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A variety of factors -- ranging from emotional stress and trauma to streptococcal infection -- can cause an episode of psoriasis. Recent research indicates that some abnormality in the immune system is the key cause of psoriasis.
These include: 1  Plaque psoriasis. 2  The most common form, plaque psoriasis causes dry, raised, red skin lesions (plaques) covered with silvery scales. 3  The plaques itch or may be painful and can occur anywhere on your body, including your genitals and the soft tissue inside your mouth. Severe cases may cause the nail to crumble. 2  Scalp psoriasis. 3  Psoriasis on the scalp appears as red, itchy areas with silvery-white scales. 4  The red or scaly areas often extend beyond the hairline. 5  You may notice flakes of dead skin in your hair or on your shoulders, especially after scratching your scalp.