A 52-year-old man with a history of hypertension is found to have a high-pitched, blowing, early diastolic murmur at the left sternal border that increases with forward lean and hand-grip. The pulse pressure is widened (systolic 160 mmHg, diastolic 60 mmHg). Which investigation is most specific for confirming the diagnosis and assessing the degree of aortic regurgitation?