A red rash on the arm considered the following diseases: 1, eczema, occurs after contact with cashmere, chemical fiber clothing caused by allergic reactions, in clinical treatment can be applied to stove glycolic lotion, benadryl cream for anti-itch treatment; if the rash is dense, fused with each other, you can apply mometasone furoate ointment and mixed with butyl hydroxy acid ointment, oral medication can be applied to lupatadine fumarate tablets, avar 2, peripapillary keratosis, can appear in the upper arm of the extensor side of the above rash, in the treatment of the application of fruit acid or azelaic acid after the application can play the effect of erosion of the hair follicle orifice embolism; 3, psoriasis, the patient can appear red rash, which has silvery white scales, treatment can be topical tacalcitol ointment, and mixed with fluticasone propionate cream coated foam.