By what coagulation pathway does clotting occur in blood sample collected in a capillary tube?
**Core Concept**
The question is testing the understanding of the intrinsic coagulation pathway, which is a complex series of biochemical reactions that ultimately lead to the formation of a blood clot. The intrinsic pathway is activated when blood comes into contact with a foreign surface, such as the glass tube used in coagulation studies.
**Why the Correct Answer is Right**
The correct answer is the intrinsic pathway because clotting occurs when blood is collected in a capillary tube. When blood comes into contact with the glass tube, it activates the intrinsic pathway, which involves a series of enzyme reactions that ultimately lead to the formation of thrombin and fibrin. The intrinsic pathway involves the activation of factor XII, which then activates factor XI, and so on. This leads to the formation of a blood clot that prevents further bleeding from the puncture site.
**Why Each Wrong Option is Incorrect**
* **Option A:** The extrinsic pathway is activated when blood is exposed to tissue factor, which is not present in a capillary tube. Therefore, this option is incorrect.
* **Option B:** The common coagulation pathway is activated when both the intrinsic and extrinsic pathways are activated. However, in this scenario, only the intrinsic pathway is activated, making this option incorrect.
* **Option D:** The coagulation pathway involving factor VIII deficiency is associated with hemophilia A, which is not related to the scenario described in the question.
**Clinical Pearl / High-Yield Fact**
When performing coagulation studies, it's essential to use a standardized collection tube to ensure accurate results. Glass tubes are often used for coagulation studies because they activate the intrinsic pathway, allowing for the evaluation of the intrinsic coagulation pathway.
**Correct Answer:** C. The intrinsic pathway.