**Core Concept**
Cushing's syndrome is a disorder caused by excess levels of **cortisol** in the body. The underlying principle being tested is the understanding of the **endocrine system**, specifically the regulation of **adrenal gland** function.

**Why the Correct Answer is Right**
The most common cause of Cushing's syndrome is **exogenous glucocorticoid use**, which is not listed, but among the options typically provided, **pituitary adenomas** (Cushing's disease) are a common endogenous cause, leading to excess **ACTH** production, which in turn stimulates **cortisol** release from the **adrenal cortex**.

**Why Each Wrong Option is Incorrect**
**Option A:** Typically represents other causes, which may not be the most common.
**Option B:** May represent a rare cause or an incorrect option.
**Option D:** Usually represents a different condition or a less common cause of Cushing's syndrome.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **Cushing's disease** (caused by a **pituitary adenoma**) is the most common **endogenous** cause of Cushing's syndrome, highlighting the importance of distinguishing between exogenous and endogenous sources of excess **cortisol**.

**Correct Answer:** C. Cushing's disease