A 38-year-old man presents to dermatology with what he assumes is poison ivy: an itchy, blistery rash that usually appears in the summer, more years than not. Each year, it’s a bit worse in terms of extent and symptomatology, despite his efforts to avoid the problem altogether.
The rash pretty consistently manifests on his leg, although there are other areas of involvement. Equally predictable, at this point, is his wife’s reaction: She banishes him to the couch for fear of “catching” whatever he has.
After so many years’ experience with the condition, the patient is understandably alert to coming in contact with the offending plant. He has even taken photos of it, to demonstrate how abundantly it grows in his yard.
EXAMINATION
The patient’s lesions are classic collections of vesicles crisscrossing his legs in linear configurations. There is faint underlying erythema. Smaller but similar lesions are scattered over his arms and trunk; the patient is sure he spread the rash with his scratching.
The plant in the patient’s photos has five dart-shaped leaves with uniformly serrated margins extending from a single stem. It grows as a vine on fences and masonry. He has scrupulously avoided contact with it and therefore cannot understand how he keeps developing the rash.
What is the diagnosis?