Inhalational agents: nitrous oxide and vitamin B12: practice question

A Anaesthesia question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Anaesthesia · Inhalational agents: nitrous oxide and vitamin B12

Sample item

Prolonged nitrous oxide exposure causes megaloblastic changes and neuropathy by inhibiting which enzyme?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Methionine synthase

Nitrous oxide oxidises the cobalt in vitamin B12 and inactivates methionine synthase, impairing DNA synthesis and myelin formation. It is avoided or limited in patients with B12 deficiency.

Option by option

  • A. Pyruvate carboxylase

    Pyruvate carboxylase is not affected.

  • B. Thymidylate synthase

    Thymidylate synthase is inhibited by 5-fluorouracil.

  • C. Methionine synthase (correct)

    Correct. Methionine synthase is inactivated.

  • D. Dihydrofolate reductase

    Dihydrofolate reductase is inhibited by methotrexate.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Anaesthesia question

What is the answer to this Anaesthesia question?

The best answer is C, Methionine synthase. Nitrous oxide oxidises the cobalt in vitamin B12 and inactivates methionine synthase, impairing DNA synthesis and myelin formation.

Why is "Pyruvate carboxylase" not the answer?

Pyruvate carboxylase is not affected.

Why is "Thymidylate synthase" not the answer?

Thymidylate synthase is inhibited by 5-fluorouracil.

Why is "Dihydrofolate reductase" not the answer?

Dihydrofolate reductase is inhibited by methotrexate.

What concept does this question test?

It tests Inhalational agents: nitrous oxide and vitamin B12 in Anaesthesia. It is avoided or limited in patients with B12 deficiency.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Inhalational agents: nitrous oxide and vitamin B12?

Make a table of each agent, its key advantage and its main side effect. Then use the flashcards and the other Anaesthesia questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.