Plaque psoriasis on a patients arm. Psoriasis occurs when skin cells are produced much faster than normal. The body cannot shed the new cells fast enough, resulting in patches (plaques) of scaly, thick skin, which are often itchy and sore. Plaque psoriasis is the most common form of psoriasis. It typically affects the elbows, knees, scalp and back, but can appear anywhere on the body. The cause of this condition is not fully understood but its thought to be due to an overactive immune system. There is no cure, however, topical treatments, anti-inflammatories and phototherapy can be used to alleviate symptoms.