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
Niacin is synthesized from: 
Select and explain. 
A. Phenylalanine, B. Tryptophan, C. Tyrosine, D. Methionine
B: Tryptophan
Answer: B: Tryptophan 
Explanation: 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.