Immunology: complement deficiency: 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: complement deficiency

Sample item

A 20-year-old man has recurrent episodes of non-itchy, non-pitting swelling of the lips and face and severe abdominal colic. Antihistamines and adrenaline give no relief, and his father had laryngeal swelling. Which laboratory finding is expected?

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

The answer and the reasoning

B. Low C4 with reduced C1 esterase inhibitor level or function

Hereditary angioedema results from C1 esterase inhibitor deficiency or dysfunction, causing uncontrolled bradykinin release and unchecked classical pathway activation. C4 is low even between attacks and is the screening test, confirmed by C1 inhibitor level and function. Attacks do not respond to antihistamines or adrenaline.

Option by option

  • A. Raised serum tryptase between attacks

    Raised tryptase marks mast cell degranulation as in anaphylaxis, which would respond to adrenaline.

  • B. Low C4 with reduced C1 esterase inhibitor level or function (correct)

    Correct. Low C4 with deficient C1 inhibitor confirms the diagnosis.

  • C. Low C3 with normal C4

    Low C3 with normal C4 suggests alternative pathway consumption, not this disorder.

  • D. Markedly raised total serum IgE

    Total IgE is raised in atopy and does not explain bradykinin-mediated swelling.

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 B, Low C4 with reduced C1 esterase inhibitor level or function. Hereditary angioedema results from C1 esterase inhibitor deficiency or dysfunction, causing uncontrolled bradykinin release and unchecked classical pathway activation.

Why is "Raised serum tryptase between attacks" not the answer?

Raised tryptase marks mast cell degranulation as in anaphylaxis, which would respond to adrenaline.

Why is "Low C3 with normal C4" not the answer?

Low C3 with normal C4 suggests alternative pathway consumption, not this disorder.

Why is "Markedly raised total serum IgE" not the answer?

Total IgE is raised in atopy and does not explain bradykinin-mediated swelling.

What concept does this question test?

It tests Immunology: complement deficiency in Microbiology. C4 is low even between attacks and is the screening test, confirmed by C1 inhibitor level and function. Attacks do not respond to antihistamines or adrenaline.

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: complement deficiency?

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.