Vitamins: B12 and folate: practice question

A Biochemistry 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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Biochemistry · Vitamins: B12 and folate

Sample item

A strict vegan has macrocytic anaemia and paraesthesiae. His serum homocysteine and methylmalonic acid are both raised. What is the biochemical explanation for the neuropathy?

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

The answer and the reasoning

C. Accumulation of methylmalonyl-CoA with abnormal fatty acid incorporation into neuronal myelin

Vitamin B12 is the cofactor for methylmalonyl-CoA mutase and methionine synthase. Its deficiency raises methylmalonic acid and homocysteine. Methylmalonyl-CoA buildup is thought to cause abnormal odd-chain fatty acids in myelin, which accounts for neuropathy. Folate deficiency does not raise methylmalonic acid or cause neuropathy.

Option by option

  • A. Defective synthesis of the haem ring from glycine and succinyl-CoA

    Haem synthesis from glycine and succinyl-CoA relates to ALA synthase and pyridoxine.

  • B. Reduced conversion of dUMP to dTMP by thymidylate synthase

    Thymidylate synthase impairment explains megaloblastosis but is shared with folate deficiency and does not explain neuropathy.

  • C. Accumulation of methylmalonyl-CoA with abnormal fatty acid incorporation into neuronal myelin (correct)

    Correct. Only B12 deficiency affects the mutase reaction and nerve myelin.

  • D. Failure of conversion of phenylalanine to tyrosine

    Phenylalanine hydroxylase relates to phenylketonuria, not B12 deficiency.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, Accumulation of methylmalonyl-CoA with abnormal fatty acid incorporation into neuronal myelin. Vitamin B12 is the cofactor for methylmalonyl-CoA mutase and methionine synthase.

Why is "Defective synthesis of the haem ring from glycine and succinyl-CoA" not the answer?

Haem synthesis from glycine and succinyl-CoA relates to ALA synthase and pyridoxine.

Why is "Reduced conversion of dUMP to dTMP by thymidylate synthase" not the answer?

Thymidylate synthase impairment explains megaloblastosis but is shared with folate deficiency and does not explain neuropathy.

Why is "Failure of conversion of phenylalanine to tyrosine" not the answer?

Phenylalanine hydroxylase relates to phenylketonuria, not B12 deficiency.

What concept does this question test?

It tests Vitamins: B12 and folate in Biochemistry. Its deficiency raises methylmalonic acid and homocysteine. Methylmalonyl-CoA buildup is thought to cause abnormal odd-chain fatty acids in myelin, which accounts for neuropathy. Folate deficiency does not raise methylmalonic acid or cause neuropathy.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Vitamins: B12 and folate?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.