A 72-year-old man with chronic hypertension and COPD is brought to the emergency department by ambulance after collapsing at home. He complains of sudden onset of severe chest and back pain. On examination, he is pale, diaphoretic, and hypotensive (BP 95/60 mmHg). There is a blood pressure difference of 20 mmHg between the right and left arms. Femoral pulses are diminished bilaterally. A bedside transthoracic echocardiogram shows a dilated aortic root with a flap-like structure in the ascending aorta. What is the most likely diagnosis?