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High doses of which gas block the metabolism of vitamin B12?
Carbon dioxide
Oxygen
Nitrous oxide
Helium
The correct answer is: Nitrous oxide
High doses of nitrous oxide block the metabolism of vitamin B12. This occurs because nitrous oxide can irreversibly oxidize vitamin B12, specifically the cobalt ion in its structure, leading to inactivation of this crucial vitamin. Vitamin B12 is essential for the synthesis of methionine from homocysteine and for the proper functioning of the nervous system. When vitamin B12 is inhibited, it can lead to various health complications, including megablastic anemia and neurological issues. In contrast, carbon dioxide, oxygen, and helium do not have these specific effects on vitamin B12 metabolism. Carbon dioxide and oxygen are gases essential for cellular respiration and overall physiological functions, whereas helium is an inert gas that does not actively participate in biochemical processes affecting vitamin metabolism. Hence, nitrous oxide is uniquely associated with the blockade of vitamin B12 metabolism among the choices given.