Psoriasis. Scaly skin on a person's knees caused by psoriasis. This chronic skin disorder affects about 1% of the population in the UK. A typical lesion of psoriasis is a thickened, red patch covered with dry, silvery scales of skin. It is a result of a ten-fold increase in the rate of new skin cell production. The cause of psoriasis is unknown, but there appears to be an inherited predisposition. It usually begins between the ages of 10 and 25, and it recurs thereafter to a greater or lesser extent. The extent of the lesions seems also to be related to stress. Management is usually by use of corticosteroids and ointments containing coal tar derivatives.