Very small white itchy bumps on half arm spreading fast around tinea versicolor patch.?

Keratosis Pilaris: It sounds like you have a condition called keratosis pilaris. This causes tiny, rough prickly bumps to form on the backs of the arms, the thighs, the buttocks and occasionally on the face. A mild exfoliating skin cream with 12% ammonium lactate used once to twice a day will smooth these areas out nicely. If you have itch, a mild cortisone cream from your doctor can help.