**Core Concept:** Addison's disease is a disorder of the endocrine system characterized by the inadequate production of cortisol and aldosterone by the adrenal glands. Cortisol and aldosterone play crucial roles in maintaining electrolyte balance, immune function, blood pressure, and glucose homeostasis.

**Why the Correct Answer is Right:** The drug of choice for Addison's disease is **hydrocortisone** (A). Hydrocortisone is a synthetic glucocorticoid that can replace the deficient cortisol and help maintain electrolyte balance, immune function, blood pressure, and glucose homeostasis.

**Why Each Wrong Option is Incorrect:**

A. **Fluids:** Adding fluids (Option B) does not directly treat the underlying adrenal insufficiency in Addison's disease. Instead, it addresses the symptoms like hypotension and dehydration caused by the disease.

C. **Fludrocortisone:** Fludrocortisone (Option C) is a mineralocorticoid used to replace aldosterone deficiency in Addison's disease. However, it is not as effective as hydrocortisone in addressing the broader range of physiological functions regulated by cortisol.

D. **Steroids:** Steroids (Option D) are a class of drugs, not a specific medication for Addison's disease. Hydrocortisone is a specific steroid that can effectively replace cortisol in patients with Addison's disease.

**Clinical Pearl:** Addison's disease should be considered in patients presenting with fatigue, weight loss, hypotension, hypokalemia, and hyperpigmentation. Early diagnosis and appropriate hormone replacement therapy are crucial to manage symptoms and prevent complications.

**Correct Answer:** The correct answer is **hydrocortisone** (A). Hydrocortisone is the drug of choice in Addison's disease as it replaces cortisol, the primary hormone affected in the disease, and addresses the broader range of physiological functions regulated by cortisol.