Psoriasis. Coloured scanning electron micrograph (SEM) of skin cells affected by psoriasis. In psoriasis, skin cells are produced much faster than normal. The body cannot shed the cells fast enough, resulting in patches (plaques) of scaly, thick skin, which are often itchy. It is thought to be triggered by emotional stress, some infections, or certain drugs, although in many cases there is no obvious cause. Magnification: x90 when printed at 10 centimetres wide.