Keratosis pilaris
A commonly seen condition, often occurring, in adolescent, in which patches of rough skin appear on the upper arms, buttocks, and thighs. The openings of the hair follicles become enlarged by plug of keratin (a fibrous, tough protein), and hair growth can be distorted.
The condition is not severe and often clears up by itself. In extreme cases applying a mixture of salicylic acid and soft paraffin and scrubbing with a scrubber can assist.