A patient with inherited protein C deficiency begins warfarin therapy without heparin bridging. In the first 24–48 hours, why is this patient paradoxically at increased thrombotic risk rather than reduced risk?
AWarfarin activates protein C before depleting clotting factors, creating a burst of anticoagulant activity
BWarfarin preferentially depletes protein C (half-life ~8 hours) before it depletes factors II and X (half-life 60–72 hours), removing the anticoagulant brake before the procoagulant factors decline
CProtein C deficiency causes warfarin resistance, so higher doses are required and clotting increases temporarily
DHeparin is contraindicated in protein C deficiency, making bridging impossible
Protein C has a much shorter half-life than the procoagulant factors warfarin also inhibits. Warfarin depletes protein C first, eliminating the anticoagulant feedback brake before factors II and X fall. This creates a window of net hypercoagulability — the 'warfarin skin necrosis' paradox — which is why heparin bridging is essential when starting warfarin, especially in patients with protein C deficiency.
Question 2 Multiple Choice
Antithrombin (AT) activity is dramatically higher on intact endothelial surfaces than in circulating plasma. The mechanism responsible is:
AAT is secreted exclusively by endothelial cells and does not enter the circulation
BHeparan sulfate proteoglycans on endothelial surfaces dramatically accelerate AT's inactivation of coagulation proteases, confining inhibitory activity to the zone where spreading coagulation would encounter healthy tissue
CPlatelets release a plasma inhibitor of AT that reduces its activity in circulating blood
DAT works by competing with thrombin for fibrinogen-binding sites, a mechanism only available on endothelial surfaces
AT circulates throughout the bloodstream but its activity is greatly enhanced by heparan sulfate proteoglycans coating intact endothelium — the same mechanism mimicked by exogenous heparin. This spatial logic confines coagulation: proteases escaping the injury site toward healthy endothelium are rapidly neutralized, while proteases at the clot site (where subendothelial tissue is exposed) act freely.
Question 3 True / False
The protein C pathway functions independently of thrombin — it is activated by vascular injury signals rather than by thrombin generated during coagulation.
TTrue
FFalse
Answer: False
The protein C pathway is activated by thrombin binding thrombomodulin on intact endothelial cells. This is precisely what makes it a self-limiting feedback mechanism: the more thrombin is generated, the more protein C is activated on adjacent endothelium, and the more the amplification machinery (factors Va and VIIIa) is dismantled. Thrombin itself triggers its own brake.
Question 4 True / False
Protein C and protein S deficiencies predominantly cause venous rather than arterial thromboembolism.
TTrue
FFalse
Answer: True
Impaired inactivation of factors Va and VIIIa sustains runaway amplification of thrombin and Xa production. This is most dangerous in venous beds where blood flow is slow and the intrinsic (contact activation) pathway is the primary driver of coagulation. Arterial thrombosis is more often driven by platelet activation at sites of endothelial disruption — a different mechanism.
Question 5 Short Answer
Explain how the protein C pathway exemplifies 'self-limiting amplification' — using the molecular roles of thrombin, thrombomodulin, and factors Va and VIIIa.
Think about your answer, then reveal below.
Model answer: Thrombin drives coagulation amplification, but when it binds thrombomodulin on intact endothelial cells adjacent to the clot site, the complex loses its ability to cleave fibrinogen and instead activates protein C. Activated protein C (with cofactor protein S) destroys factors Va and VIIIa — the two co-factors that dramatically amplify both thrombin generation and factor Xa production. The more thrombin is generated, the more protein C is activated and the more the amplification machinery is dismantled. The very molecule driving amplification triggers its own shutdown.
This feedback loop is a textbook example of product-inhibited amplification: the amplification product (thrombin) activates the mechanism that destroys the amplification co-factors. Clinical consequences — protein C/S deficiency causing VTE, warfarin skin necrosis — all follow directly from disruptions of this feedback loop.