Additional Study Information: Over time, children and young adults who have had a Fontan operation may find they are more tired, have difficulty breathing and cannot exercise as well as before. Currently, there are no approved medicines to prevent this decline. The purpose of this research study is to learn whether children who have had the Fontan operation can improve their ability to exercise by taking a medicine called Udenafil. The medicine works by making it easier for blood to flow through the lungs and back to the heart. The medicine may also improve overall circulation, including function of the blood vessels in the body and the heart itself.