Psoriatic arthritis is a form of inflammatory arthritis that can affect some of the millions of Americans who have psoriasis. Psoriasis is a skin disease that causes a red, scaly rash, most commonly over the elbows, knees, ankles, feet, hands, and other areas. Asymmetric psoriatic arthritis: Asymmetric arthritis typically involves one to three joints in the body -- large or small -- such as the knee, hip, or one or several fingers. 2  Asymmetric psoriatic arthritis does not affect matching pairs of joints on opposite sides of the body.