Medical Definition of urticaria pigmentosa
: a skin disorder usually occurring in children that is caused by the proliferation of mast cells and is marked by small, itchy, reddish-brown spots or bumps on the skin Urticaria pigmentosa is associated with indolent systemic mastocytosis in more than 80% of all cases; scratching of these reddish brown macules, which are scattered over the body except for the palms, soles, and scalp, triggers urtication and erythema …—Mandakolathur R. Murali et al., The New England Journal of Medicine, 24 Mar. 2011
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