Answer: A. Synthesis of vitamin C
Explanation:
Humans cannot synthesize vitamin C due to a mutation in gulonolactone oxidase gene that codes for a protein or enzyme needed for production of vitamin C. Since we cannot produce vitamin C ourselves we have to consume it from our diet such as green pepper, tomatoes and spinach.
Some animals due produce their own vitamin C by converting glucose into ascorbate but this is not achieved by the integumentary system, it is done by the kidney and liver.
