A 72-year-old man with coronary artery disease comes to the ph | FreeMedSite

A 72-year-old man with coronary artery disease comes to the physician because of intermittent episodes of substernal chest pain and shortness of breath. The episodes occur only when walking up stairs and resolves after resting for a few minutes. He is a delivery man and is concerned because the chest pain has impacted his ability to work. His pulse is 98/min and blood pressure is 132/77 mm Hg. Physical examination is unremarkable. An ECG shows no abnormalities. A drug that blocks which of the following receptors is most likely to prevent future episodes of chest pain from occurring?