Haematology: cold agglutinin haemolysis: practice question

A Pathology 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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Pathology · Haematology: cold agglutinin haemolysis

Sample item

A 25-year-old man recovering from Mycoplasma pneumoniae infection develops pallor and jaundice, and his fingers turn blue on exposure to cold. Which antibody and test result characterise his haemolysis?

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

The answer and the reasoning

B. IgM against the I antigen, with direct Coombs test positive for C3

Cold agglutinin disease is caused by IgM autoantibodies, commonly against the I antigen, which bind red cells at low temperatures and fix complement. Cells coated with C3b are removed by liver macrophages, and the direct Coombs test is positive for C3. Associations are Mycoplasma pneumoniae, infectious mononucleosis and lymphoproliferative disorders.

Option by option

  • A. Drug-dependent IgG acting as a hapten after penicillin

    Hapten-type drug-induced haemolysis follows a drug exposure such as penicillin, which is absent here.

  • B. IgM against the I antigen, with direct Coombs test positive for C3 (correct)

    Correct. Cold-reactive IgM with complement fixation explains the Coombs result and cold-induced acrocyanosis.

  • C. Biphasic IgG against the P antigen (Donath-Landsteiner antibody)

    The Donath-Landsteiner antibody causes paroxysmal cold haemoglobinuria, typically in children after viral infection.

  • D. IgG against Rh antigens, with direct Coombs test positive for IgG

    Warm IgG autoantibodies against Rh antigens cause warm haemolysis with a Coombs test positive for IgG.

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.

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Questions about this Pathology question

What is the answer to this Pathology question?

The best answer is B, IgM against the I antigen, with direct Coombs test positive for C3. Cold agglutinin disease is caused by IgM autoantibodies, commonly against the I antigen, which bind red cells at low temperatures and fix complement.

Why is "Drug-dependent IgG acting as a hapten after penicillin" not the answer?

Hapten-type drug-induced haemolysis follows a drug exposure such as penicillin, which is absent here.

Why is "Biphasic IgG against the P antigen (Donath-Landsteiner antibody)" not the answer?

The Donath-Landsteiner antibody causes paroxysmal cold haemoglobinuria, typically in children after viral infection.

Why is "IgG against Rh antigens, with direct Coombs test positive for IgG" not the answer?

Warm IgG autoantibodies against Rh antigens cause warm haemolysis with a Coombs test positive for IgG.

What concept does this question test?

It tests Haematology: cold agglutinin haemolysis in Pathology. Cells coated with C3b are removed by liver macrophages, and the direct Coombs test is positive for C3. Associations are Mycoplasma pneumoniae, infectious mononucleosis and lymphoproliferative disorders.

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 Haematology: cold agglutinin haemolysis?

Learn each disease as a bundle: cause, key histology, markers, and the clinical hallmark. Then use the flashcards and the other Pathology 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.