A 3-year-old girl with a history of atopic dermatitis presents to the emergency department with sudden onset of swelling and color change of the left third fingertip (Figs 1 and 2). Atopic dermatitis has been previously treated with various modalities, including bleach baths and wet wraps. Her atopic dermatitis is currently active and managed with topical corticosteroids and topical calcineurin inhibitors. The patient’s immunizations are up to date. Historical food sensitivities include scrambled eggs (leading to face swelling) and peanut butter (leading to vomiting). On her emergency department physical examination, a tender hemorrhagic bulla is noted on the pad of her left third fingertip. She has no history of underlying trauma, fever, or systemic signs of illness, although notably her mother was recently treated for streptococcal pharyngitis.

Given an unclear etiology of finger swelling, a radiograph is obtained to rule out fracture. The radiograph does not show evidence...

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