**Core Concept:** Cushing's Syndrome is a clinical condition caused by long-standing hypercortisolism, which leads to the symptoms and signs associated with this hormonal imbalance. The primary cause of Cushing's Syndrome is a tumor in the pituitary gland, specifically a pituitary adenoma, that secretes excessive amounts of adrenocorticotropic hormone (ACTH).

**Why the Correct Answer is Right:** The correct answer, **C. Pituitary Tumor**, is right because it is the primary cause of Cushing's Syndrome. When a pituitary tumor (adenoma) secretes excessive amounts of ACTH, it leads to excessive cortisol production in the adrenal glands, resulting in the characteristic features of Cushing's Syndrome.

**Why Each Wrong Option is Incorrect:**

A. **Adrenal Tumor (Option A)** is incorrect because, although an adrenal tumor can cause cortisol excess, it is not the typical cause of Cushing's Syndrome. The primary tumor producing ACTH, which leads to Cushing's Syndrome, is usually located in the pituitary gland (as mentioned in Option C).

B. **Exogenous Corticosteroids (Option B)** is incorrect because the question asks about a tumor associated with Cushing's Syndrome, not the cause. Exogenous corticosteroids refer to the intake of corticosteroids from external sources, such as medications, and are not a tumor.

D. **Ectopic ACTH Syndrome (Option D)** is incorrect because this is a rare variant of Cushing's Syndrome, caused by tumors outside the pituitary and adrenal glands. However, it is not the primary cause of Cushing's Syndrome as mentioned in Option C.

**Clinical Pearl / High-Yield Fact:** It is essential to understand the distinction between the primary causes of Cushing's Syndrome, such as pituitary adenomas (Cushing's Syndrome) and ectopic ACTH syndrome (a rare variant). This understanding helps in accurately diagnosing and treating the condition, ensuring appropriate management for patients with symptoms of Cushing's Syndrome.