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 load test helps in the evaluation of deficiency of the Vitamin 
Select and explain. 
A. Folic acid, B. Niacinamide, C. Pyridoxine, D. Cyanocobolamine
B: Niacinamide
Answer: B: Niacinamide 
Explanation: The synthesis of niacin coenzymes (NAD+ and NADP+) from tryptophan is dependent on PLP The enzyme kynureninase requires PLP. In B6 deficiency, 3-hydroxy anthranilic acid is diveed to xanthurenic acid Increased excretion of xanthurenic acid in urine is an indication of B6 deficiency. ref satyanarayana 4th ed page 145