A 52-year-old man is brought to the emergency department 30 minutes after he had an episode of chest pain radiating to his jaw while shoveling snow. His pulse is 80/min, and blood pressure is 130/70 m

A 52-year-old man is brought to the emergency department 30 minutes after he had an episode of chest pain radiating to his jaw while shoveling snow. His pulse is 80/min, and blood pressure is 130/70 mm Hg. The lungs are clear to auscultation. Cardiac examination shows an S4. While undergoing an ECG, the patient says that he feels the chest pain returning. The most appropriate immediate treatment is a drug with which of the following mechanisms of action?