Question

multiple choice question
what begins the intrinsic pathway of coagulation?

platelets release factor xii.

platelets stick to collagen.

prostacyclin binds to platelets.

white blood cells respond to chemoattractants.

Explanation:

To determine what starts the intrinsic coagulation pathway, we analyze each option:

• "Platelets release factor XII": Factor XII (Hageman factor) is activated in the intrinsic pathway, often when blood contacts collagen (exposed during injury), and platelets' interaction with collagen can lead to factor XII activation. But the more direct initial event related to the intrinsic pathway's start is platelets sticking to collagen (exposing the subendothelium, initiating the cascade). However, among the options, the key event for the intrinsic pathway's initiation is the exposure of collagen, leading to platelets adhering (sticking) to it, which then triggers the coagulation cascade.
• "Platelets stick to collagen": When blood vessel damage occurs, the subendothelial collagen is exposed. Platelets adhere (stick) to this collagen, which is a key initial step in the intrinsic coagulation pathway (as well as platelet activation). This adhesion triggers a series of events including the activation of coagulation factors (like factor XII) in the intrinsic pathway.
• "Prostacyclin binds to platelets": Prostacyclin is an inhibitor of platelet aggregation, so this is not an initiating event for coagulation.
• "White blood cells respond to chemoattractants": This is related to immune response/inflammation, not coagulation pathway initiation.

So the correct option is the one where platelets stick to collagen as it initiates the intrinsic coagulation pathway by exposing the subendothelial matrix and triggering factor activation.

Answer:

B. Platelets stick to collagen (assuming the options are labeled A - D with this as option B; adjust label if needed, but the content is "Platelets stick to collagen")