A patient receiving N2O-based anaesthesia for a 4-hour bowel surgery develops postoperative megaloblastic changes. The underlying mechanism is:

• A Direct bone marrow suppression via GABA receptor
• B Inhibition of folate absorption in the jejunum
• C Irreversible oxidation of cobalt in vitamin B12 (cobalamin) ✓
• D Competitive antagonism of methyltetrahydrofolate

Explanation

Nitrous oxide irreversibly oxidizes the cobalt atom of vitamin B12 from its active Co(I) state to the inactive Co(III) state, thereby inactivating methionine synthase. This enzyme is required for the conversion of homocysteine to methionine and for maintaining adequate 5-methyltetrahydrofolate; its inhibition leads to impaired DNA synthesis and eventually megaloblastic anaemia with prolonged or repeated N2O exposure.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.