Haematology: aPTT mixing study: 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: aPTT mixing study

Sample item

A 6-year-old boy has recurrent haemarthroses. PT is normal and aPTT is 70 seconds against a control of 30 seconds. When his plasma is mixed 1:1 with normal plasma, the aPTT falls to 33 seconds. What does this correction indicate?

AContamination of the sample with heparinBPresence of a lupus anticoagulantCPresence of a specific factor inhibitor antibodyDDeficiency of an intrinsic pathway clotting factor

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

The answer and the reasoning

D. Deficiency of an intrinsic pathway clotting factor

A factor-deficient plasma is repaired by the factors in normal plasma, so the mix corrects. An inhibitor or lupus anticoagulant continues to block the added factors, so the aPTT stays prolonged.

Option by option

A. Contamination of the sample with heparin
Heparin would continue to prolong the mixed sample and would not account for the history of haemarthroses.
B. Presence of a lupus anticoagulant
A lupus anticoagulant also stays prolonged after mixing and is not associated with recurrent haemarthroses.
C. Presence of a specific factor inhibitor antibody
A factor inhibitor persists after mixing and keeps the aPTT prolonged.
D. Deficiency of an intrinsic pathway clotting factor (correct)
Correct. Normal plasma supplies the missing factor, as in haemophilia A or B, and the aPTT corrects.

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 Pathology question

What is the answer to this Pathology question?

The best answer is D, Deficiency of an intrinsic pathway clotting factor. A factor-deficient plasma is repaired by the factors in normal plasma, so the mix corrects.

Why is "Contamination of the sample with heparin" not the answer?

Heparin would continue to prolong the mixed sample and would not account for the history of haemarthroses.

Why is "Presence of a lupus anticoagulant" not the answer?

A lupus anticoagulant also stays prolonged after mixing and is not associated with recurrent haemarthroses.

Why is "Presence of a specific factor inhibitor antibody" not the answer?

A factor inhibitor persists after mixing and keeps the aPTT prolonged.

What concept does this question test?

It tests Haematology: aPTT mixing study in Pathology. An inhibitor or lupus anticoagulant continues to block the added factors, so the aPTT stays prolonged.

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: aPTT mixing study?

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.