A 49-year-old, previously healthy woman reports having suddenly lost consciousness four times in the past 6 months. In three instances, she was unconsciousness for only a few minutes. After the fourth episode 1 month ago, she was unconscious for 6 hours and had weakness in her left arm and difficulty speaking. On physical examination, she is afebrile, and her blood pressure is normal. She has good carotid puls es with no bruits. Which of the following cardiac lesions is most likely to be present? (A) Pericardial effusion (B) Left atrial myxoma (C) Bicuspid aortic valve (D) Mitral valve stenosis (E) Left anterior descending artery thrombosis