A 30-year-old woman with a history of intravenous drug abuse is admitted to a hospital for rapidly progressive malaise, fever, and chills. On physical examination, subungual splinter hemorrhages and a systolic murmur are detected. Blood cultures are positive for Staphylococcus aureus. She died of myocardial infarction on her third hospital day. Which of the following was the underlying condition leading to the patient's demise?