A 52-year-old man with a known history of pulmonary tuberculosis (treated 2 years ago) presents to the outpatient clinic with a 6-week history of progressive weakness, anorexia, and recurrent episodes of dizziness. His wife reports that he has become increasingly withdrawn and forgetful. On examination, he is found to be hypotensive (BP 92/58 mmHg) and has darkening of the skin over the palms and creases. Serum sodium is 132 mEq/L and potassium is 5.2 mEq/L. A morning cortisol level is 3.5 µg/dL (normal > 10 µg/dL) and ACTH is 620 pg/mL. Which of the following is the most appropriate next step in management?