A 72-year-old man presents with 3 weeks of intensely pruritic, tense blisters on his trunk and proximal extremities. He reports a burning sensation but denies oral pain or difficulty swallowing. Physical exam shows multiple large, tense bullae on an erythematous base; gentle lateral pressure on uninvolved skin does not cause epidermal sloughing. A punch biopsy demonstrates a subepidermal split, and direct immunofluorescence shows linear deposition of IgG and C3 along the dermal-epidermal junction. Which of the following is the most appropriate first-line therapy?

A. Begin high-potency topical corticosteroids B. Begin systemic prednisone therapy C. Start dapsone and eliminate gluten from the diet D. Order anti–tissue transglutaminase antibody testing E. Initiate a strict gluten-free diet only