Psoriasis is a medical condition that occurs when skin cells grow too quickly and not shed quickly leading to their piling up. Faulty signals in the immune system cause new skin cells to form in days rather than weeks. The body does not shed these excess skin cells, so the cells pile up on the surface of the skin and lesions form. Psoriasis causes patches of thick, scaly skin that may be white, silvery, or red. Called plaques (plax), these patches can develop anywhere on the skin. The most common areas to find plaques are the elbows, knees, lower back, and scalp.