Immunology: ABO haemolytic disease of the newborn: practice question

A Microbiology 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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Microbiology · Immunology: ABO haemolytic disease of the newborn

Sample item

A group O, Rh-positive mother delivers her first child, who is group A, Rh-positive. The baby becomes jaundiced on the second day, with spherocytes on the smear and a weakly positive direct antiglobulin test. Which statement best explains why this can occur in a first pregnancy?

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

The answer and the reasoning

C. Group O mothers often have pre-existing IgG anti-A and anti-B that cross the placenta

Group O individuals commonly make IgG anti-A and anti-B after exposure to similar antigens on gut bacteria and environmental material. IgG, unlike IgM, crosses the placenta through the neonatal Fc receptor, so ABO disease can occur in the first pregnancy. Because fetal red cells express A antigen weakly, haemolysis is usually mild and the direct antiglobulin test only weakly positive.

Option by option

  • A. Fetal A antigen is poorly expressed, so haemolysis is usually severe

    Weak expression of A antigen on fetal cells makes ABO disease milder, not more severe.

  • B. Sensitisation occurred at the delivery of an earlier pregnancy

    Sensitisation at an earlier delivery explains Rh disease, but it is not required for ABO disease.

  • C. Group O mothers often have pre-existing IgG anti-A and anti-B that cross the placenta (correct)

    Naturally acquired IgG antibodies cross the placenta, so prior pregnancy is not needed.

  • D. Group O mothers have IgM anti-A antibodies that cross the placenta freely

    IgM is a pentamer and does not cross the placenta.

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 Microbiology question

What is the answer to this Microbiology question?

The best answer is C, Group O mothers often have pre-existing IgG anti-A and anti-B that cross the placenta. Group O individuals commonly make IgG anti-A and anti-B after exposure to similar antigens on gut bacteria and environmental material.

Why is "Fetal A antigen is poorly expressed, so haemolysis is usually severe" not the answer?

Weak expression of A antigen on fetal cells makes ABO disease milder, not more severe.

Why is "Sensitisation occurred at the delivery of an earlier pregnancy" not the answer?

Sensitisation at an earlier delivery explains Rh disease, but it is not required for ABO disease.

Why is "Group O mothers have IgM anti-A antibodies that cross the placenta fr…" not the answer?

IgM is a pentamer and does not cross the placenta.

What concept does this question test?

It tests Immunology: ABO haemolytic disease of the newborn in Microbiology. IgG, unlike IgM, crosses the placenta through the neonatal Fc receptor, so ABO disease can occur in the first pregnancy. Because fetal red cells express A antigen weakly, haemolysis is usually mild and the direct antiglobulin test only weakly positive.

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 Immunology: ABO haemolytic disease of the newborn?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology 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.