SBA — Single Best Answer · Review
Question 312
Physiology → Haematology
Classification
Question
SSBAOptions
Explanation
The intrinsic pathway starts with exposure to collagen and factor XII, whereas the extrinsic pathway starts with tissue thromboplastin. Both ultimately contribute to the coagulation sequence leading to thrombin and fibrin formation.
Option Validity
A) This reverses the initiating mechanisms. Collagen with factor XII initiates the intrinsic pathway, whereas tissue thromboplastin initiates the extrinsic pathway.
B) Factor VII is associated with the extrinsic pathway, while factor VIII participates in the intrinsic pathway, but this pairing does not identify the initiating mechanisms.
C) Factor XIII with calcium stabilises the fibrin mesh after fibrin formation, while serotonin participates in vascular and platelet haemostasis. Neither represents initiation of the intrinsic or extrinsic pathway.
D) Thrombin and fibrinogen participate later in the coagulation sequence. They are not the distinct initiating events for the intrinsic and extrinsic pathways.
Further Reading
Saxena. Textbook for MRCOG-1. Physiology - Haematology - Blood Coagulation, pp.70-71.