The Fontan operation allows survival for children with single ventricle congenital heart disease. In the acute postoperative period, perioperative insults and drastic changes in vascular pressures can potentially cause ischemic liver injury. We present a 3-year-old female with congenital heart disease presenting post-Fontan procedure complicated by altered mental status due to elevated ammonia levels. Etiology of the hyperammonemia was unknown and relatively controlled with medication. Further investigation, however, revealed a congenital portosystemic shunt. Congenital portosystemic shunts, more specifically Abernethy malformations, are rare conditions characterized as intrahepatic or extrahepatic, resulting in diversion of portal flow to systemic.