Blood and immunity: paroxysmal nocturnal haemoglobinuria: 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 · Blood and immunity: paroxysmal nocturnal haemoglobinuria

Sample item

A 30-year-old man has episodes of dark morning urine, unexplained abdominal vein thrombosis and a negative Coombs test. Which basic abnormality causes his red cells to be lysed by his own complement?

ALoss of GPI-anchored complement regulators CD55 and CD59BInherited absence of C3 proteinCAutoantibody coating the red cell membraneDDeficiency of the C1 esterase inhibitor

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

The answer and the reasoning

A. Loss of GPI-anchored complement regulators CD55 and CD59

In paroxysmal nocturnal haemoglobinuria an acquired PIGA mutation in a stem cell prevents synthesis of the GPI anchor, so CD55 (decay-accelerating factor) and CD59 are missing. Without them complement activation proceeds unchecked and the membrane attack complex lyses red cells.

Option by option

A. Loss of GPI-anchored complement regulators CD55 and CD59 (correct)
Correct. Missing CD55 and CD59 allow unchecked complement lysis.
B. Inherited absence of C3 protein
C3 absence would impair opsonisation and cause infections instead of complement-mediated haemolysis.
C. Autoantibody coating the red cell membrane
Antibody coating would give a positive Coombs test, which is negative here.
D. Deficiency of the C1 esterase inhibitor
C1 inhibitor deficiency causes hereditary angioedema, not haemolysis.

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 Physiology question

What is the answer to this Physiology question?

The best answer is A, Loss of GPI-anchored complement regulators CD55 and CD59. In paroxysmal nocturnal haemoglobinuria an acquired PIGA mutation in a stem cell prevents synthesis of the GPI anchor, so CD55 (decay-accelerating factor) and CD59 are missing.

Why is "Inherited absence of C3 protein" not the answer?

C3 absence would impair opsonisation and cause infections instead of complement-mediated haemolysis.

Why is "Autoantibody coating the red cell membrane" not the answer?

Antibody coating would give a positive Coombs test, which is negative here.

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

C1 inhibitor deficiency causes hereditary angioedema, not haemolysis.

What concept does this question test?

It tests Blood and immunity: paroxysmal nocturnal haemoglobinuria in Physiology. Without them complement activation proceeds unchecked and the membrane attack complex lyses red cells.

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 Blood and immunity: paroxysmal nocturnal haemoglobinuria?

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.