Immunology: complement regulation: 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 regulation

Sample item

A 28-year-old woman has episodes of dark morning urine, unexplained abdominal vein thrombosis and pancytopenia. Which test best establishes the diagnosis?

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

The answer and the reasoning

D. Flow cytometry for CD55 and CD59 on blood cells

Paroxysmal nocturnal haemoglobinuria arises from acquired PIGA mutation in a stem cell, leading to loss of GPI-anchored proteins including CD55 (decay-accelerating factor) and CD59. Without these complement regulators, red cells undergo complement-mediated intravascular haemolysis. Flow cytometry is the diagnostic test.

Option by option

  • A. Haemoglobin electrophoresis

    Electrophoresis diagnoses haemoglobinopathies such as thalassaemia.

  • B. Direct antiglobulin test

    A direct antiglobulin test detects antibody on red cells in autoimmune haemolysis, and is negative here.

  • C. Osmotic fragility test

    Osmotic fragility is increased in hereditary spherocytosis.

  • D. Flow cytometry for CD55 and CD59 on blood cells (correct)

    Correct. Flow cytometry demonstrates the missing complement regulators.

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.

More Microbiology questions

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is D, Flow cytometry for CD55 and CD59 on blood cells. Paroxysmal nocturnal haemoglobinuria arises from acquired PIGA mutation in a stem cell, leading to loss of GPI-anchored proteins including CD55 (decay-accelerating factor) and CD59.

Why is "Haemoglobin electrophoresis" not the answer?

Electrophoresis diagnoses haemoglobinopathies such as thalassaemia.

Why is "Direct antiglobulin test" not the answer?

A direct antiglobulin test detects antibody on red cells in autoimmune haemolysis, and is negative here.

Why is "Osmotic fragility test" not the answer?

Osmotic fragility is increased in hereditary spherocytosis.

What concept does this question test?

It tests Immunology: complement regulation in Microbiology. Without these complement regulators, red cells undergo complement-mediated intravascular haemolysis. Flow cytometry is the diagnostic test.

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 regulation?

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.