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Quick Question: Extrinsic Pathway

Our September, 2018 Quick Question was a quiz with one correct answer addressing our knowledge of the coagulation cascade: Which coagulation factor is the first activated in the extrinsic pathway? With 144 responding, the answers were:

  1. Factor XII: 16 (11%)
  2. Factor IX: 9 (6%)
  3. Factor VII: 105 (73%)
  4. Prothrombin: 14 (10%)

Congratulations to the overwhelming majority who chose the correct answer, factor VII, the factor that is first activated when exposed to subendothelial tissue factor which is exposed at the site of a wound. Activated VII, termed VIIa, bound to tissue factor, then activates its substrate, factor X, to initiate the common pathway.

Factor XII is considered the coagulation factor first activated in the intrinsic pathway. Upon activation, XIIa acts upon factor XI, and the cascade proceeds through factor IX to also activate factor X to trigger the common pathway. Factor XII activation is largely an in vitro phenomenon, as it becomes activated upon exposure to a negatively charged surface. Factor XII does not appear to have a significant role in in vivo coagulation, as factor XII deficiency has no clinical consequence.

Prothrombin, also labeled factor II, appears near the end of the pathway. When activated, prothrombin becomes thrombin, which digests fibrinogen to form fibrin polymer, the basis for the fibrin clot.

Thanks to all for participating!

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