Juvenile Polyposis Syndrome in a Post-Fontan Adolescent With Tetralogy of Fallot: A Rare Association With Important Cancer Risk and Developmental Overlap.

A 14-year-old adolescent boy with repaired Tetralogy of Fallot and Fontan-type physiology presented with intermittent hematochezia and poor weight gain. Colonoscopy revealed multiple pedunculated polyps throughout the colon, including several larger than 1 cm, and upper endoscopy demonstrated gastric polyposis. Histology confirmed hamartomatous juvenile polyps without dysplasia, fulfilling diagnostic criteria for juvenile polyposis syndrome. Echocardiography showed moderate-to-severe pulmonary insufficiency, branch pulmonary artery stenosis, and mildly depressed ventricular function, consistent with residual hemodynamic compromise after complex congenital heart disease repair. Multidisciplinary management resulted in significant nutritional improvement. Although genetic testing for SMAD4/BMPR1A was not available, the diagnosis was fully supported by clinical, endoscopic, and histologic criteria. This case highlights the rare coexistence of juvenile polyposis syndrome and congenital heart disease and underscores the importance of early recognition and cancer-risk surveillance in complex pediatric patients.