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': 'All of the following are poor prognostic factors of vitiligo except: \nSelect and explain. \nA. Long standing skin lesion, B. Underlying thyroid disease, C. Vitiligo on bony prominence, D. Black hair', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Black hair follicle has melanocyte precursors which is a good prognostic sign.\nWhite hair or leucotrichia is a poor prognostic sign.', 'role': 'assistant'}]