right:**
The most common cause of Cushing's syndrome is exogenous corticosteroid use (Option D). When patients use corticosteroids for prolonged periods, they can develop Cushing's syndrome due to the suppression of the hypothalamic-pituitary-adrenal (HPA) axis. This leads to increased cortisol secretion, causing the symptoms and signs associated with hypercortisolism.

**Why Each Wrong Option is Incorrect:**

A. Adrenal carcinoma (Option A) and adrenal adenoma (Option B) are rare causes of Cushing's syndrome. Adrenal tumors usually secrete cortisol excessively, leading to primary adrenal hypercortisolism. However, they are not the most common cause of the condition.

C. Pituitary adenoma (Option C) can secrete ACTH, leading to ectopic ACTH syndrome, which is a rare cause of Cushing's syndrome. It is not the most common cause, which is exogenous corticosteroid use.

**Clinical Pearl:**
While adrenal and pituitary tumors can cause Cushing's syndrome, the most common cause is exogenous corticosteroid use. In this scenario, the HPA axis is suppressed, leading to increased cortisol secretion and the clinical features of hypercortisolism. Understanding the underlying mechanism helps differentiate between the various causes of Cushing's syndrome and appropriate management strategies.