A 65-year-old male with 10-year history of coronary artery disease (CAD) experienced severe retrosternal pain 6 hours ago, described as crushing and radiating to the left arm. Took nitroglycerin 4 times with slight relief. Now presents with restlessness and sweating. Examination shows acute distress, temperature 36.5°C, BP 100/70 mmHg, pulse rate 110 bpm; normal cardiac size, regular rhythm, diminished heart sounds, no gallop or murmurs; few crackles in both lungs; liver/spleen not palpable. On day 3 of admission, sudden severe chest pain and orthopnea occurred. Echocardiography reveals significant mitral regurgitation. Which finding is most diagnostic on physical examination?