<prompt> A 5 yr. old male child has multiple hyperpigmented macules over the trunk. On rubbing the lesion with rounded end of pen, he developed urticarial wheal, confined to border of lesion, most likely diagnosis is:Fixed drug eruptionLichen planusUrticaria pigmentosaUrticarial vasculitis  <response>: Ans. C. Urticaria pigmentosa It is urticaria pigmentosa. It is cutaneous mastocytosis presenting with pigmented lesions. On rubbing the lesions mast cells degranulate releasing histamine. This produces the wheal and flare of the lesion. It is known as Dariers sign.Epidermal acantholysis in mid epidermis with dilapidated or crumbling brick wall appearance. Dariers disease is associated with ATP2A2 defect 