A 50-year-old woman presents with fatigue and shortness of breath. Physical examination shows evidence of pulmonary edema, enlargement of the left atrium, and calcification of the mitral valve. A CT scan demonstrates a large obstructing mass in the left atrium. Before open heart surgery can be performed to remove the tumor, the patient suffers a stroke and expires. Which of the following hemodynamic disorders best explains the pathogenesis of stroke in this patient?