**Core Concept**
Conn's disease is a rare endocrine disorder characterized by an excess of aldosterone, a mineralocorticoid hormone produced by the adrenal cortex. This excess leads to sodium retention, potassium excretion, and hypertension.

**Why the Correct Answer is Right**
The excess production of aldosterone in Conn's disease is due to an aldosterone-producing adenoma (APA) or hyperplasia of the zona glomerulosa in the adrenal cortex. The excess aldosterone promotes sodium retention in the distal convoluted tubules and collecting ducts of the kidneys, leading to volume expansion and hypertension. The increased aldosterone also stimulates potassium excretion, resulting in hypokalemia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a hormone or substance that is directly related to Conn's disease. While hyperaldosteronism is a key feature of the condition, the specific excess substance is aldosterone, not a more general term.

**Option B:** This option is incorrect because it refers to an excess of cortisol, which is a glucocorticoid hormone produced by the adrenal cortex. While an excess of cortisol can lead to Cushing's syndrome, it is not directly related to Conn's disease.

**Option C:** This option is incorrect because it refers to an excess of insulin, which is a hormone produced by the pancreas. Insulin excess is related to conditions such as insulinoma, not Conn's disease.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of Conn's disease is the presence of hypokalemia, which can lead to muscle weakness, fatigue, and cardiac arrhythmias. The diagnosis of Conn's disease can be confirmed by measuring the levels of aldosterone and potassium in the blood.

**Correct Answer:** A. Aldosterone.