A 42-year-old African-American woman presents with highly symptomatic blisters on her arms. She has no explanation for their recent appearance.

An initial visit to urgent care netted her a diagnosis of probable poison ivy and a prescription for topical steroid cream (triamcinolone 0.1%) and calamine lotion. Neither was of any help. She followed up with her primary care provider, who expedited referral to dermatology.

The patient denies any recent history of exposure to the great outdoors. She is not ill; she has neither fever nor malaise—just considerable discomfort from the itching. She denies any oral lesions or hoarseness. No one else in her household is similarly affected.

Examination reveals a dozen fluid-filled bullae, measuring 8 mm to 3 cm, scattered across both forearms (predominantly on the flexural surfaces). The lesions are distinct and discrete but randomly configured. The fluid is clear and yellow. There is no tenderness or erythema. Testing for Nikolsky sign is negative (ie, the lesions cannot be extended or ruptured with digital pressure).

Punch biopsy is performed on both perilesional and uninvolved skin; some samples are submitted in formalin-based fixative for default hematoxylin and eosin staining and others in Michel’s medium (a simple salt solution that neither kills organisms nor alters cell architecture) for direct immunofluorescent studies. The report shows IgG deposited at the dermoepidermal junction.