This research study is designed to test the use of ranolazine in patients with angina (chest discomfort due to reduced blood supply to the heart) due to microvascular coronary dysfunction (MCD; abnormalities in the small blood vessels of the heart). This drug is approved by the U.S. Food and Drug Administration (FDA) for treatment of chronic angina. The FDA has approved this drug based on studies primarily on patients with chronic angina with major blockages of the arteries.