A 63-year-old man with long-standing hypertension presents to the ED with sudden onset of severe chest pain that he describes as “tearing” and radiating to his back. He is diaphoretic and anxious. BP is 198/112 mm Hg in the right arm and 182/108 mm Hg in the left arm; HR is 118/min. He becomes increasingly somnolent, and his oxygen saturation falls despite supplemental oxygen. Serum creatinine is 3.1 mg/dL (baseline 2.8 mg/dL). Chest radiograph shows a widened mediastinum. The team suspects an acute aortic dissection. Which of the following is the most appropriate diagnostic study at this time?

(Assume immediate availability of all listed modalities.)