Psoriasis is a chronic relapsing and remitting disease of the skin that may affect any site. It is one of the commonest of all skin diseases, with a reported incidence of 1–2% in Caucasians. It is rare among blacks, Japanese, and native North and South american populations. Males and females are affected equally. Although psoriasis may occur at any age, it most frequently presents in the teens and in early adult life (type I psoriasis). A second peak in which the disease is often milder appears around the sixth decade (type II psoriasis).
The classic cutaneous lesion of psoriasis vulgaris (plaque psoriasis), developing in about 85–90% of patients with psoriasis, is raised, sharply demarcated, with a silvery scaly surface. The underlying skin has a glossy, erythematous appearance. If the parakeratotic scales are removed with the fingernail, small droplets of blood may appear on the surface (auspitz’s sign); this is diagnostic. plaques, when multiple, are often symmetrical and annular lesions due to central clearing are a common finding. the scalp, the extensor surfaces (mainly the knees and elbows), the lower back, and around the umbilicus are particularly affected. the clinical features, however, show regional variation: scalp involvement often shows very marked plaque formation, whereas on the penis scaling is commonly minimal and the features may be mistaken for Bowen’s disease. Linear lesions (linear psoriasis) follow previous trauma (koebnerization).
Source: P. McKee, J. Calonje – McKee’s Pathology of the Skin (Elsevier)