Doctor administering an intralesional injection of the anti-inflammatory steroid triamcinolone acetonide to psoriasis on a patients elbow. The injections deliver higher concentrations than topical application. Psoriasis is a skin condition where excessive growth of skin cells can lead to itchy, scaly, thickened patches (plaques) of skin which may become infected. Psoriasis is sometimes triggered by emotional stress, some infections or drugs, although often the cause is unknown.