This research study is to see if a drug called *udenafil* helps teenagers who have had a special heart surgery called the Fontan procedure. Udenafil is a medicine that is taken by mouth and works by blocking a specific enzyme, PDE5, to improve blood flow. The study will compare udenafil to a *placebo*, which is a pill that looks like the drug but doesn’t contain any medicine, over 26 weeks (about 6 months).

The main goal is to check if udenafil improves exercise ability, specifically by measuring how much oxygen the body uses during exercise. Participants must be between 12 and 19 years old and have had the Fontan procedure. They should not have serious heart problems or other medical issues that might make participating unsafe.

• The study lasts for 26 weeks with regular visits for tests and check-ups.
• No previous udenafil use in the past year is allowed.
• Participants must not be pregnant and should avoid grapefruit during the study.

If you are interested, you or your guardian need to give permission, and you'll need to be able to attend all required visits and follow the study rules.