A 72-year-old man presents with 3 weeks of intense pruritus and several large blisters on his lower abdomen and proximal thighs. The blisters are tense and filled with clear fluid; a few have ruptured leaving shallow erosions. He denies oral pain or difficulty swallowing. On exam, there are multiple tense bullae on an erythematous base; gentle lateral pressure on uninvolved skin does not induce epidermal sloughing. A punch biopsy shows a subepidermal split, and direct immunofluorescence demonstrates linear deposition of immunoglobulin along the basement membrane zone. Which of the following is the most appropriate first-line therapy?

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