A 48-year-old, previously healthy woman reports having suddenly lost consciousness four times in the past 6 months. In three instances, she was unconscious for only a few minutes. After the fourth episode 1 month ago, she was unconscious for 6 hours and had weakness in her right arm and difficulty speaking. On physical examination, she is afebrile, and her blood pressure is normal. No murmurs are auscultated. She has good carotid pulses with no bruits. Which of the following cardiac lesions is most likely to be present in this woman?

Atrial myxoma is the most common primary cardiac neoplasm. On the left side of the heart, it can produce a ball-valve effect that intermittently occludes the mitral valve, leading to syncopal episodes and possible strokes from embolization to cerebral arteries. Calcification of a bicuspid valve can lead to stenosis and heart failure, but this condition is progressive. Coronary artery thrombosis results in an acute ischemic event, typically with chest pain. By the time left atrial enlargement with mural thrombosis and risk of embolization occurs from mitral stenosis, this patient would have been symptomatic for years. Most pericardial effusions are not large and do not cause major problems. Large effusions could lead to tamponade, but this is not an intermittent problem.