The sudden red patch on the arm, consider the following possibilities: the first, fixed drug rash, mainly due to taking antipyretic and analgesic drugs, or contraceptives, anti-diarrheal drugs, etc., resulting in an allergic reaction to drugs, the appearance of well-defined, dark red edematous erythema. In clinical treatment, one can take oral olopatadine hydrochloride tablets, one tablet in the morning and one tablet in the evening, and combine with Denide ointment 2 times/day to apply the treatment. The second type of psoriasis, in addition to erythema, can be covered with silvery white scales, and scraping away the scales can be seen as film phenomenon and punctate bleeding phenomenon, accompanied by a certain itching sensation. In clinical treatment, it can be treated with oral amineptine tablets and external application of mometasone furoate ointment once/day.