**Question:** A common cause of Cushing syndrome is:

A. Adrenal adenoma
B. Pituitary adenoma
C. Exogenous corticosteroid use
D. Unknown cause

**Core Concept:**
Cushing syndrome is a condition characterized by long-term exposure to high levels of cortisol, a hormone produced by the adrenal glands. It is classified into three types: Cushing's disease (e.g., due to ACTH-secreting pituitary adenoma), ectopic ACTH syndrome (e.g., due to lung or pancreas tumors), and exogenous corticosteroid use (e.g., due to long-term corticosteroid therapy).

**Why the Correct Answer is Right:**
Cushing syndrome due to pituitary adenoma (B) is known as Cushing's disease. Adrenocorticotropic hormone (ACTH) is produced by the pituitary gland and acts on the adrenal glands to stimulate cortisol production. When a pituitary tumor (adenoma) secretes excessive ACTH, it leads to high cortisol levels, resulting in Cushing syndrome.

**Why Each Wrong Option is Incorrect:**
A. Adrenal adenoma (A) is a rare cause of Cushing syndrome. These tumors secrete cortisol directly, bypassing the ACTH pathway and resulting in autonomous cortisol secretion.

C. Exogenous corticosteroid use (C) directly stimulates the adrenal glands to produce cortisol, mimicking the effects of ACTH and leading to Cushing syndrome. However, it is not a cause of endogenous Cushing syndrome.

D. Unknown cause (D) refers to cases where the underlying cause of Cushing syndrome remains undetermined despite thorough investigations. This option is not specific to a particular cause and is a more appropriate answer for cases without a known cause.

**Clinical Pearl:**
Cushing syndrome can be a challenging diagnosis due to its diverse etiologies. In clinical practice, it is essential to consider the different causes (pituitary, adrenal, and exogenous corticosteroid) and initiate appropriate investigations (e.g., cortisol suppression test, ACTH stimulation test) for a proper diagnosis.