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 radiating to the back. On examination, his right radial pulse is weaker than the left, and blood pressure is 180/95 mm Hg in the left arm and 150/85 mm Hg in the right arm. A new, high-pitched decrescendo diastolic murmur is heard along the left sternal border. Chest radiograph demonstrates a widened mediastinum. He is hemodynamically stable, and his serum creatinine is 0.9 mg/dL.

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