Immunology: immunoglobulin classes: 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: immunoglobulin classes

Sample item

A newborn has a markedly raised IgM in cord blood, with no maternal infection recognised. What is the reasoning behind suspecting intrauterine infection?

AIgM does not cross the placenta, so it must have been synthesised by the fetusBIgM crosses the placenta by FcRn, so it reflects maternal immunityCCord IgM is always raised in healthy neonates after deliveryDIgM is secreted into breast milk and absorbed before birth

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

The answer and the reasoning

A. IgM does not cross the placenta, so it must have been synthesised by the fetus

Only IgG crosses the placenta, using the neonatal Fc receptor. IgM is a large pentamer that cannot cross, so IgM above about 20 mg/dL in cord blood or specific IgM in a newborn indicates fetal antibody synthesis, as in congenital TORCH infections.

Option by option

A. IgM does not cross the placenta, so it must have been synthesised by the fetus (correct)
Correct. Fetal IgM production signals prenatal exposure to antigen.
B. IgM crosses the placenta by FcRn, so it reflects maternal immunity
FcRn transports IgG, and IgM cannot use this route.
C. Cord IgM is always raised in healthy neonates after delivery
Healthy neonates have very low IgM at birth, so raised levels are abnormal.
D. IgM is secreted into breast milk and absorbed before birth
Breast milk is not available before birth, and its main antibody is secretory IgA.

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 Microbiology question

What is the answer to this Microbiology question?

The best answer is A, IgM does not cross the placenta, so it must have been synthesised by the fetus. Only IgG crosses the placenta, using the neonatal Fc receptor.

Why is "IgM crosses the placenta by FcRn, so it reflects maternal immunity" not the answer?

FcRn transports IgG, and IgM cannot use this route.

Why is "Cord IgM is always raised in healthy neonates after delivery" not the answer?

Healthy neonates have very low IgM at birth, so raised levels are abnormal.

Why is "IgM is secreted into breast milk and absorbed before birth" not the answer?

Breast milk is not available before birth, and its main antibody is secretory IgA.

What concept does this question test?

It tests Immunology: immunoglobulin classes in Microbiology. IgM is a large pentamer that cannot cross, so IgM above about 20 mg/dL in cord blood or specific IgM in a newborn indicates fetal antibody synthesis, as in congenital TORCH infections.

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 Immunology: immunoglobulin classes?

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.