Final answer:
Tetralogy of Fallot involves a ventricular septal defect, which is an abnormal opening in the wall between the ventricles.
Step-by-step explanation:
Tetralogy of Fallot is a congenital heart condition characterized by four defects. One of these defects is a ventricular septal defect, which is an abnormal opening in the interventricular septum (wall between the ventricles). This allows blood that is relatively low in oxygen from the right ventricle to mix with the blood that is relatively high in oxygen in the left ventricle.