A 62-year-old man with a history of long-standing uncontrolled hypertension presents to the emergency department with sudden onset of severe, “tearing” chest pain that radiates to the back. He is diaphoretic and anxious. Blood pressure is 88/54 mm Hg, pulse is 122/min, and oxygen saturation is 95% on room air. Radial pulses are asymmetric. ECG shows sinus tachycardia without ST-segment changes. Chest x-ray demonstrates a widened mediastinum. Serum creatinine is 3.1 mg/dL (baseline unknown). The team strongly suspects an aortic dissection. Which of the following is the most appropriate next diagnostic study to confirm the diagnosis in this patient?

(Note: management is not being asked; choose the best diagnostic test given the clinical constraints.)