Primary Adrenal Insufficiency (Addison Disease)

A 42-year-old man with a 3-month history of fatigue, weight loss, and postural dizziness is found to have serum sodium of 128 mEq/L, potassium of 5.8 mEq/L, and fasting blood glucose of 65 mg/dL. ACTH level is elevated at 85 pg/mL (normal <46). A non-contrast CT abdomen shows bilateral adrenal enlargement. Which of the following is the most likely diagnosis?