**Core Concept**
Addison's disease is a rare endocrine disorder characterized by primary adrenal insufficiency, where the adrenal glands fail to produce sufficient cortisol and aldosterone. This can be due to various causes affecting the adrenal glands or the hypothalamic-pituitary-adrenal (HPA) axis.

**Why the Correct Answer is Right**
The most common cause of Addison's disease is autoimmune adrenalitis, where the immune system mistakenly attacks the adrenal glands, leading to their destruction. This condition is often associated with autoimmune polyendocrine syndrome (APS) type 1 or 2, where multiple endocrine glands are affected. The autoimmune response targets the 21-hydroxylase enzyme, a crucial component of the steroidogenesis pathway, resulting in cortisol and aldosterone deficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because tuberculosis is a secondary cause of Addison's disease, typically occurring in areas with high prevalence of TB. While it is a significant cause, it is not the most common one.
**Option B:** This is incorrect because congenital adrenal hyperplasia (CAH) is a genetic disorder affecting the steroidogenesis pathway, but it is not the most common cause of Addison's disease.
**Option C:** This is incorrect because metastatic cancer can cause secondary adrenal insufficiency, but it is not the most common cause of primary Addison's disease.

**Clinical Pearl / High-Yield Fact**
Autoimmune adrenalitis is often associated with other autoimmune disorders, such as type 1 diabetes, Hashimoto's thyroiditis, or vitiligo. Recognizing these associations can help clinicians diagnose and manage patients with Addison's disease.

**Correct Answer: A. Autoimmune adrenalitis.**