Psoriasis and ethnic melanonychia of the fingernail. Psoriasis is a condition in which 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 and may become infected. Psoriasis is triggered by many factors including emotional stress, some infections or certain drugs, although in many cases there is no obvious cause. Treatment is aimed at the cause, if known. If the cause is not known, then emollient creams, phototherapy and drug treatments can be used. Melanonychia is a nail discolouration caused by the pigment melanin. It may be the equivalent of a mole on the skin, be due to infection or injury, or a sign of disease such as skin cancer.