You are working in a dermatology clinic, seeing a patient with a rash that is possibly psoriasis. Outside the room, the attending asks if you noticed any evidence of the “Koebner phenomenon” or an “Auspitz sign” when you examined the patient. What are these?

The isomorphic or Koebner phenomenon is the development of a cutaneous eruption at the site of physical trauma (scratch, surgical wound, or sunburn). Other skin conditions that exhibit the Koebner phenomenon include lichen planus, lichen nitidus, and vitiligo. Patients with psoriasis should be warned of this tendency before subjecting themselves to cosmetic procedures involving physical trauma (such as having a tattoo).

The Auspitz sign is the presence of small bleeding points seen on a psoriatic lesion when the scales are removed. This bleeding is due to thinning of the epidermis between the elongated rete ridges. Note that it is not a good idea to attempt to elicit these two signs on your psoriatic patients.