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  Hi Dana,   It is difficult to diagnose your mom's rash without seeing it and getting more background history. Blistering rashes occur in several different subtypes of bullous skin eruptions. Bullous is an adjective for bulla which is a thin walled, fluid filled blister, usually more than 5 mm in diameter. Skin biopsy and subsequent staining with immunofluorescence (a special type of staining process that identifies antibodies) helps to confirm the diagnosis.   There are other diseases associated with blistering that are related to inborn defects in making the protein in red blood cells that carry iron, heme. The most common one is Porphyria Cutania Tarda.   Some phototoxic reactions may cause blistering. This occurs when certain drugs absorb into the skin and then become toxic after exposure to specific wavelengths of sun light.   Some inflammatory skin conditions require repeated courses of oral steroids. Although there are risks associated with prolonged or frequent steroid use, untreated blistering diseases may be fatal because of the risk of infection.   My recommendation is to have your mom evaluated by a dermatologist. A fresh blister should be biopsied and properly stained to establish the diagnosis after the history and physical exam. A university based dermatology clinic should be considered if she has already seen a skin specialist.    Yours,   J. Thompson, MD