A previously healthy man in his late teens and of Japanese ancestry presented with a 4-month history of a pruritic eruption that started on his neck, upper back, and buttocks (Figure 1A). He felt well otherwise, denying fever, gastrointestinal upset, or any antecedent illness or exposure. He denied any family history of similar eruptions and was not taking any medications aside from whey protein supplementation. He was eating a normal diet. The cutaneous eruption had continued to spread despite topical and oral corticosteroid therapy, and diphenhydramine provided no relief from the pruritus.