Immunology: C3 nephritic factor: 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: C3 nephritic factor

Sample item

A 12-year-old girl has haematuria, proteinuria and loss of subcutaneous fat from the face and upper trunk. Serum C3 is persistently very low with normal C4, and biopsy shows a tram-track basement membrane. Which autoantibody explains the complement pattern?

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

The answer and the reasoning

D. Antibody that stabilises the alternative pathway C3 convertase

C3 nephritic factor binds the alternative pathway convertase C3bBb and prevents its decay, so C3 is consumed continuously. This gives low C3 with normal C4, membranoproliferative glomerulonephritis and sometimes partial lipodystrophy.

Option by option

  • A. Antibody against double-stranded DNA forming immune complexes

    Anti-dsDNA lupus complexes consume classical components, so C4 is low along with C3.

  • B. Antibody against type IV collagen of the basement membrane

    Anti-GBM disease gives linear IgG staining with usually normal complement levels.

  • C. Antibody against C1q that depletes the classical pathway

    Anti-C1q disease lowers C1q and C4 as well, because it acts on the classical pathway.

  • D. Antibody that stabilises the alternative pathway C3 convertase (correct)

    Correct. C3 nephritic factor stabilises C3bBb and drives continuous C3 consumption.

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 D, Antibody that stabilises the alternative pathway C3 convertase. C3 nephritic factor binds the alternative pathway convertase C3bBb and prevents its decay, so C3 is consumed continuously.

Why is "Antibody against double-stranded DNA forming immune complexes" not the answer?

Anti-dsDNA lupus complexes consume classical components, so C4 is low along with C3.

Why is "Antibody against type IV collagen of the basement membrane" not the answer?

Anti-GBM disease gives linear IgG staining with usually normal complement levels.

Why is "Antibody against C1q that depletes the classical pathway" not the answer?

Anti-C1q disease lowers C1q and C4 as well, because it acts on the classical pathway.

What concept does this question test?

It tests Immunology: C3 nephritic factor in Microbiology. This gives low C3 with normal C4, membranoproliferative glomerulonephritis and sometimes partial lipodystrophy.

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: C3 nephritic factor?

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.