Final answer:
A ventricular septal defect (VSD) occurs in the interventricular septum of the heart, allowing blood to flow between the left and right ventricles. In tetralogy of Fallot, a VSD is one of the components of the condition, contributing to a decrease in oxygen saturation.
Step-by-step explanation:
A ventricular septal defect (VSD) occurs in the interventricular septum of the heart. The interventricular septum separates the right and left ventricles, and a VSD is an abnormal opening in this septum. This opening allows blood to flow from the left ventricle, which carries oxygenated blood, to the right ventricle, which carries deoxygenated blood.
For example, in the case of tetralogy of Fallot, one of the four components of this condition is a VSD. The presence of a VSD in this condition allows oxygenated and deoxygenated blood to mix, leading to a decrease in oxygen saturation in the body.