Which is the most common cause of Cushing syndrome
**Core Concept**
Cushing syndrome is a rare endocrine disorder caused by excess levels of cortisol in the body. It can result from various etiologies, including exogenous steroid use, adrenal tumors, and pituitary pathologies.
**Why the Correct Answer is Right**
The most common cause of Cushing syndrome is **ACTH-dependent Cushing syndrome**, specifically due to **Cushing's disease**, which is caused by an **adenoma of the pituitary gland**. This condition leads to excessive production of **Adrenocorticotropic hormone (ACTH)**, which in turn stimulates the adrenal glands to produce excessive cortisol. The pituitary adenoma secretes ACTH autonomously, resulting in high cortisol levels.
**Why Each Wrong Option is Incorrect**
**Option A:** Exogenous steroid use is a common cause of Cushing syndrome, but it is not the most common cause, as it requires intentional administration of steroids.
**Option B:** Adrenal tumors, such as adrenal adenomas or carcinomas, can cause Cushing syndrome, but they are less common than pituitary adenomas.
**Option D:** Ectopic ACTH-producing tumors, such as small cell lung carcinoma, can also cause Cushing syndrome, but they are relatively rare compared to Cushing's disease.
**Clinical Pearl / High-Yield Fact**
Cushing's disease, the most common cause of Cushing syndrome, often presents with subtle symptoms, such as weight gain, hypertension, and glucose intolerance. It is essential to consider Cushing's disease in the differential diagnosis of patients with these symptoms, particularly those with a history of menstrual irregularities or infertility.
**Correct Answer: C. Adenoma of the pituitary gland.**