A 62-year-old man with a history of long-standing, poorly controlled hypertension presents to the emergency department with abrupt onset of severe, sharp chest pain that he describes as "tearing". The pain began suddenly while he was watching television and now radiates to his interscapular back. He is diaphoretic and anxious. BP is 210/110 mm Hg in the right arm and 170/95 mm Hg in the left arm; heart rate is 118/min. Radial pulses are asymmetric. A portable chest radiograph shows a widened mediastinum. He is hemodynamically stable and has normal serum creatinine. Which of the following is the most appropriate next diagnostic study to confirm the suspected diagnosis?

(Assume no contrast allergy.)