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
All of the following are poor prognostic factors of vitiligo except: 
Select and explain. 
A. Long standing skin lesion, B. Underlying thyroid disease, C. Vitiligo on bony prominence, D. Black hair
D: Black hair
Answer: D: Black hair 
Explanation: Black hair follicle has melanocyte precursors which is a good prognostic sign.
White hair or leucotrichia is a poor prognostic sign.