(BMJ)—A 50-year-old nurse presented to the ED with a 2-day hx of a mildly itchy rash on her torso that began the day after eating a stir-fried “mushroom medley.” Exam: low BP, most likely due to volume depletion from diarrhea; florid, red, linear rash on abdomen and low back. She otherwise felt well. Labs including creatinine kinase and ANA: normal. What’s the dx?
Dermatomyositis
Photodermatitis
Flagellate dermatitis
Still dz
Allergic reaction
You are correct. Flagellate dermatitis is caused by a toxic reaction to lentinan, a component of shiitake mushrooms, which are safely consumed if cooked thoroughly; the condition can also be caused by bleomycin chemo. A normal creatinine kinase level and negative ANA helped exclude dermatomyositis and adult Still dz. This patient was treated with a mild topical steroid and antihistamine for sx relief. Her rash resolved spontaneously within 3 weeks.

BMJ 2020;368:m418