A 62-year-old man with a history of hypertension is brought to the emergency room with severe left chest and back pain. His blood pressure is 80/50 mm Hg. Physical examination shows pallor, diaphoresis, and a murmur of aortic regurgitation. An ECG does not show myocardial infarction. An X-ray film of the chest reveals mediastinal widening. Which of the following is the most likely diagnosis?