In which patient should nitrous oxide sedation be used with particular caution?

Correct answer: B. A patient with vitamin B12 deficiency

• A. ❌ Vitamin C deficiency (scurvy) affects collagen synthesis and causes gingival bleeding, but has no interaction with nitrous oxide.
• B. ✅ CORRECT — Nitrous oxide irreversibly oxidizes the cobalt ion of VITAMIN B12, inactivating methionine synthase. In a patient who is already B12-deficient this can precipitate megaloblastic anemia and, with repeated or prolonged exposure, subacute combined degeneration of the spinal cord — a neuropathy that may be irreversible.
• C. ❌ Vitamin D deficiency affects bone mineralization, with no nitrous oxide interaction.
• D. ❌ Iron deficiency anemia is not affected by nitrous oxide.

Note: Other nitrous cautions: first trimester of pregnancy, COPD/severe respiratory disease, recent middle-ear or ophthalmic surgery with gas bubble, bleomycin therapy, severe psychiatric illness, and nasal obstruction.