A 47-year-old woman presents to the emergency department with symptoms of new-onset transient right arm weakness and word-finding difficulty lasting 3 hours. She is also experiencing exertional dyspnea, and had a syncopal event 1 month ago. Her medical history is only remarkable for 2 uneventful pregnancies, and she is not taking any medications.Physical examination reveals normal vital signs, and no residual focal neurological deficits. The ECG and CT brain are normal but an echocardiogram reveals a cardiac tumor in the left atrium, it is pedunculated, and attached to the endocardium. Which of the following is the most likely cause of this lesion?