Blood and immunity: initiation of coagulation: 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: initiation of coagulation

Sample item

After a vessel is cut, which event is the main trigger for coagulation in vivo?

AExposure of tissue factor, which binds factor VIIaBThrombin activation of factor V in plasmaCRelease of factor VIII from platelet granulesDContact of factor XII with subendothelial collagen

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

The answer and the reasoning

A. Exposure of tissue factor, which binds factor VIIa

Tissue factor on subendothelial cells and fibroblasts is exposed when the vessel is injured. It binds factor VII and activates it, and the complex activates factors X and IX. The small amount of thrombin generated then amplifies the response. The contact (intrinsic) pathway is more important in vitro than in vivo.

Option by option

A. Exposure of tissue factor, which binds factor VIIa (correct)
Correct. Tissue factor with factor VIIa starts the extrinsic pathway in vivo.
B. Thrombin activation of factor V in plasma
Thrombin feeds back to activate factor V, which amplifies clotting after it has started.
C. Release of factor VIII from platelet granules
Factor VIII is made largely by endothelium and the liver, and platelets do not release it as the trigger.
D. Contact of factor XII with subendothelial collagen
Contact activation of factor XII is the in vitro trigger of the aPTT and is not the physiological start.

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 Physiology question

What is the answer to this Physiology question?

The best answer is A, Exposure of tissue factor, which binds factor VIIa. Tissue factor on subendothelial cells and fibroblasts is exposed when the vessel is injured.

Why is "Thrombin activation of factor V in plasma" not the answer?

Thrombin feeds back to activate factor V, which amplifies clotting after it has started.

Why is "Release of factor VIII from platelet granules" not the answer?

Factor VIII is made largely by endothelium and the liver, and platelets do not release it as the trigger.

Why is "Contact of factor XII with subendothelial collagen" not the answer?

Contact activation of factor XII is the in vitro trigger of the aPTT and is not the physiological start.

What concept does this question test?

It tests Blood and immunity: initiation of coagulation in Physiology. It binds factor VII and activates it, and the complex activates factors X and IX. The small amount of thrombin generated then amplifies the response. The contact (intrinsic) pathway is more important in vitro than in vivo.

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: initiation of coagulation?

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.