A 62-year-old man is hospitalized for an acute ST-elevation myocardial infarction and undergoes successful percutaneous coronary intervention. Ten days later, he develops new-onset palpitations and brief episodes of lightheadedness. Telemetry shows runs of nonsustained ventricular tachycardia. Physical examination reveals a new systolic murmur. Transthoracic echocardiography demonstrates a saccular outpouching adjacent to the left ventricular wall with a narrow neck; color Doppler suggests blood flow into a cavity that appears to be bounded by pericardium rather than myocardium. Which of the following findings is most strongly associated with this condition?