Aspirin - does it inhibit enzyme of thromboxane?
Aspirin and some other NSAIDs inhibit cyclooxygenase which is the first enzyme in the prostaglandin/thromboxane biosynthesis pathway if you consider first substance is arachidonic acid (a fatty acid). Generally, arachidonic acid is present in the form of a phospholipid, and if you consider that in the pathway, substance-Y is arachidonic acid and enzyme-A is cyclooxygenase. 




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  From: Heckmann, Lars-Henrik, et al. "Outlining eicosanoid biosynthesis in the crustacean Daphnia." Front. Zool 5.11 (2008).