Pustular psoriasis on a 45 year old Hispanic female patient's arm. 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. Pustular psoriasis is when pus forms in the skin as a complication of psoriasis. Psoriasis is sometimes triggered by emotional stress, some infections or drugs, although often the cause is unknown.