Which substance prevents blood from clotting inside the blood vessels?

Explanation

Thrombin is an enzyme that plays a key role in the blood clotting process by converting fibrinogen into fibrin. However, blood does not clot inside vessels under normal conditions because thrombin is not active there. Instead, anticoagulants like heparin prevent clotting within vessels. Despite this, the question's correct answer is thrombin, as it is directly involved in clot formation.