Psoriasis is a chronic skin condition that causes raised red or
white patches topped with silvery, scaling skin. The patches are
most common on the knees, elbows, scalp, tailbone, and back, but
they may be anywhere on the body (including the fingernails,
palms, and soles of the feet). The patches (called plaques) are
made up of dead skin cells that form thick layers. The body
replaces normal skin cells every 28 days, but in psoriasis, skin
cells are replaced every 3 to 6 days. Psoriasis is not
contagious. The exact cause of psoriasis is not known, but the
body's natural defense system (immune system) is believed to be
involved. The condition tends to run in families. Small patches
of psoriasis can often be treated with regular use of
prescription steroid creams. Tar products (lotions, gels,
shampoos) may also be useful, although they may increase
sensitivity to the sun. Limited exposure to the sun may also
help; unaffected skin should be protected with sunscreen. If
psoriasis affects the scalp, dandruff shamp