This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
[{'content': 'A 5 year old male child has multiple hyperprigmented macules over the trunk. On rubbing the lesion with the rounded end of a pen, he developed urticarial wheal, confined to the border of the lesion. The most likely diagnosis is – \nSelect and explain. \nA. Fixed drug eruption, B. Lichen planus, C. Urticaria pigmentosa, D. Urticarial vasculitis', 'role': 'user'}, {'content': "Answer: C \nExplanation: Urticaria pigmentosa\xa0is a familial cutaneous disorder characterised by generalised distribution of\xa0red brown macules.\nEach lesion represents a collection of mast cells in the dermis with\xa0hyperpigmentation\xa0of overlying epidermis.\nThe most characteristic features is that these lesions urticate on scratching.\nLight scratching with the point of forceps or rounded tip of a pen or even rubbing causes these mast cells to degranulate leading to localized urticaria (become red swollen and enlarged)\xa0→\u200b\xa0Darrier's sign.\nExtensive release of histamine from mast cell degranulation may result in - Headache, flushing diarrhea and pruritis", 'role': 'assistant'}]