A 26-year-old man is referred after a harsh systolic murmur is heard at a pre-season sports medical. He has no symptoms and plays club football. Echocardiography shows an aortic valve with two cusps that opens poorly, with a high-velocity systolic jet across it. On the same study the valve itself is seen lying deep to the left half of the sternum, at the level of the third costal cartilage.
This man's systolic murmur will be heard with greatest intensity over which one of the following areas of the chest wall?