What is the Koebner phenomenon?

Fig. 2.8 Koebner (isomorphic) phenomenon. Patient with acute explosive psoriasis demonstrating restriction of lesions to the site of minor trauma in the form of a sunburn. (Courtesy of the William L. Weston, M.D. collection.)

Traumatizing the epidermis of a patient with a certain preexisting skin disease will cause the same skin disease to form in the traumatized skin. Noticing this skin finding is helpful when creating a differential diagnosis. Only certain diseases are associated with a Koebner phenomenon; lichen planus, lichen nitidus, and psoriasis (Fig. 2-8) are examples.