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: Biochemistry
[{'content': 'Niacin is synthesized from: \nSelect and explain. \nA. Phenylalanine, B. Tryptophan, C. Tyrosine, D. Methionine', 'role': 'user'}, {'content': 'Answer: B \nExplanation: The liver can synthesize niacin from the essential amino acid tryptophan, but the synthesis is extremely inefficient; 60 mg of tryptophan are required to make one milligram of niacin. The 5-membered aromatic heterocycle of the essential amino acid, tryptophan, is cleaved and rearranged with the alpha amino group of tryptophan into the 6-membered aromatic heterocycle of niacin.', 'role': 'assistant'}]