The Lone Lump: Cutaneous Rosai-Dorfman Disease as an Isolated Upper Arm Lesion.

Rosai-Dorfman disease (RDD) is a rare disorder characterized by excessive growth of histiocytes. We present a case of a 14-year-old female with cutaneous RDD who had a subcutaneous lump on her left arm for three years. The lump became tender and progressively larger over the past year.