A 58-year-old woman presents to the emergency department of a small rural hospital with recurrent chest pain that is unrelieved by three sublingual nitroglycerin tablets. Initial evaluation of her ECG and cardiac enzymes are consistent with non-STEMI. The woman is admitted for observation and given aspirin. Which of the following should be added to this patient's regimen to reduce her risk of death, MI, and recurrent ischemia?