A 52-year-old man, after suffering a heart attack, was put on 81 mg of aspirin daily by his cardiologist. The purpose of this treatment is to reduce the levels of which one of the following?

Platelet aggregation is often a determining factor in heart attacks. Thromboxane A2 , produced by platelets, promotes platelet aggregation when clotting is required, and inhibition of thromboxane A2 synthesis by aspirin reduces the potential for inappropriate clot formation, and further heart attacks. Thromboxane A2 is produced from arachidonic acid by the action of COX, the enzyme covalently modified and irreversibly inhibited by aspirin. Leukotrienes are also synthesized from arachidonic acid, but utilize lipoxygenase in their synthesis, which is not inhibited by aspirin. Cholesterol, triglyceride, and cytokine synthesis do not require COX activity.