A 72-year-old Indian woman with a 2-month history of recurrent blisters on the extensor surfaces of her elbows and knees presents to the dermatology clinic. The blisters are preceded by urticarial and eczematous lesions. She reports intense pruritus. On examination, some blisters have ruptured into erosions, but the majority remain intact and tense. Serum testing reveals negative anti-desmoglein 3 and anti-desmoglein 1 antibodies. Indirect immunofluorescence (IIF) on patient serum shows circulating IgG antibodies against the basement membrane zone. What is the next most appropriate diagnostic step?