A 68-year-old man with a long history of poorly controlled hypertension presents to the emergency department with sudden-onset, severe “tearing” chest pain that radiates to his back. He is diaphoretic and anxious. Blood pressure is 210/110 mm Hg in the right arm and 160/95 mm Hg in the left arm; pulses are diminished in the left radial artery compared with the right. He becomes progressively confused during evaluation. Serum creatinine is 3.0 mg/dL (baseline unknown). ECG shows nonspecific ST-T changes. Chest radiograph demonstrates a widened mediastinum. Which of the following is the preferred diagnostic study to confirm the suspected diagnosis in this patient?

(Assume imaging is immediately available.)