A 6 month-old boy is brought to your pediatric cardiology clinic for evaluation of a murmur heard by her primary care doctor. Her mother informs you that he is healthy and active. He is playful, no shortness of breath, and no childhood infections, not more than usual for other kids of his age. On physical examination, you note that the boy is well nourished and appropriately sized for his age. On auscultation of his heart, you heard a harsh, holosystolic murmur at the left lower sternal border. There is no cyanosis, no clubbing. What is the most valuable diagnostic tool to detect the cardiac anomaly in this boy?