A 62-year-old man with a history of long-standing, poorly controlled hypertension presents to the emergency department with abrupt onset of severe chest pain that he describes as “sharp and tearing.” The pain began suddenly while he was watching television and now radiates between his shoulder blades. He is diaphoretic and anxious. Blood pressure is 198/112 mm Hg in the right arm and 164/96 mm Hg in the left arm. Pulse is 108/min. Cardiac exam is notable for a new early diastolic murmur. A chest radiograph shows a widened mediastinum. He is hemodynamically stable and his creatinine is 0.9 mg/dL.

Which of the following is the most appropriate next diagnostic study to confirm the suspected diagnosis?