urticaria pigmentosa

noun

urticaria pig·​men·​to·​sa -ˌpig-mən-ˈtō-sə, -(ˌ)men-, -zə 

: 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

—abbreviation UP