Haematology: vitamin B12 deficiency: practice question

A Pathology 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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Pathology · Haematology: vitamin B12 deficiency

Sample item

A 62-year-old woman has MCV 118 fL, hypersegmented neutrophils, tingling of the feet and loss of vibration sense. Which biochemical pattern would be expected, and would distinguish her cause from folate deficiency?

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

The answer and the reasoning

A. Raised homocysteine and raised methylmalonic acid

Vitamin B12 is a cofactor for methionine synthase (homocysteine to methionine) and for methylmalonyl-CoA mutase. Deficiency therefore raises both homocysteine and methylmalonic acid, and causes subacute combined degeneration through defective myelin. Folate deficiency raises homocysteine only, and does not typically cause the neurological damage seen with B12 deficiency.

Option by option

  • A. Raised homocysteine and raised methylmalonic acid (correct)

    Correct. B12 deficiency impairs both enzymes, so both metabolites rise.

  • B. Normal homocysteine with raised methylmalonic acid

    Normal homocysteine with raised methylmalonic acid would suggest an isolated mutase defect, such as inherited methylmalonic aciduria.

  • C. Normal homocysteine and normal methylmalonic acid

    Normal levels of both would argue against a functional deficiency of B12 or folate.

  • D. Raised homocysteine with normal methylmalonic acid

    Raised homocysteine with normal methylmalonic acid is seen in isolated folate 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 Pathology question

What is the answer to this Pathology question?

The best answer is A, Raised homocysteine and raised methylmalonic acid. Vitamin B12 is a cofactor for methionine synthase (homocysteine to methionine) and for methylmalonyl-CoA mutase.

Why is "Normal homocysteine with raised methylmalonic acid" not the answer?

Normal homocysteine with raised methylmalonic acid would suggest an isolated mutase defect, such as inherited methylmalonic aciduria.

Why is "Normal homocysteine and normal methylmalonic acid" not the answer?

Normal levels of both would argue against a functional deficiency of B12 or folate.

Why is "Raised homocysteine with normal methylmalonic acid" not the answer?

Raised homocysteine with normal methylmalonic acid is seen in isolated folate deficiency.

What concept does this question test?

It tests Haematology: vitamin B12 deficiency in Pathology. Deficiency therefore raises both homocysteine and methylmalonic acid, and causes subacute combined degeneration through defective myelin. Folate deficiency raises homocysteine only, and does not typically cause the neurological damage seen with B12 deficiency.

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 Haematology: vitamin B12 deficiency?

Learn each disease as a bundle: cause, key histology, markers, and the clinical hallmark. Then use the flashcards and the other Pathology 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.