This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: AIIMS 2018, Subject name: Biochemistry
Due to which of the following enzyme deficiency, vitamin C cannot be synthesised in humans? 
Select and explain. 
A. L-Glucuronic acid oxidase, B. L-Gulonic acid reductase, C. L-Gulonolactone oxidase, D. L-Gulonolactone reductase
C: L-Gulonolactone oxidase
Answer: C: L-Gulonolactone oxidase 
Explanation: L-Gulonolactone oxidase produces Vitamin C Uronic acid pathway, but only in plants & most animals. Humans cannot make vitamin C due to absence of L-Gulonolactone oxidase enzyme.(See fig) Uronic acid pathway- glucose is conveed to glucuronic acid, pentoses and, in some animals, to ascorbic acid (not in man). ADDITIONAL EDGE: Uronic Acid Pathway synthesize Glucuronic Acid, Pentoses and Vit C Uses of Glucuronic acid: Incorporated into proteoglycans (Glucuronate used) Acts as a conjugating agent (Phase II conjugation reactions like bilirubin conjugation)