Immunology: C3 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: C3 deficiency

Sample item

A 2-year-old has had repeated pneumonias and a bacteraemia due to Streptococcus pneumoniae and Haemophilus influenzae, plus a proliferative glomerulonephritis. Immunoglobulin levels and neutrophil function are normal, but both CH50 and AH50 haemolytic assays are almost zero. Which complement component is most likely deficient?

AC3BC5CC9DC1q

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

The answer and the reasoning

A. C3

C3 lies at the junction of all pathways, and its deficiency removes opsonisation by C3b, causing severe recurrent pyogenic infections with encapsulated bacteria and immune-complex disease. Both classical and alternative haemolytic assays are absent. Terminal component deficiencies mainly cause Neisseria infections instead.

Option by option

A. C3 (correct)
Correct. Loss of C3 abolishes opsonisation and both pathway assays, with severe pyogenic infections and immune-complex nephritis.
B. C5
C5 deficiency also lowers both assays but mainly causes recurrent Neisseria infections without prominent pyogenic sepsis.
C. C9
C9 deficiency is usually mild or asymptomatic, with some increased risk of Neisseria infection.
D. C1q
C1q deficiency lowers the classical pathway only and is associated with lupus-like disease, while AH50 remains normal.

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.

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is A, C3. C3 lies at the junction of all pathways, and its deficiency removes opsonisation by C3b, causing severe recurrent pyogenic infections with encapsulated bacteria and immune-complex disease.

Why is "C5" not the answer?

C5 deficiency also lowers both assays but mainly causes recurrent Neisseria infections without prominent pyogenic sepsis.

Why is "C9" not the answer?

C9 deficiency is usually mild or asymptomatic, with some increased risk of Neisseria infection.

Why is "C1q" not the answer?

C1q deficiency lowers the classical pathway only and is associated with lupus-like disease, while AH50 remains normal.

What concept does this question test?

It tests Immunology: C3 deficiency in Microbiology. Both classical and alternative haemolytic assays are absent. Terminal component deficiencies mainly cause Neisseria infections instead.

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 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.