Questions: Hemostasis and Coagulation Pathophysiology

Question 1

A patient with von Willebrand disease has reduced platelet adhesion to the subendothelium after vascular injury. Which step of normal hemostasis is primarily impaired?

• A. Vascular spasm and vasoconstriction
• B. Primary hemostasis — platelet plug formation
• C. The intrinsic coagulation pathway
• D. Fibrinolysis

Answer: (B)

Von Willebrand factor (vWF) bridges platelet surface receptors (GPIb) to exposed subendothelial collagen, enabling initial platelet adhesion — the first step of primary hemostasis. Without vWF, platelets cannot adhere, so the platelet plug that forms before the coagulation cascade is initiated fails to form properly. This is a primary hemostasis defect, distinct from coagulation factor deficiencies.

Question 2

The extrinsic coagulation pathway is called 'extrinsic' because it is triggered by clotting factors that circulate freely in the bloodstream rather than factors released from injured tissue.

• T. True
• F. False

Answer: False

The extrinsic pathway is initiated by tissue factor (TF), a transmembrane protein expressed by cells in the vessel wall and surrounding tissue that is exposed upon vascular injury. TF is 'extrinsic' to the circulation in the sense that it is not normally present in blood, but it is not a circulating factor — it comes from damaged tissue, not an external source.

Question 3

A patient has normal platelet counts and normal clotting times on standard tests, but still bleeds abnormally. What hemostatic mechanism might explain this?

Model answer: Platelet function defects: platelets may be present in adequate numbers but fail to activate, aggregate, or release granule contents properly when stimulated by agonists like thrombin, ADP, or collagen. Disorders like Glanzmann thrombasthenia (GPIIb/IIIa deficiency) impair aggregation despite normal counts.

Standard coagulation tests (PT, aPTT) assess the coagulation cascade, and platelet counts assess quantity — but neither measures platelet function directly. Qualitative platelet defects require platelet aggregometry or flow cytometry to detect. This illustrates why hemostasis must be evaluated at each step, not just globally.