psoriasis (sôrĪˈəsĭs) [key], occasionally acute but usually chronic and recurrent inflammation of the skin. The exact cause is unknown, but the disease appears to be an inherited, possibly autoimmune disorder that causes the overproduction of skin cells. Psoriasis may occur at any age but is uncommon in children. The characteristic lesion is a scaly "mother-of-pearl" patch, appearing anywhere on the body. Involvement may range from a single plaque to numerous patches that cover most of the skin. A variety of treatments are used for patients with mild to moderate cases. Treatments directed at the symptoms include the application of ointments and exposure to sunlight and ultraviolet (UVB) light. Retinoids help stabilize follicular epithelial cells. Vitamin D analogs and metabolites, although effective in treatment, have side effects. Photochemotherapy (psoralen combined with UVA radiation) is also effective, but increases the risk of skin cancer. Alfacept and other drugs that interfere with T-cell (see immunity) activation, and etanercept, infliximab, and other drugs that block tumor-necrosis factor are effective in many patients with moderate to severe psoriasis.