Fontan Udenafil Exercise Longitudinal Assessment Trial - 2 (FUEL-2)

Inclusion Criteria: Males and females with Fontan physiology who are 12 to less than 19 years of age at enrollment. Participant consent or parental/guardian consent and participant assent. Participant fluency in primary language of country in which study is being conducted. Current antiplatelet or anticoagulant therapy. Exclusion Criteria: Height < 132 cm. Hospitalization for acute decompensated heart failure within the last 12 months. Current intravenous inotropic drugs. Undergoing evaluation for heart transplantation or listed for transplantation. Diagnosis of active protein losing enteropathy or plastic bronchitis within the last 3 years, or a history of liver cirrhosis. Known Fontan baffle obstruction, branch pulmonary artery stenosis, or pulmonary vein stenosis resulting in a mean gradient of > 4 mmHg between the regions proximal and distal to the obstruction as measured by either catheterization or echocardiography, obtained prior to screening for the trial. Single lung physiology with greater than 80% flow to one lung. Peak minute oxygen consumption (VO2) less than 45% or ≥ 80% of predicated for age and gender at enrollment. Severe ventricular dysfunction assessed qualitatively by clinical echocardiography within 6 months prior to enrollment. Severe valvar regurgitation, ventricular outflow obstruction, or severe aortic arch obstruction assessed by clinical echocardiography within six months prior to enrollment. History of significant renal (serum creatinine > 2.0), hepatic (serum AST and/or ALT > 3 times upper limit of normal), gastrointestinal or biliary disorders that could impair absorption, metabolism or excretion of orally administered medications. Inability to complete exercise testing at baseline screening. History of PDE-5 inhibitor use within 12 months prior to enrollment. History of any other medication for treatment of pulmonary hypertension within 3 months before study onset. Known intolerance to oral udenafil. Frequent use of medications or other substances that inhibit or induce CYP3A4. Current use of alpha-blockers or nitrates. Ongoing or planned participation in another research protocol that would either prevent successful completion of planned study testing or invalidate its results. Noncardiac medical, psychiatric, and/or social disorder that would prevent successful completion of planned study testing or would invalidate its results. Cardiac care, ongoing or planned, at a non-study center that would impede study completion. For females: Pregnancy at the time of screening, pregnancy planned before study completion, or refusal to use an acceptable method of contraception for study duration if sexually active. Unable to abstain or limit intake of grapefruit juice and grapefruit containing drinks during the duration of the trial. Refusal to provide written informed consent/assent. In the opinion of the investigator, the subject is likely to be non-compliant with the study protocol. History of clinically significant thromboembolic event, in the option of the site Principal Investigator, that may put the subject at increased risk of a subsequent event while participating in the study. Coronavirus disease 2019 (COVID-19) vaccination or symptoms of COVID-19 infection within 7 days of Visit 1. Not taking antiplatelet or anticoagulant therapy.