Painful Oral Erosions and a Rash

An 86 year-old woman presents to the Emergency Department with a 4-day history of rash on her back and legs, as well as painful sores in her mouth. She denies having fevers or chills and has not experienced any arthralgias, dyspnea on exertion, or diarrhea. She has been able to keep up with her oral fluid intake but feels pain when she swallows. The patient has a past medical history of chronic atrial fibrillation, hypertension, glaucoma, and mild dementia. The patient has had no recent change in her medication regimen and is taking lisinopril, metoprolol, aspirin, warfarin, timolol, and folic acid.

On physical examination, the patient is afebrile with blood pressure and heart rate within normal limits. Her oxygen saturation while breathing room air is 99%, and her respiratory rate is 12 breaths per minute. The head, neck, and chest examination is significant for bilateral buccal mucosal erosions that involve the posterior pharynx, tongue, and the floor of the mouth (see Images 1-2). The cardiovascular, pulmonary, and abdominal examinations are unremarkable. A detailed skin examination (see Image 3) reveals an erythematous rash with raised plaques mixed in with patches on the lower back, buttocks, and both thighs. A few small bullae are also noted. The scalp, face, conjunctivae, anterior chest, and upper extremities do not exhibit symptoms.

What is the diagnosis?

What is the diagnosis?
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The presentation is suggestive of an autoimmune blistering disease.