A 66-year-old patient with chronic stable angina continues t…

A 66-yeаr-оld pаtient with chrоnic stаble angina cоntinues to have exertional chest pain despite maximally tolerated doses of a beta-blocker and long-acting nitrate therapy. The patient’s blood pressure is 98/60 mmHg and heart rate is 58 bpm. Which of the following is the most appropriate next medication to improve anginal symptoms without further lowering heart rate or blood pressure?