**Core Concept**
Adrenal Cushing syndrome, also known as ACTH-independent Cushing syndrome, is a condition characterized by excess cortisol production due to an adrenal gland disorder. This occurs when there is an overproduction of cortisol from the adrenal glands, which is not controlled by the normal regulatory mechanism of ACTH (Adrenocorticotropic hormone).

**Why the Correct Answer is Right**
The leading cause of ACTH-independent Cushing syndrome is adenoma of the adrenal gland, specifically a benign tumor that produces cortisol. This tumor can be unilateral (occurring in one adrenal gland) or bilateral (occurring in both adrenal glands). The adenoma produces cortisol autonomously, meaning it does not respond to normal feedback inhibition by cortisol, leading to an overproduction of cortisol.

**Why Each Wrong Option is Incorrect**
* **Option A:** Carcinoma of the adrenal gland is a rare cause of ACTH-independent Cushing syndrome, and not the leading cause.
* **Option B:** Pheochromocytoma is a type of tumor that occurs in the adrenal medulla and produces catecholamines, but it is not typically associated with ACTH-independent Cushing syndrome.
* **Option D:** Hyperplasia of the adrenal gland can lead to an overproduction of cortisol, but it is not the leading cause of ACTH-independent Cushing syndrome.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that ACTH-independent Cushing syndrome is typically associated with a unilateral adrenal mass, and imaging studies such as CT or MRI can help identify the location and size of the adenoma.

**Correct Answer: C. Adrenal adenoma. Adrenal adenoma.**