**Core Concept**
Cushing's Syndrome is a disorder caused by excess cortisol levels, often resulting from a tumour affecting the adrenal gland or hypothalamic-pituitary-adrenal axis. The underlying pathophysiology involves an overproduction of cortisol, leading to various systemic effects.

**Why the Correct Answer is Right**
The correct answer is related to the production of cortisol, a glucocorticoid hormone produced by the adrenal cortex. In Cushing's Syndrome, a tumour can lead to an overproduction of cortisol, which can be caused by an adrenal gland tumour (e.g., adrenal adenoma or carcinoma) or a pituitary tumour (e.g., ACTH-secreting adenoma). The excess cortisol production can be due to the increased expression of the enzyme steroidogenic acute regulatory protein (StAR) and the enzyme P450scc (side-chain cleavage enzyme), which are involved in the biosynthesis of cortisol.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not directly related to the production of cortisol in Cushing's Syndrome. While aldosterone is another hormone produced by the adrenal cortex, its overproduction is associated with a different condition, primary aldosteronism.

**Option B:** This option is not a correct answer, as the production of antidiuretic hormone (ADH) is associated with the posterior pituitary gland, not the adrenal gland. ADH is involved in water reabsorption in the kidneys, not cortisol production.

**Option D:** This option is not a correct answer, as the production of growth hormone (GH) is associated with the pituitary gland, not the adrenal gland. GH is involved in growth and development, not cortisol production.

**Clinical Pearl / High-Yield Fact**
In Cushing's Syndrome, it's essential to distinguish between the different causes of excess cortisol production, as the treatment and management strategies vary depending on the underlying cause. For example, an adrenal adenoma may require surgical resection, while an ACTH-secreting pituitary adenoma may require radiation therapy or medication to reduce cortisol production.

**Correct Answer: A. Cortisol**