A patient with Addison disease treated with hydrocortisone is experiencing dehydration and hyponatremia. Which drug is best to add to the patient's therapy?

In the case of a patient with Addison's disease, the primary issue is the deficiency of adrenal hormones, particularly cortisol and aldosterone. Hydrocortisone is often prescribed to replace cortisol; however, it does not sufficiently address the lack of aldosterone, which regulates sodium and fluid balance in the body.

Adding fludrocortisone to the treatment regimen is the most appropriate choice for this patient experiencing dehydration and hyponatremia. Fludrocortisone is a synthetic mineralocorticoid that mimics the effect of aldosterone. It promotes sodium reabsorption in the kidneys, which helps to correct hyponatremia (low sodium levels), and it aids in fluid retention, thereby alleviating dehydration.

The other options, including dexamethasone, prednisone, and triamcinolone, are forms of glucocorticoids that primarily replace cortisol but do not provide the mineralocorticoid effects necessary to manage sodium and hydration levels. Therefore, fludrocortisone is the best addition to the therapy to specifically target the symptoms related to aldosterone deficiency in Addison's disease.