A 72-year-old man presents with 3 weeks of progressively worsening, intensely pruritic blisters on his trunk and proximal extremities. He reports burning and itching before the blisters appear. Exam shows multiple large, tense bullae on an erythematous base and several urticarial plaques. No oral lesions are present. Gentle lateral pressure on uninvolved skin does not cause epidermal sloughing. A skin biopsy shows a subepidermal split with numerous eosinophils; direct immunofluorescence demonstrates linear deposition of IgG and C3 along the basement membrane zone.

Which of the following is the most appropriate first-line treatment?