Background: In Fontan Associated Liver Disease (FALD), congestion of blood in the liver causes cirrhosis. This condition can cause death. Researchers want to understand what triggers this process and find new treatments for it.

Objective: To understand how long-term congestion of blood in the liver causes liver scarring that eventually leads to cirrhosis.

Eligibility: People aged 18 and older who are at risk of developing FALD from the Fontan procedure.

Design: Participants will be screened with: Medical history Physical exam Blood and urine tests Liver ultrasound. This uses sound waves to take pictures of the body. Participants will have an outpatient visit within 12 weeks after screening. Within 24 weeks later, they will have a 3-day hospital stay. About 2 weeks later, they will have a follow-up visit. Visits will include repeats of the screening tests and: Heart tests Stool collection Questionnaires MRI of the liver. Participants will lie on a bed that slides in and out of the scanner. They will receive a contrast agent injected into a vein. While in the scanner, they will also have an MRCP to view the bile ducts and the pancreatic duct. Fibroscan exam. This is an ultrasound that uses a special probe to look at the toughness of the liver. Upper endoscopy. This uses a thin scope to look inside the upper digestive tract. Liver biopsy. This will be taken through large vein in the neck or through the chest. Just before the biopsy, participants will have pressure measurements inside their liver. For this, a catheter will be inserted into a neck vein and guided into the liver.