Thrombosis is initiated by:

Correct Answer: C. Endothelial injury

Thrombosis is fundamentally initiated by endothelial injury, which is the primary trigger in Virchow's triad. When the endothelium is damaged—whether by atherosclerotic plaque rupture, trauma, infection, or indwelling catheters—the subendothelial collagen and tissue factor are exposed. This exposure activates platelets directly and simultaneously initiates the extrinsic coagulation pathway. In Indian clinical practice, acute coronary syndromes (ACS) and stroke represent the most common thrombotic emergencies, both stemming from endothelial disruption. The intact endothelium normally provides an anticoagulant surface via heparan sulfate, thrombomodulin, and prostacyclin; loss of this barrier is the initiating event. Platelet activation and coagulation cascade are secondary consequences of endothelial injury, not primary initiators. Vasoconstriction may accompany thrombosis but does not initiate it. Per Robbins pathology, endothelial injury is the most common and important cause of thrombosis, accounting for the majority of clinically significant thrombi in arterial and venous systems.

Why the other options are wrong

A. Platelet activation — Platelet activation is a secondary event that occurs after endothelial injury exposes subendothelial collagen and von Willebrand factor. While essential for thrombus propagation, platelets cannot initiate thrombosis without an underlying trigger—they require adhesion to exposed matrix or tissue factor signaling. This is the NBE trap: students confuse the sequence and think platelet activation is primary. B. Vasoconstriction of vessels — Vasoconstriction is a consequence of thrombosis (via serotonin and thromboxane release), not an initiator. While it may accompany thrombotic events and reduce blood flow, it does not directly trigger the coagulation cascade or platelet adhesion. Vasoconstriction alone, without endothelial injury, does not cause thrombosis—blood flow stasis alone is insufficient without vessel wall damage. D. Coagulation cascade — The coagulation cascade is activated downstream of endothelial injury via tissue factor exposure (extrinsic pathway). It is not the initiating event but rather a propagation mechanism. Tissue factor, released from damaged endothelium, triggers Factor VII and the cascade—making endothelial injury the true primary event. Students often confuse the sequence of Virchow's triad components.

High-Yield Facts

Mnemonics

VET (Virchow's Endothelial Triad) Vessel wall injury (endothelial injury) → Extrinsic pathway (tissue factor) → Thrombosis. Endothelial injury is listed first because it is the primary initiator; the other two factors (stasis and hypercoagulability) are secondary modifiers. SEAT (Sequence of thrombosis initiation) Subendothelial exposure → Endothelial injury (primary) → Activation of platelets (secondary) → Tissue factor cascade (secondary). Use this to remember the temporal sequence.

NBE Trap

NBE pairs "platelet activation" with thrombosis to trap students who memorize platelet function in isolation and forget that platelets require an endothelial trigger (collagen, vWF, TF) to initiate adhesion. The question tests understanding of sequence, not just components.

Clinical Pearl

In acute coronary syndromes (the leading cause of mortality in urban India), thrombosis is initiated when atherosclerotic plaque ruptures and exposes endothelial injury—not by platelet dysfunction. This is why dual antiplatelet therapy (aspirin + P2Y12 inhibitor) works: it blocks the secondary platelet response, but the endothelial injury has already occurred. Understanding this sequence is critical for interpreting why anticoagulation (targeting the coagulation cascade) is also needed in ACS.

_Reference: Robbins and Cotran Pathologic Basis of Disease, Ch. 4 (Hemostasis and Thrombosis)_