Derm Dx: Mouth rash and painful skin blisters

A Hispanic man, aged 60 years, complains of 12 months of painful blisters and erosions on his mouth and skin. He was treated in Mexico with oral prednisone, but the eruption returned on cessation of the medication. He was recently restarted on prednisone 50 mg daily. The sores make it difficult to eat. No other significant medical or family history ...