38-year-old woman with a past medical history of rheumatic fever and endocarditis presents with progressive dyspnea on exertion associated with palpitations and intermittent episodes of left-sided chest pain. Both symptoms resolve at rest. Her physical exam reveals resting tachycardia and a widened pulse pressure. The cardiac exam is notable for a decrescendo diastolic high-pitched murmur, loudest at the left sternal border and accentuated with the patient leaning forward in full expiration. Abrupt distention and quick collapse are observed upon palpation of the peripheral arterial pulses. Booming systolic and diastolic sounds are acultated over the femoral arteries. What is the diagnostic test of choice to confirm this patient's most likely diagnosis?