This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of protein and amino acid, Subject name: Biochemistry
Nicotinamide is derived from 
Select and explain. 
A. Histidine, B. Methionine, C. Tryptophan, D. Phenylalanine
C: Tryptophan
Answer: C: Tryptophan 
Explanation: The enzyme QP Is the rate limiting step in the formation of nicotinamide from Tryptophan.Ref: DM Vasudevan - Textbook of Biochemistry, 7th edition, page no: 240