Immunology: complement levels in active lupus: 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 levels in active lupus

Sample item

A patient with systemic lupus erythematosus develops a flare with nephritis, and serum C3 and C4 are both markedly low. What is the best explanation?

AImmune complexes consume complement through the classical pathwayBLoss of CD59 from red cellsCInherited absence of C1 inhibitorDFailure of hepatic synthesis of all complement proteins

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

The answer and the reasoning

A. Immune complexes consume complement through the classical pathway

In active lupus, circulating and deposited immune complexes activate the classical pathway, consuming C1q, C4 and C3, so levels fall as disease activity rises. Serial complement levels and anti-dsDNA titres are used to follow disease activity. Low C4 with normal C3 is more characteristic of hereditary angioedema.

Option by option

A. Immune complexes consume complement through the classical pathway (correct)
Correct. Classical pathway activation by immune complexes depletes C4 and C3.
B. Loss of CD59 from red cells
CD59 loss is the defect in paroxysmal nocturnal haemoglobinuria and does not lower serum complement.
C. Inherited absence of C1 inhibitor
C1 inhibitor deficiency causes hereditary angioedema, with low C4 but usually normal C3.
D. Failure of hepatic synthesis of all complement proteins
Hepatic synthetic failure is not the mechanism of low complement in a lupus flare.

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, Immune complexes consume complement through the classical pathway. In active lupus, circulating and deposited immune complexes activate the classical pathway, consuming C1q, C4 and C3, so levels fall as disease activity rises.

Why is "Loss of CD59 from red cells" not the answer?

CD59 loss is the defect in paroxysmal nocturnal haemoglobinuria and does not lower serum complement.

Why is "Inherited absence of C1 inhibitor" not the answer?

C1 inhibitor deficiency causes hereditary angioedema, with low C4 but usually normal C3.

Why is "Failure of hepatic synthesis of all complement proteins" not the answer?

Hepatic synthetic failure is not the mechanism of low complement in a lupus flare.

What concept does this question test?

It tests Immunology: complement levels in active lupus in Microbiology. Serial complement levels and anti-dsDNA titres are used to follow disease activity. Low C4 with normal C3 is more characteristic of hereditary angioedema.

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 levels in active lupus?

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.