**Core Concept**
Addison's disease is a rare endocrine disorder caused by the inadequate production of cortisol and, in some cases, aldosterone hormones. This condition arises due to the destruction or failure of the adrenal cortex, leading to a deficiency in these vital hormones.

**Why the Correct Answer is Right**
Addison's disease is characterized by a deficiency of cortisol and aldosterone hormones, which are crucial for maintaining blood pressure, electrolyte balance, and the body's response to stress. The adrenal cortex produces these hormones through the action of enzymes such as 17α-hydroxylase and aldosterone synthase. Cortisol plays a vital role in regulating metabolism, immune response, and blood pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as Addison's disease is indeed characterized by hyperkalemia due to the decreased production of aldosterone, which regulates potassium levels in the body.

**Option B:** This option is incorrect as Addison's disease is not typically associated with an increased level of cortisol. Instead, patients with Addison's disease often present with hypokalemia due to the decreased production of aldosterone.

**Option C:** This option is incorrect as Addison's disease is characterized by a deficiency of cortisol and aldosterone hormones, leading to symptoms such as fatigue, weight loss, and hypotension.

**Clinical Pearl / High-Yield Fact**
It's essential to note that patients with Addison's disease may present with non-specific symptoms, making it crucial to consider this diagnosis in patients with unexplained weight loss, fatigue, or hypotension.

**Correct Answer:**
C.