Haematology: methaemoglobinaemia: 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: methaemoglobinaemia

Sample item

A 25-year-old man on dapsone has dyspnoea and cyanosis not corrected by oxygen. Pulse oximetry reads 85 percent, arterial PaO2 is normal and the blood sample is chocolate brown. What is the underlying mechanism?

AReduced red cell 2,3-BPG shifting the dissociation curve leftwardBInhibition of mitochondrial cytochrome c oxidaseCCompetitive binding of carbon monoxide to haemDOxidation of haem iron from the ferrous to the ferric state

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

The answer and the reasoning

D. Oxidation of haem iron from the ferrous to the ferric state

In methaemoglobinaemia haem iron is oxidised to Fe3+, so haemoglobin cannot bind oxygen and the remaining haem binds oxygen more tightly. Oxidant drugs such as dapsone, nitrites and local anaesthetics are causes. Patients appear cyanosed with a normal PaO2 and chocolate-brown blood.

Option by option

A. Reduced red cell 2,3-BPG shifting the dissociation curve leftward
Low 2,3-BPG causes a left shift but does not produce chocolate-brown blood or a fall in measured saturation.
B. Inhibition of mitochondrial cytochrome c oxidase
Cytochrome oxidase inhibition by cyanide blocks tissue oxygen use and leaves venous blood bright red.
C. Competitive binding of carbon monoxide to haem
Carbon monoxide poisoning forms carboxyhaemoglobin and gives cherry-red skin, not chocolate-brown blood.
D. Oxidation of haem iron from the ferrous to the ferric state (correct)
Correct. Ferric haem cannot carry oxygen, giving cyanosis with normal PaO2.

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.

Questions about this Pathology question

What is the answer to this Pathology question?

The best answer is D, Oxidation of haem iron from the ferrous to the ferric state. In methaemoglobinaemia haem iron is oxidised to Fe3+, so haemoglobin cannot bind oxygen and the remaining haem binds oxygen more tightly.

Why is "Reduced red cell 2,3-BPG shifting the dissociation curve leftward" not the answer?

Low 2,3-BPG causes a left shift but does not produce chocolate-brown blood or a fall in measured saturation.

Why is "Inhibition of mitochondrial cytochrome c oxidase" not the answer?

Cytochrome oxidase inhibition by cyanide blocks tissue oxygen use and leaves venous blood bright red.

Why is "Competitive binding of carbon monoxide to haem" not the answer?

Carbon monoxide poisoning forms carboxyhaemoglobin and gives cherry-red skin, not chocolate-brown blood.

What concept does this question test?

It tests Haematology: methaemoglobinaemia in Pathology. Oxidant drugs such as dapsone, nitrites and local anaesthetics are causes. Patients appear cyanosed with a normal PaO2 and chocolate-brown blood.

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: methaemoglobinaemia?

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.