Immunology: Omenn syndrome: 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: Omenn syndrome

Sample item

A 2-month-old has generalised erythroderma, lymphadenopathy, hepatosplenomegaly, eosinophilia and a high serum IgE with chronic diarrhoea. B cells are absent, and a few oligoclonal activated T cells are present. Which defect is most likely?

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

The answer and the reasoning

A. Hypomorphic mutation in RAG1 or RAG2

Omenn syndrome is a form of severe combined immunodeficiency in which partly functional RAG1 or RAG2 allow a few oligoclonal, autoreactive T cells to develop. The result is erythroderma, lymphadenopathy, eosinophilia and high IgE, with absent B cells and low immunoglobulins other than IgE. Haematopoietic stem cell transplant is curative.

Option by option

  • A. Hypomorphic mutation in RAG1 or RAG2 (correct)

    Correct. Leaky RAG function permits oligoclonal T cells that infiltrate skin and gut, giving the Omenn picture.

  • B. Mutation of the WAS gene

    WAS mutation causes eczema, thrombocytopenia and infections but not erythroderma with lymphadenopathy in infancy.

  • C. Mutation of BTK

    BTK mutation causes X-linked agammaglobulinaemia with absent B cells but no erythroderma or T-cell expansion.

  • D. Mutation of the CD40 ligand gene

    CD40 ligand deficiency causes hyper IgM syndrome with normal or high IgM and recurrent opportunistic infections.

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 A, Hypomorphic mutation in RAG1 or RAG2. Omenn syndrome is a form of severe combined immunodeficiency in which partly functional RAG1 or RAG2 allow a few oligoclonal, autoreactive T cells to develop.

Why is "Mutation of the WAS gene" not the answer?

WAS mutation causes eczema, thrombocytopenia and infections but not erythroderma with lymphadenopathy in infancy.

Why is "Mutation of BTK" not the answer?

BTK mutation causes X-linked agammaglobulinaemia with absent B cells but no erythroderma or T-cell expansion.

Why is "Mutation of the CD40 ligand gene" not the answer?

CD40 ligand deficiency causes hyper IgM syndrome with normal or high IgM and recurrent opportunistic infections.

What concept does this question test?

It tests Immunology: Omenn syndrome in Microbiology. The result is erythroderma, lymphadenopathy, eosinophilia and high IgE, with absent B cells and low immunoglobulins other than IgE. Haematopoietic stem cell transplant is curative.

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: Omenn syndrome?

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.