Haematology and oncology: paroxysmal cold haemoglobinuria: practice question

A General Medicine 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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General Medicine · Haematology and oncology: paroxysmal cold haemoglobinuria

Sample item

A 6-year-old boy has sudden dark urine and pallor after playing in cold water, two weeks after a viral illness. The direct antiglobulin test shows complement C3 only, cold agglutinin titre is low, and haemolysis occurs when his serum is first chilled with red cells and then warmed. Which antibody is responsible?

AIgG anti-Rh that coats red cells at body temperatureBIgM anti-I that agglutinates red cells in the coldCIgM anti-i found in infectious mononucleosisDIgG anti-P that binds in the cold and fixes complement on warming

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

The answer and the reasoning

D. IgG anti-P that binds in the cold and fixes complement on warming

Paroxysmal cold haemoglobinuria is caused by the biphasic Donath-Landsteiner IgG antibody against the P antigen. It binds red cells in the cold, and complement-mediated lysis occurs on warming. It follows viral infection in children and is usually self-limiting.

Option by option

A. IgG anti-Rh that coats red cells at body temperature
IgG anti-Rh causes warm autoimmune haemolysis, with IgG on the direct antiglobulin test.
B. IgM anti-I that agglutinates red cells in the cold
IgM anti-I causes cold agglutinin disease with agglutination and a high cold agglutinin titre.
C. IgM anti-i found in infectious mononucleosis
IgM anti-i is a cold agglutinin seen with Epstein-Barr infection and does not cause a biphasic lysis.
D. IgG anti-P that binds in the cold and fixes complement on warming (correct)
Correct. The biphasic IgG anti-P haemolysin explains haemolysis after chilling and warming.

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 General Medicine question

What is the answer to this General Medicine question?

The best answer is D, IgG anti-P that binds in the cold and fixes complement on warming. Paroxysmal cold haemoglobinuria is caused by the biphasic Donath-Landsteiner IgG antibody against the P antigen.

Why is "IgG anti-Rh that coats red cells at body temperature" not the answer?

IgG anti-Rh causes warm autoimmune haemolysis, with IgG on the direct antiglobulin test.

Why is "IgM anti-I that agglutinates red cells in the cold" not the answer?

IgM anti-I causes cold agglutinin disease with agglutination and a high cold agglutinin titre.

Why is "IgM anti-i found in infectious mononucleosis" not the answer?

IgM anti-i is a cold agglutinin seen with Epstein-Barr infection and does not cause a biphasic lysis.

What concept does this question test?

It tests Haematology and oncology: paroxysmal cold haemoglobinuria in General Medicine. It binds red cells in the cold, and complement-mediated lysis occurs on warming. It follows viral infection in children and is usually self-limiting.

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 Haematology and oncology: paroxysmal cold haemoglobinuria?

For each disease know the classic presentation, the best test, and the first-line treatment. Then use the flashcards and the other General Medicine 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.