Review Question - QID 109554

A 26-year-old man is brought to the emergency department due to right-sided facial and upper extremity weakness and aphasia. The patient was in his usual state of health until two hours prior to presentation, when he was eating breakfast with a friend and acutely developed the aforementioned symptoms. Medical history is unremarkable except for mild palpitations that occur during times of stress or when drinking coffee. Physical examination is consistent with the clinical presentation. Laboratory testing is unremarkable and a 12-lead electrocardiogram is normal. A non-contrast head CT and diffusion-weighted MRI shows no intracranial hemorrhage and an isolated superficial cerebral infarction. Transthoracic echocardiography with agitated saline mixed with air shows microbubbles in the left heart. There is a possible minor effusion surrounding the heart and the ejection fraction is within normal limits. Which of the following is most likely the cause of this patient's clinical presentation?