Normal shedding of old skin can't keep up, so cells begin to accumulate and form patches of thick, raised skin. Patches are red or dark pink and usually covered with white or silvery scales, which may flake off. The knees, elbows, scalp, and trunk are most commonly affected, but psoriasis can occur anywhere, even under fingernails and toenails. Nails may become pitted and ridged. In some people, a form of arthritis develops in affected areas.
Psoriasis seems to run in families and occurs most often in teenagers and young adults.
Doctors can often diagnose it just by inspecting the areas. If there's any doubt, a small piece of skin may be taken for testing. Not one that can make it go away for good. You'll probably have recurrences, especially in the winter when your skin is most likely to be dry and get little sunlight. But there are methods to relieve attacks, and your doctor can help decide what will work best for you.