The main cause of a patch of dense bumps on the arm is considered to be related to keratosis pilaris, eczema, or contact dermatitis. It is necessary to combine with other symptoms and examinations to make a clear diagnosis. 1. Keratosis pilaris: If there is a problem of keratosis pilaris, it is easy to produce local skin bumps, related to genetic factors, local except dense small bumps, no other discomfort. 2. Eczema: eczema can lead to localized skin with small dense pimples, usually accompanied by a certain degree of itching, and the continued development of flaking, oozing, vesicles and so on. 3. contact dermatitis: arm skin contact with metal products, animal fur and other allergic substances produce allergic reaction or irritation reaction, local skin can appear patchy dense pimples phenomenon, can be accompanied by redness and itching performance. A piece of dense bumps on the arm may also be due to other reasons, if the symptoms last longer, or progressive aggravation of the phenomenon, you need to go to the dermatology department as soon as possible to avoid delaying the condition.