This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nutrition and Health, Subject name: Social & Preventive Medicine
[{'content': 'Niacin is synthesized from:(1994) \nSelect and explain. \nA. Tryptophan, B. Tyrosine, C. Methionine, D. Phenyl alanine', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans: aRef: Park, 17th ed, p. 420 & 18th ed, p. 446', 'role': 'assistant'}]