Blood and immunity: fetal haemoglobin and sickling: 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 and immunity: fetal haemoglobin and sickling

Sample item

A child with sickle cell disease stays well during the first months of life and has the first painful crisis after about 6 months. Which feature of infant red cells explains the early protection?

AHigh 2,3-BPG, which keeps haemoglobin in the oxygenated stateBMaternal IgG coating the red cells and preventing deoxygenationCAbsence of the mutated beta-globin gene until about 6 monthsDHigh fetal haemoglobin, whose gamma chains do not enter the HbS polymer

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

The answer and the reasoning

D. High fetal haemoglobin, whose gamma chains do not enter the HbS polymer

Sickling needs polymerisation of deoxygenated HbS. Fetal haemoglobin has gamma chains in place of beta chains and interferes with polymer formation, so symptoms emerge only as HbF falls over the first months. This is also the rationale for hydroxyurea, which raises HbF.

Option by option

A. High 2,3-BPG, which keeps haemoglobin in the oxygenated state
2,3-BPG favours the deoxy form, so more of it would promote sickling.
B. Maternal IgG coating the red cells and preventing deoxygenation
Maternal IgG does not alter red cell deoxygenation or polymerisation.
C. Absence of the mutated beta-globin gene until about 6 months
The mutated gene is present from conception; the beta gene is simply expressed more as HbF declines.
D. High fetal haemoglobin, whose gamma chains do not enter the HbS polymer (correct)
Correct. HbF dilutes HbS and blocks polymer formation.

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 Physiology question

What is the answer to this Physiology question?

The best answer is D, High fetal haemoglobin, whose gamma chains do not enter the HbS polymer. Sickling needs polymerisation of deoxygenated HbS.

Why is "High 2,3-BPG, which keeps haemoglobin in the oxygenated state" not the answer?

2,3-BPG favours the deoxy form, so more of it would promote sickling.

Why is "Maternal IgG coating the red cells and preventing deoxygenation" not the answer?

Maternal IgG does not alter red cell deoxygenation or polymerisation.

Why is "Absence of the mutated beta-globin gene until about 6 months" not the answer?

The mutated gene is present from conception; the beta gene is simply expressed more as HbF declines.

What concept does this question test?

It tests Blood and immunity: fetal haemoglobin and sickling in Physiology. Fetal haemoglobin has gamma chains in place of beta chains and interferes with polymer formation, so symptoms emerge only as HbF falls over the first months. This is also the rationale for hydroxyurea, which raises HbF.

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 and immunity: fetal haemoglobin and sickling?

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.