A 72-year-old man presents with 2 weeks of severe pruritus and a new blistering rash on his lower abdomen and flexor forearms. He denies recent travel or new soaps. Past medical history is notable for hypertension and osteoarthritis. Examination shows multiple large, tense bullae on an erythematous base with a few erosions from scratching. There are no oral lesions. Gentle lateral pressure on uninvolved skin does not cause epidermal sloughing (negative Nikolsky sign). A punch biopsy is performed.

Which of the following is the most appropriate first-line therapy for this patient?