IMProving DRug Dosing and Outcomes for Single VEntricle Patients With Fontan Associated Liver Disease

Inclusion Criteria: * \> 8 years * Status Post Fontan Completion * Ability to provide informed permission-assent (\<18 years) or consent (≥18 years) * Fasting overnight (\~8 hours) Exclusion Criteria: * Pregnancy * Non-fasting * Non-removable metal in body or where magnetic resonance imaging (MRI) is considered unsafe * Sildenafil and/or Pravastatin therapy within last 2 months * History of underlying or laboratory evidence of underlying intestinal, metabolic, autoimmune, renal disease that can alter Sildenafil and Pravastatin disposition (absorption, metabolism, distribution, or clearance) * Pharmacotherapy that interacts with Sildenafil (cytochrome P450 3A4 and P450 3A5 (CYP3A4/5) inducers/inhibitors) and/or Pravastatin (organic anion transporting polypeptide (OATP1B1) inducers/inhibitors) * Inability to swallow a tablet * \>5X the age-specific upper limit of normal for aspartate aminotransferase (AST) and alanine aminotransferase (ALT), total and conjugated bilirubin * Diarrhea in the last 24 hours \*History of solid organ transplantation