A 77-year-old woman fell and fractured her ankle. She has spent most of her time in bed for the past 16 days. She develops sudden chest pain, dyspnea, and diaphoresis. On examination, she has left thigh swelling and tenderness. A chest CT shows areas of decreased attenuation in the right and left pulmonary arteries. A day later she has difficulty speaking. MR angiography shows focal occlusion of a left middle cerebral artery branch. Which of the following cardiac abnormalities is she most likely to have?