Acne on the arms usually occurs on the outer edge of the upper arms on both sides and has a pimple-like appearance. The perifollicular keratosis is mostly caused by abnormal keratinization around the hair follicles due to genetic factors. The abnormal perifollicular keratinization inevitably clogs the surrounding sebaceous duct openings, and the sebaceous glands are unable to discharge sebum effectively, so sebum is prone to accumulate, and a certain amount of acne-like structures appear around the hair follicles.