## **Core Concept**
Cushing's syndrome is a clinical condition caused by excess levels of **corticosteroids**, particularly **cortisol**, in the body. This can result from various etiologies, including exogenous steroid use, ACTH-dependent and ACTH-independent causes. The question aims to identify the most common cause among these.

## **Why the Correct Answer is Right**
The correct answer, **C. Iatrogenic (steroid use)**, is the most common cause of Cushing's syndrome. Iatrogenic Cushing's syndrome occurs due to the prolonged use of **glucocorticoid** medications, which are prescribed for their potent anti-inflammatory and immunosuppressive effects. This form of Cushing's syndrome is more prevalent than the endogenous forms, which include ACTH-dependent (e.g., Cushing's disease due to pituitary adenoma) and ACTH-independent (e.g., adrenal tumors) causes.

## **Why Each Wrong Option is Incorrect**
- **Option A: Pituitary tumor (Cushing's disease)**: While this is a common endogenous cause of Cushing's syndrome, it is less common than iatrogenic causes. Cushing's disease accounts for about 15-20% of endogenous Cushing's syndrome cases.
- **Option B: Adrenal tumor**: Adrenal tumors causing Cushing's syndrome are relatively rare compared to iatrogenic causes. They can be benign (adenomas) or malignant (carcinomas) and lead to ACTH-independent Cushing's syndrome.
- **Option D: Ectopic ACTH-producing tumor**: This is a rare cause of Cushing's syndrome, usually associated with certain types of cancer, such as small cell lung cancer. It is much less common than iatrogenic causes.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that iatrogenic Cushing's syndrome, caused by the long-term use of **steroids**, is the most common form of the syndrome. When evaluating patients for Cushing's syndrome, it's crucial to first inquire about any history of steroid use, as this can often explain the clinical presentation.

## **Correct Answer: C. Iatrogenic (steroid use)**