Immunity: complement: practice question

A Physiology 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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Physiology · Immunity: complement

Sample item

A 20-year-old man has had two episodes of meningococcal meningitis. A total haemolytic complement (CH50) assay is near zero. Which defect is most likely?

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

The answer and the reasoning

B. Deficiency of a terminal component, C5 to C9, preventing membrane attack complex formation

The membrane attack complex formed by C5b to C9 punctures the outer membrane of Gram-negative diplococci such as Neisseria. Terminal complement deficiency causes recurrent Neisseria infection with a very low CH50. Other complement defects present differently.

Option by option

  • A. Deficiency of C1 esterase inhibitor

    C1 esterase inhibitor deficiency causes hereditary angioedema, not recurrent meningitis.

  • B. Deficiency of a terminal component, C5 to C9, preventing membrane attack complex formation (correct)

    Correct. Loss of the membrane attack complex leaves Neisseria insufficiently lysed.

  • C. Deficiency of decay accelerating factor (CD55)

    CD55 deficiency causes complement-mediated lysis of red cells in paroxysmal nocturnal haemoglobinuria.

  • D. Deficiency of the immunoglobulin heavy chain gene

    A heavy chain defect gives agammaglobulinaemia, with a normal CH50.

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 Physiology question

What is the answer to this Physiology question?

The best answer is B, Deficiency of a terminal component, C5 to C9, preventing membrane attack complex formation. The membrane attack complex formed by C5b to C9 punctures the outer membrane of Gram-negative diplococci such as Neisseria.

Why is "Deficiency of C1 esterase inhibitor" not the answer?

C1 esterase inhibitor deficiency causes hereditary angioedema, not recurrent meningitis.

Why is "Deficiency of decay accelerating factor (CD55)" not the answer?

CD55 deficiency causes complement-mediated lysis of red cells in paroxysmal nocturnal haemoglobinuria.

Why is "Deficiency of the immunoglobulin heavy chain gene" not the answer?

A heavy chain defect gives agammaglobulinaemia, with a normal CH50.

What concept does this question test?

It tests Immunity: complement in Physiology. Terminal complement deficiency causes recurrent Neisseria infection with a very low CH50. Other complement defects present differently.

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 Immunity: complement?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology 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.