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: Social & Preventive Medicine
[{'content': 'True regarding vitamin B3 are all except: March 2012 \nSelect and explain. \nA. Tryptophan is a precursor, B. Milk is a rich source, C. Pellagra results due to deficiency, D. Not excreted in urine', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans: B i.e. Milk is a rich source', 'role': 'assistant'}]