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
The amino acid from which niacin synthesized is: 
Select and explain. 
A. Tyrosine, B. Tryptophan, C. Threonine, D. Histidine
B: Tryptophan
Answer: B: Tryptophan 
Explanation: None