A 52-year-old woman from Mumbai with a history of acute rheumatic fever (ARF) in childhood presents with exertional chest discomfort, syncope, and a systolic ejection murmur best heard at the right upper sternal border that radiates to the neck. Echocardiography shows a bicuspid aortic valve with restricted leaflet opening. Which investigation is most appropriate to assess the hemodynamic severity and guide intervention?