A 52-year-old man with a 10-year history of hypertension presents with a 3-month history of progressive dyspnea on exertion and orthopnea. On cardiac examination, a late systolic murmur is heard at the apex, which increases with standing. Echocardiography shows an ejection fraction of 35%, severe mitral regurgitation, and left ventricular end-systolic diameter (LVESd) of 52 mm. He is currently on lisinopril 10 mg daily. What is the most appropriate next step in management?