Client restrictions apply, please speak to your account manager for details. Urticaria pigmentosa in a 3 year old Hispanic male patient. Urticaria pigmentosa is the most common form of mastocytosis. It is a skin disorder that is most common in children and infants. It causes itchy brown skin patches due to abnormal collections of mast cells, a type of white blood cell of the body's immune system. Treatment is with oral antihistamine tablets or topical steroid cream.