This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Vitamins and Minerals, Subject name: Biochemistry
Vitamin C cannot be produced in humans due to lack of: 
Select and explain. 
A. L-gulonolactone oxidase, B. Xylutitol reductase, C. Pyruvate dehydrogenase, D. UDP glucose dehydrogenase
A: L-gulonolactone oxidase
Answer: A: L-gulonolactone oxidase 
Explanation: (a) L-gulonolactone oxidaseRef: Harper's illustrated biochemistry, 30th ed., pg. 200-201In liver, the uronic acid pathway catalyzes the conversion of glucose to glucuronic acid, ascorbic acid (except in human beings and other species for which ascorbate is a vitamin, vitamin C), andpentoses. It is also an alternative oxidative pathway for glucose that, like the pentose phosphate pathway, does not lead to the formation of ATP. Glucuronate is reduced to 1-gulonate, the direct precursor of ascorbate in those animals capable of synthesizing this vitamin, in an NADPH dependent reaction. In human beings and other primates, as well as guinea pigs, bats, and some birds and fishes, ascorbic acid cannot be synthesized because of the absence of l-gulonolactone oxidase.