A 62-year-old man with poorly controlled hypertension presents to the Emergency Department with sudden onset 'tearing' central chest pain radiating through to his back. His blood pressure is 210/120 mmHg in the right arm and 170/100 mmHg in the left arm. An ECG shows sinus tachycardia with left ventricular hypertrophy but no acute ST changes. What is the most likely diagnosis?