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
Which of the following vitamin is synthesized in vivo, in the body by humans? 
Select and explain. 
A. Niacin, B. Pantothenic acid, C. Cyanocobalamin, D. Folic acid
A: Niacin
Answer: A: Niacin 
Explanation: Ans: A. Niacin(Ref: Harper 30/e p547, 556)Niacin:Not strictly a vitamin.Can be synthesized in body from essential amino acid tryptophan.Two compounds, nicotinic acid & nicotinamide - Similar biologic activity of niacin.Metabolic function is as the nicotinamide ring of coenzymes NAD & NADP in oxidation-reduction reactions.