Blood: methaemoglobinaemia: practice question

A Physiology 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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Physiology · Blood: methaemoglobinaemia

Sample item

A 30-year-old man on dapsone for dermatitis herpetiformis is cyanosed with pulse oximetry reading 85% that does not improve with oxygen. His arterial PaO2 is normal and his blood is chocolate-brown. Why is intravenous methylene blue effective?

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

The answer and the reasoning

C. It is reduced by NADPH-dependent methaemoglobin reductase and then reduces Fe3+ to Fe2+

Dapsone oxidises haem iron from Fe2+ to Fe3+, producing methaemoglobin that cannot carry oxygen and shifts the curve to the left. Methylene blue is reduced to leucomethylene blue by NADPH-dependent methaemoglobin reductase, which then donates electrons to methaemoglobin. This pathway depends on the pentose phosphate pathway, so methylene blue is ineffective and risky in G6PD deficiency.

Option by option

  • A. It raises 2,3-BPG and shifts the dissociation curve to the right

    Altering 2,3-BPG does not correct the oxidised iron and would not restore oxygen-carrying capacity.

  • B. It chelates ferric iron and removes it from the haem group

    Chelation would be the action of a drug like desferrioxamine and does not regenerate haemoglobin.

  • C. It is reduced by NADPH-dependent methaemoglobin reductase and then reduces Fe3+ to Fe2+ (correct)

    Correct. Leucomethylene blue reduces methaemoglobin back to functional haemoglobin.

  • D. It competes with oxygen for binding to ferrous haem

    Competition at the haem site is how carbon monoxide acts; methylene blue removes the oxidised state instead.

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 Physiology question

What is the answer to this Physiology question?

The best answer is C, It is reduced by NADPH-dependent methaemoglobin reductase and then reduces Fe3+ to Fe2+. Dapsone oxidises haem iron from Fe2+ to Fe3+, producing methaemoglobin that cannot carry oxygen and shifts the curve to the left.

Why is "It raises 2,3-BPG and shifts the dissociation curve to the right" not the answer?

Altering 2,3-BPG does not correct the oxidised iron and would not restore oxygen-carrying capacity.

Why is "It chelates ferric iron and removes it from the haem group" not the answer?

Chelation would be the action of a drug like desferrioxamine and does not regenerate haemoglobin.

Why is "It competes with oxygen for binding to ferrous haem" not the answer?

Competition at the haem site is how carbon monoxide acts; methylene blue removes the oxidised state instead.

What concept does this question test?

It tests Blood: methaemoglobinaemia in Physiology. Methylene blue is reduced to leucomethylene blue by NADPH-dependent methaemoglobin reductase, which then donates electrons to methaemoglobin. This pathway depends on the pentose phosphate pathway, so methylene blue is ineffective and risky in G6PD 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 Blood: methaemoglobinaemia?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology 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.