A 65 year old man presents with a sudden onset of severe central chest pain, which radiates to the back. It is sharp and tearing in nature, initially in the middle of the chest, but has since radiated to the upper abdominal area. On examination, his blood pressure is 174/102 mmHg and he has a diastolic murmur. A subsequent ECG shows no acute changes to previous investigations.
What is the most likely diagnosis?