Darier sign in mastocytoma

A 1‐year‐old boy presented with a 6‐month history of a brown plaque on his left forearm (Figure, A). His parents reported variation in the size of the plaque and a single episode of unprovoked generalised flushing. On examination, stroking the plaque led to an immediate urticated reaction (Figure, B) — a response known as Darier sign, observed in cutaneous mastocytosis. Given the episode of flushing, a serum tryptase test was completed, returning a normal result (12.6 µg/L; reference interval, < 13.5 µg/L). His presentation was in keeping with a mastocytoma. Cutaneous mastocytosis in childhood generally follows a self‐resolving course but adult presentations require exclusion of mast cell leukaemia.