Which of the following tests is most likely to be abnormal in patients taking aspirin?

In patients taking aspirin, the test most likely to show abnormal results is bleeding time. Aspirin works by inhibiting thromboxane A2 synthesis in platelets, which affects platelet aggregation and prolongs the time it takes for bleeding to stop during injury. As a result, patients on aspirin therapy may experience a longer bleeding time due to reduced platelet function, despite having a normal platelet count and morphology.

The other tests listed, such as platelet morphology and platelet count, typically remain normal in the context of aspirin use, as aspirin does not affect the number or the structure of platelets directly. Prothrombin time, which measures the extrinsic pathway of coagulation, is also not influenced by aspirin. Hence, the most relevant test indicating the effect of aspirin on hemostasis is bleeding time, making it the most likely to be abnormal in these patients.