Detail of monomorphic urticaria pigmentosa

This picture shows lesions of monomorphic urticaria pigmentosa (UP), also called maculopapular cutaneous mastocytosis (MPCM), in a patient with systemic mastocytosis. MPCM lesions are small, yellow-tan to reddish-brown macules or slightly raised papules, most commonly on the upper and lower extremities. Lesions of MPCM display Darier's sign, which is the development of localized urticaria and erythema at the site a few minutes after a lesion is rubbed or scratched. This occurs because these lesions are rich in mast cells, and physical irritation causes the cells to release inflammatory mediators. Note that newly formed lesions are reddish and become more brown in coloration with time and that Darier's sign is best elicited from a newer, reddish lesion. The two shiny, raised papules in this picture are not typical of MPCM and may be nevi or other unrelated lesions.