Psoriasis is a chronic condition that affects less than 2% of the U.S. population. Although the exact cause of psoriasis is unknown, it is believed to have a strong genetic component. Psoriasis most often manifests as a silvery, scaly, red rash that occurs in clumps. The rash is seen most often on the extensor surface of the elbows and knees, buttock crease, and scalp. Psoriasis of the scalp can result in hair loss. There are several forms of psoriasis.