**Core Concept**
Addison's disease is a rare endocrine disorder characterized by the inadequate production of cortisol and, in some cases, aldosterone by the adrenal cortex. This results from the destruction or dysfunction of the adrenal glands, often due to autoimmune disease, infections, or tumors.

**Why the Correct Answer is Right**
The drug of choice for Addison's disease is hydrocortisone, a synthetic glucocorticoid that mimics the body's natural cortisol production. Hydrocortisone is preferred because it has both glucocorticoid and mineralocorticoid effects, making it suitable for replacing the deficient hormones in Addison's disease. Hydrocortisone is administered orally and is usually given in a split dose regimen to mimic the body's natural circadian rhythm of cortisol secretion.

**Why Each Wrong Option is Incorrect**
* **Option A:** Fludrocortisone is a synthetic mineralocorticoid, but it lacks significant glucocorticoid activity, making it unsuitable as the primary treatment for Addison's disease.
* **Option B:** Prednisolone is a synthetic glucocorticoid, but it lacks mineralocorticoid activity, making it inadequate for replacing both deficient hormones in Addison's disease.
* **Option C:** Dexamethasone is a potent synthetic glucocorticoid, but it has a long duration of action and is not suitable for replacement therapy in Addison's disease due to its lack of mineralocorticoid activity and potential for causing hypernatremia.

**Clinical Pearl / High-Yield Fact**
In patients with Addison's disease, it is essential to replace both glucocorticoids and mineralocorticoids to avoid complications such as hypotension, hyponatremia, and hyperkalemia. Hydrocortisone's dual activity makes it an ideal choice for this purpose.

**Correct Answer:** C. Hydrocortisone