**Core Concept:** The presented case describes a patient with Addison's disease, also known as primary adrenal insufficiency. Addison's disease involves the hypothalamic-pituitary-adrenal axis dysfunction, characterized by low cortisol and aldosterone levels, and specific clinical manifestations. Cosyntropin stimulation test measures cortisol response to synthetic corticotropin, while aldosterone level can be helpful in diagnosing primary adrenal insufficiency.

**Why the Correct Answer is Right:** The correct answer is C (hydrocortisone). This patient has primary adrenal insufficiency (Addison's disease), which is characterized by deficient cortisol production and normal aldosterone levels. Hydrocortisone is a synthetic glucocorticoid used to replace deficient cortisol in adrenal insufficiency cases, thus addressing the primary deficiency in this patient.

**Why Each Wrong Option is Incorrect:**

A. Fludrocortisone (aldosterone analog) is not the appropriate treatment for Addison's disease as it compensates for the deficient aldosterone, not cortisol.
B. Dexamethasone is a synthetic glucocorticoid, but it is not the standard therapy for primary adrenal insufficiency. Hydrocortisone is the preferred synthetic glucocorticoid for this purpose.
D. Prednisolone is another synthetic glucocorticoid, but it is not the first-choice medication for treating primary adrenal insufficiency due to its different pharmacokinetic and pharmacodynamic properties compared to hydrocortisone.

**Clinical Pearl:** It is essential to differentiate between aldosterone and cortisol deficiencies, as their treatment strategies differ. In Addison's disease, hydrocortisone replaces cortisol deficiency, while fludrocortisone addresses aldosterone deficiency.

**Why the Correct Answer is Right:** Hydrocortisone is the appropriate treatment for primary adrenal insufficiency (Addison's disease) as it is a synthetic glucocorticoid that replaces deficient cortisol production. Hydrocortisone mimics the physiological cortisol action and provides cortisol replacement therapy.

**Why Each Wrong Option is Incorrect:**

A. Fludrocortisone is an aldosterone analog and does not address cortisol deficiency. In primary adrenal insufficiency, treatment involves cortisol replacement, not aldosterone replacement.
B. Prednisolone is a synthetic glucocorticoid but is not the first-choice medication for primary adrenal insufficiency treatment due to its different pharmacokinetic and pharmacodynamic properties compared to hydrocortisone.
D. Dexamethasone is another synthetic glucocorticoid but also not the first-choice medication for primary adrenal insufficiency treatment since it has different pharmacokinetic and pharmacodynamic properties compared to hydrocortisone.