Blood and immunity: role of factor VIII: 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: role of factor VIII

Sample item

A boy with recurrent haemarthroses has a prolonged aPTT with a normal PT and normal platelet count, and his factor VIII activity is under 1 percent. In the coagulation cascade, what role does factor VIII normally play?

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

The answer and the reasoning

B. Cofactor that accelerates factor X activation by factor IXa

Factor VIII, activated by thrombin, forms the tenase complex with factor IXa on the platelet surface and greatly increases the rate of factor X activation. Without it, propagation of coagulation is poor, giving the joint and muscle bleeding of haemophilia A.

Option by option

  • A. Transglutaminase that cross-links fibrin polymers

    This describes factor XIII, which stabilises the fibrin clot.

  • B. Cofactor that accelerates factor X activation by factor IXa (correct)

    Correct. Factor VIIIa is the cofactor of IXa in the tenase complex.

  • C. Serine protease that converts factor X to Xa

    This describes factor IXa, the protease that factor VIII assists.

  • D. Cofactor for factor Xa in converting prothrombin to thrombin

    This describes factor V, the cofactor of the prothrombinase complex.

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 Physiology question

What is the answer to this Physiology question?

The best answer is B, Cofactor that accelerates factor X activation by factor IXa. Factor VIII, activated by thrombin, forms the tenase complex with factor IXa on the platelet surface and greatly increases the rate of factor X activation.

Why is "Transglutaminase that cross-links fibrin polymers" not the answer?

This describes factor XIII, which stabilises the fibrin clot.

Why is "Serine protease that converts factor X to Xa" not the answer?

This describes factor IXa, the protease that factor VIII assists.

Why is "Cofactor for factor Xa in converting prothrombin to thrombin" not the answer?

This describes factor V, the cofactor of the prothrombinase complex.

What concept does this question test?

It tests Blood and immunity: role of factor VIII in Physiology. Without it, propagation of coagulation is poor, giving the joint and muscle bleeding of haemophilia A.

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 Blood and immunity: role of factor VIII?

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.