This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: vitamins, Subject name: Biochemistry
Tryptophan is required for synthesis of which vitamin? 
Select and explain. 
A. Folic acid, B. Thiamine, C. Niacin, D. Riboflavin
C: Niacin
Answer: C: Niacin 
Explanation: Tryptophan is required for the synthesis of Niacin. In the major pathway, kynureninase is an enzyme dependent on pyridoxal phosphate (enzyme 2, Fig.17.8). Therefore, in vitamin B6 deficiency, the pathway is blocked at this level. This leads to niacin deficiency and manifestations of pellagra. 7. Quinolinate phosphoribosyl transferase is the rate-limiting enzyme in the conversion of niacin to NAD+. About 60 mg of tryptophan is equivalent to 1 mg of niacin. Reference: Vasudevan Textbook of Biochemistry pg no. 211, 395