This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Proteins and Amino Acids, Subject name: Biochemistry
The amino acid that can be converted into a vitamin is: 
Select and explain. 
A. Glycine, B. Tryptophan, C. Phenylalanine, D. Lysine
B: Tryptophan
Answer: B: Tryptophan 
Explanation: B Harper, 26th ed. p. 258 & 25th ed, p. 630, Fig (52.4)