A 50-year-old male with a known heart murmur for 20 years presents with exertional palpitations and shortness of breath for the past 6 months. Examination reveals BP 110/70 mmHg, enlarged cardiac borders, heart rate 132 bpm with irregular rhythm, variable S1 intensity, pulse deficit, grade III pansystolic murmur at the apex radiating to the axilla, mild diastolic rumbling murmur, and basal rales. The most accurate diagnosis is: