Blood: fetal haemoglobin: 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: fetal haemoglobin

Sample item

A newborn's umbilical venous blood shows a higher oxygen affinity than maternal blood, even though both are exposed to the same 2,3-bisphosphoglycerate concentration. Which property of fetal haemoglobin explains this?

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

The answer and the reasoning

D. Gamma chains lack the histidine that binds 2,3-BPG, so the molecule binds it poorly

In adult haemoglobin, histidine 143 on the beta chains forms part of the 2,3-BPG binding pocket. Gamma chains carry serine at this position, so HbF binds 2,3-BPG weakly and stays in the high-affinity (relaxed) state. This left-shifted curve helps transfer oxygen across the placenta.

Option by option

  • A. Alpha chains are replaced by gamma chains, which increases the Bohr effect

    HbF is made of two alpha and two gamma chains, so alpha chains are not replaced, and a stronger Bohr effect would not raise affinity.

  • B. Fetal haemoglobin has a higher haem iron content per molecule

    Each haemoglobin molecule has four haem groups in both HbF and HbA.

  • C. Fetal haemoglobin binds carbon monoxide in preference to oxygen

    Carbon monoxide affinity is not the reason for the left shift, and it is not preferentially bound by HbF.

  • D. Gamma chains lack the histidine that binds 2,3-BPG, so the molecule binds it poorly (correct)

    Correct. Reduced 2,3-BPG binding raises oxygen affinity and favours placental oxygen transfer.

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 D, Gamma chains lack the histidine that binds 2,3-BPG, so the molecule binds it poorly. In adult haemoglobin, histidine 143 on the beta chains forms part of the 2,3-BPG binding pocket.

Why is "Alpha chains are replaced by gamma chains, which increases the Bohr e…" not the answer?

HbF is made of two alpha and two gamma chains, so alpha chains are not replaced, and a stronger Bohr effect would not raise affinity.

Why is "Fetal haemoglobin has a higher haem iron content per molecule" not the answer?

Each haemoglobin molecule has four haem groups in both HbF and HbA.

Why is "Fetal haemoglobin binds carbon monoxide in preference to oxygen" not the answer?

Carbon monoxide affinity is not the reason for the left shift, and it is not preferentially bound by HbF.

What concept does this question test?

It tests Blood: fetal haemoglobin in Physiology. Gamma chains carry serine at this position, so HbF binds 2,3-BPG weakly and stays in the high-affinity (relaxed) state. This left-shifted curve helps transfer oxygen across the placenta.

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 Blood: fetal haemoglobin?

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.