Shedding light on an unusual blister

It could be a burn, bullous insect bite reaction, fixed drug reaction, epidermolysis bullosa acquisita (EBA), porphyria cutanea tarda or a porphyria-like drug reaction. A very specific cause of something like this is blisters during dialysis.

A good history always helps. This lady had severe hepatitis C infection, which predisposes to porphyria, but her porphyrins were normal. She denied any burn or insect bite. NSAIDs often cause pseudoporphyria with normal porphyrins in the blood. To investigate this patient further, an incisional biopsy should be taken from