Haemoglobin: methaemoglobinaemia: 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 · Haemoglobin: methaemoglobinaemia

Sample item

A 3-month-old bottle-fed infant given milk prepared with well water becomes cyanosed with SpO2 of 85% that does not improve with oxygen. Blood is chocolate brown. What change in haemoglobin explains this?

AOxidation of heme iron from the ferrous to the ferric stateBIncreased 2,3-BPG shifting the curve to the rightCDissociation of the tetramer into free globin chainsDBinding of carbon monoxide to ferrous iron

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

The answer and the reasoning

A. Oxidation of heme iron from the ferrous to the ferric state

Nitrates in contaminated well water are converted by gut bacteria to nitrites, which oxidise Fe2+ in haem to Fe3+. Methaemoglobin cannot carry oxygen and also shifts the curve of the remaining haem to the left. Methylene blue, which is reduced by NADPH-dependent methaemoglobin reductase, is the antidote.

Option by option

A. Oxidation of heme iron from the ferrous to the ferric state (correct)
Correct. Ferric haemoglobin cannot bind oxygen and gives brown blood.
B. Increased 2,3-BPG shifting the curve to the right
Increased 2,3-BPG would release more oxygen and would not cause unresponsive cyanosis.
C. Dissociation of the tetramer into free globin chains
Tetramer dissociation does not occur in this scenario and would not explain chocolate-brown blood.
D. Binding of carbon monoxide to ferrous iron
Carboxyhaemoglobin gives cherry-red colour and a normal-looking pulse oximetry reading.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Oxidation of heme iron from the ferrous to the ferric state. Nitrates in contaminated well water are converted by gut bacteria to nitrites, which oxidise Fe2+ in haem to Fe3+.

Why is "Increased 2,3-BPG shifting the curve to the right" not the answer?

Increased 2,3-BPG would release more oxygen and would not cause unresponsive cyanosis.

Why is "Dissociation of the tetramer into free globin chains" not the answer?

Tetramer dissociation does not occur in this scenario and would not explain chocolate-brown blood.

Why is "Binding of carbon monoxide to ferrous iron" not the answer?

Carboxyhaemoglobin gives cherry-red colour and a normal-looking pulse oximetry reading.

What concept does this question test?

It tests Haemoglobin: methaemoglobinaemia in Biochemistry. Methaemoglobin cannot carry oxygen and also shifts the curve of the remaining haem to the left. Methylene blue, which is reduced by NADPH-dependent methaemoglobin reductase, is the antidote.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 Haemoglobin: methaemoglobinaemia?

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.