Psoriasis is a skin condition caused by an abnormally functioning immune system. New skin cells are made too quickly causing them to pile up on the skin surface. These areas appear as rough and scaly plaques most commonly found on the elbows, knees, scalp and lower back. In addition to skin changes psoriasis may affect the joints leading to arthritis. There are many treatments for psoriasis including creams, light therapy, oral medication and injectable medications called biologics.