A 51 year old woman was seen because of a three day history of itchy bumps on her back. She said that she woke three days before with itching on her back and noticed red bumps on her back only. She had no idea how the rash started and had no previous history of a similar rash. She was otherwise in good health, had no fever, chills, or other systemic symptoms, and was not taking any medications. She had no history of allergy to medications, foods, or other substances. She said that she had not had any unusual foods and had not started to use any new soaps, laundry detergents, or topical preparations. She had not worn any new clothing in the past week. She had not gone camping or hiking in the past month. Examination showed an erythematous papular eruption with possible pustules confined to the back (see figure).