Psoriasis (dithranol treatment). Close-up of the arm of a young patient with psoriasis, showing dark brown patches of skin. This skin condition is characterised by thickened plaques of inflamed skin, covered with silvery scales. It is a result of localised disturbance of skin production. Here, the patient has been successfully treated with dithranol, the most potent topical preparation available for psoriasis. The drug is applied to the skin patches and covered with a dressing for an hour. The strength of dithranol is gradually increased to minimise the risk of a sensitivity reaction to the drug, particularly common in fair skin.