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
VITAMIN WHICH IS NOT SYNTHESIZED BY THE BACTERIAL FLORA IN THE INESTINE IS? 
Select and explain. 
A. vitamin k, B. Biotin, C. Niacin, D. Pantothenic acid
C: Niacin
Answer: C: Niacin 
Explanation: Endogenously synthesized Vitamins are 1. Synthesized by the body- Niacin: synthesized from an amino acid Tryptophan Vitamin D: synthesized from 7 Dehydrocholesterol by exposure to UVB rays(290-315nm)in the skin. 2. Synthesized in the body(by Bacterial flora in the intestine) Vitamin K Biotin Pantothenic Acid Reference: Harpers illustrated biochemistry 30th edition