In platelet aggregation studies, the second phase of biphasic aggregation is directly related to?

• A. Formation of fibrin
• B. Changes in platelet shape
• C. Release of endogenous ADP
• D. Release of platelet factor 3

Answer: (C)

During platelet aggregation studies, platelets show two waves of response. The first wave is rapid and largely reflects platelet shape change and reversible aggregation. The second, sustained wave is driven by the release of mediators from platelet dense granules, with endogenous ADP as the key amplifier. When ADP is released, it acts on nearby platelets via P2Y receptors, promoting further activation, recruitment, and aggregation to strengthen the plug. This positive feedback is what produces the second phase of biphasic aggregation.

Fibrin formation belongs to the coagulation cascade and helps stabilize a clot later, not the immediate second wave of platelet aggregation. The initial phase is tied to shape change, not the second-phase amplification. Platelet factor 3 influences coagulation factor complex assembly on the platelet surface, rather than driving the secondary aggregation surge.