A 38-year-old woman presents with progressive fatigue, weight loss, and hyperpigmentation. Investigations reveal serum sodium 128 mEq/L, potassium 5.8 mEq/L, and a low morning cortisol of 3 µg/dL with elevated ACTH of 85 pg/mL. A diagnosis of primary adrenal insufficiency (Addison disease) is confirmed. What is the drug of choice for glucocorticoid replacement in this patient?