<s>[INST] context: Blistering diseases are potentially serious conditions. Blisters that are smaller than 0.5 cm are termed vesicles; larger lesions are called bullae. The proper diagnosis and treatment of bullous disorders are paramount in order to prevent disability and even death from burn-like denudation of the skin and associated infection. Although many skin diseases such as allergic contact dermatitis, erythema multiforme, and bullous impetigo can cause blisters, this patient is more likely to have bullous pemphigoid or pemphigus. These are immunologically mediated disorders. Skin biopsy with immunofluorescence staining will reveal antibodies at the basal layer of the epidermis (bullous pemphigoid) or within the epidermis (pemphigus). Mucosal, especially oral, involvement is characteristic of pemphigus. Immunosuppressive agents including systemic corticosteroids are often necessary to treat these conditions. Antihistamines, sometimes helpful if itching is prominent, will not treat the underlying condition. It is no longer felt that bullous dermatoses are indicative of underlying malignancy, so a "shotgun" search for occult malignancy is not recommended. Dermatitis herpetiformis and porphyria cutanea tarda are other skin diseases that can be associated with blisters. [/INST] question: A 68-year-old man complains of several blisters arising over the back and trunk for the preceding 2 weeks. He takes no medications and has not noted systemic symptoms such as fever, sore throat, weight loss, or fatigue. The general physical examination is normal. The oral mucosa and the lips are normal. Several 2- to 3-cm bullae are present over the trunk and back. A few excoriations where the blisters have ruptured are present. The remainder of the skin is normal, without erythema or scale. What is the best diagnostic approach at this time? | options: A: Culture of vesicular fluid for herpes viruses B: Trial of corticosteroids C: Biopsy of the edge of a bulla with some surrounding intact skin D: CT scan of the chest and abdomen looking for occult malignancy | answer: C </s>