A 32-year-old woman presents to the emergency department for evaluation of a worsening rash that begins at her navel and continues down both of her legs. The rash first appeared approximately 10 days earlier and has gotten progressively worse; initially the rash was itchy, but now is painful. The patient is not receiving any medications and denies recent travel. She has not had a fever or other medical complaints, and has never experienced a rash like this before. She has a dog and 2 cats in her home.

The patient’s vital signs and physical examination are normal except for the rash (Figure). Differential diagnosis includes flea bites with a severe allergic reaction, vasculitis, disseminated gonorrhea, and necrotic erythema multiforme (EM). Laboratory testing (complete blood count, basal metabolic panel, and liver function test) are all normal.

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