Vitamins and minerals: folic acid masking cobalamin deficiency: 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 and minerals: folic acid masking cobalamin deficiency

Sample item

A strict vegan with macrocytic anaemia and distal paraesthesiae is given folic acid alone. His haemoglobin rises, but six months later he has gait ataxia and loss of vibration sense. Why did the neurological disease progress?

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

The answer and the reasoning

D. Folic acid fixes the blood count but not the cobalamin-dependent mutase step

In cobalamin deficiency, trapped methyl-tetrahydrofolate lowers the folate pool needed for DNA synthesis, and extra folic acid bypasses this block and corrects the anaemia. Methylmalonyl-CoA mutase still needs cobalamin, and its failure is linked to subacute combined degeneration. This is why folate alone can mask the deficiency while neurological damage advances.

Option by option

  • A. Folic acid is directly toxic to myelin sheaths at standard therapeutic doses

    Folic acid at therapeutic doses is not myelin toxic.

  • B. Folic acid competitively blocks the ileal uptake of any remaining cobalamin

    Folic acid does not compete for intrinsic factor-mediated cobalamin uptake.

  • C. Folic acid cannot be reduced to tetrahydrofolate in neurons and glial cells

    Folate is reduced to tetrahydrofolate normally; the anaemia improved, showing the pathway works.

  • D. Folic acid fixes the blood count but not the cobalamin-dependent mutase step (correct)

    Correct. Folate corrects the DNA synthesis defect but cannot replace cobalamin in the mutase reaction.

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 D, Folic acid fixes the blood count but not the cobalamin-dependent mutase step. In cobalamin deficiency, trapped methyl-tetrahydrofolate lowers the folate pool needed for DNA synthesis, and extra folic acid bypasses this block and corrects the anaemia.

Why is "Folic acid is directly toxic to myelin sheaths at standard therapeuti…" not the answer?

Folic acid at therapeutic doses is not myelin toxic.

Why is "Folic acid competitively blocks the ileal uptake of any remaining cob…" not the answer?

Folic acid does not compete for intrinsic factor-mediated cobalamin uptake.

Why is "Folic acid cannot be reduced to tetrahydrofolate in neurons and glial…" not the answer?

Folate is reduced to tetrahydrofolate normally; the anaemia improved, showing the pathway works.

What concept does this question test?

It tests Vitamins and minerals: folic acid masking cobalamin deficiency in Biochemistry. Methylmalonyl-CoA mutase still needs cobalamin, and its failure is linked to subacute combined degeneration. This is why folate alone can mask the deficiency while neurological damage advances.

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 and minerals: folic acid masking cobalamin deficiency?

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.