Immunology: physiological hypogammaglobulinaemia of infancy: 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: physiological hypogammaglobulinaemia of infancy

Sample item

A thriving 5-month-old breastfed infant with only two mild colds has a low serum IgG level on a screening test, with normal IgM. There is no history of serious infection. What is the best explanation?

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

The answer and the reasoning

C. Maternal IgG has declined while the infant's own IgG production is still maturing

Maternal IgG crosses the placenta and has a half-life of about three weeks, so levels fall during the first months. The infant's own IgG synthesis rises more slowly, producing a physiological nadir at about 3 to 6 months. An otherwise healthy child needs no treatment, whereas recurrent serious infections suggest a primary antibody deficiency.

Option by option

  • A. Selective loss of IgG from the gut

    A protein-losing enteropathy would be accompanied by diarrhoea and low albumin.

  • B. Absent B cells due to a BTK mutation

    BTK deficiency would produce serious infections once maternal antibody wanes and an absence of B cells.

  • C. Maternal IgG has declined while the infant's own IgG production is still maturing (correct)

    Correct. The nadir reflects catabolism of maternal IgG before adequate endogenous production.

  • D. Defective class switching due to CD40 ligand deficiency

    CD40 ligand deficiency causes recurrent infections with normal or raised IgM, rather than a healthy infant.

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, Maternal IgG has declined while the infant's own IgG production is still maturing. Maternal IgG crosses the placenta and has a half-life of about three weeks, so levels fall during the first months.

Why is "Selective loss of IgG from the gut" not the answer?

A protein-losing enteropathy would be accompanied by diarrhoea and low albumin.

Why is "Absent B cells due to a BTK mutation" not the answer?

BTK deficiency would produce serious infections once maternal antibody wanes and an absence of B cells.

Why is "Defective class switching due to CD40 ligand deficiency" not the answer?

CD40 ligand deficiency causes recurrent infections with normal or raised IgM, rather than a healthy infant.

What concept does this question test?

It tests Immunology: physiological hypogammaglobulinaemia of infancy in Microbiology. The infant's own IgG synthesis rises more slowly, producing a physiological nadir at about 3 to 6 months. An otherwise healthy child needs no treatment, whereas recurrent serious infections suggest a primary antibody deficiency.

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: physiological hypogammaglobulinaemia of infancy?

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.