Blood groups: ABO haemolytic disease: 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 groups: ABO haemolytic disease

Sample item

A group A newborn of a group O, Rh-positive mother becomes jaundiced within 24 hours of birth, with a positive direct antiglobulin test and spherocytes. Why can this occur in a first pregnancy?

AGroup O mothers commonly carry IgG anti-A and anti-B, which cross the placentaBNatural anti-A in group O mothers is mainly IgM, which crosses the placentaCFetal A antigen is absent on red cells, so sensitisation is not neededDMaternal sensitisation in a previous pregnancy is always required

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

The answer and the reasoning

A. Group O mothers commonly carry IgG anti-A and anti-B, which cross the placenta

Group O individuals have anti-A and anti-B that include IgG, which crosses the placenta, so even a first baby of group A or B can be affected. Mothers of group A or B carry mainly IgM anti-B or anti-A, which does not cross. ABO disease is usually milder than Rh disease because fetal red cells carry fewer A or B sites.

Option by option

A. Group O mothers commonly carry IgG anti-A and anti-B, which cross the placenta (correct)
Correct. IgG isohaemagglutinins cross the placenta without prior sensitisation.
B. Natural anti-A in group O mothers is mainly IgM, which crosses the placenta
IgM is a large pentamer that does not cross the placenta.
C. Fetal A antigen is absent on red cells, so sensitisation is not needed
Fetal red cells do express A antigen, which is the target.
D. Maternal sensitisation in a previous pregnancy is always required
Pre-existing natural antibodies mean a first pregnancy can be affected, unlike Rh disease.

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 A, Group O mothers commonly carry IgG anti-A and anti-B, which cross the placenta. Group O individuals have anti-A and anti-B that include IgG, which crosses the placenta, so even a first baby of group A or B can be affected.

Why is "Natural anti-A in group O mothers is mainly IgM, which crosses the pl…" not the answer?

IgM is a large pentamer that does not cross the placenta.

Why is "Fetal A antigen is absent on red cells, so sensitisation is not needed" not the answer?

Fetal red cells do express A antigen, which is the target.

Why is "Maternal sensitisation in a previous pregnancy is always required" not the answer?

Pre-existing natural antibodies mean a first pregnancy can be affected, unlike Rh disease.

What concept does this question test?

It tests Blood groups: ABO haemolytic disease in Physiology. Mothers of group A or B carry mainly IgM anti-B or anti-A, which does not cross. ABO disease is usually milder than Rh disease because fetal red cells carry fewer A or B sites.

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 groups: ABO haemolytic disease?

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.