A platelet aggregation agent that characteristically yields a biphasic curve in optimal concentration is?

• A. Arachidonic acid
• B. Collagen
• C. Epinephrine
• D. Ristocetin

Answer: (C)

The biphasic pattern seen with this agent comes from two sequential ways it activates platelets. At an optimal concentration, epinephrine first stimulates platelets directly through adrenergic receptors, causing an initial wave of aggregation. As platelets activate, they release dense-granule contents like ADP and other mediators, which then drive a second, second-wave aggregation. The result is two distinct peaks in the aggregation curve.

Other agents tend to produce a single, monotonic aggregation response. Arachidonic acid acts through the thromboxane A2 pathway to give a more uniform rise, collagen generally yields a strong single-phase curve, and ristocetin causes vWF-dependent agglutination with a different pattern altogether. That dual, two-phase response is the hallmark of epinephrine in platelet aggregation testing.