An 82-year-old man presents with a 1 and a half-week history of severe pruritus and eczema. His past medical history includes atopic dermatitis, thyroid disease, and multiple sclerosis. On physical examination, he has dozens of 1-2 cm tense bullae and several erosions over his upper and lower extremities. His oral mucosa is not involved. Nikolsky sign is negative. A skin biopsy is taken and anti-BP 180 is detected in his blood. A potent topical steroid is prescribed. The most likely diagnosis is