A recent outbreak of small-plaque and guttate psoriasis was confined primarily to the arms of this 32-year-old woman. The slightly scaly, brick-red, linear plaques and clusters consisted of 3- to 10-mm papules, some of which were surrounded by a faint whitish ring. It was quickly ascertained that many of these lesions corresponded to areas where the patient had been scratched by her cat. This case demonstrates two classic features of psoriasis: Koebner's phenomenon (the isomorphic response), which consists of typical lesions at sites of skin trauma or injury, such as scratches; and Woronoff's ring, an area of concentric blanching around healing psoriatic lesions. The causes of these responses are not well understood, but their recognition facilitates the diagnosis of psoriasis.
Drs Eric J. Lewis and Charles E. Crutchfield III of Minneapolis point out that Koebner's phenomenon is not specific for psoriasis. It can also be observed in such conditions as lichen planus and contact dermatitis.