The echocardiogram (ultrasound of the heart) reveals that Isabelle's murmur is caused by something called a ventricular septal defect (VSD). A VSD is a hole in the interventricular septum, the wall of the muscle that separates the left ventricle from the right ventricle. The ECG shows sinus rhythm with no conduction of abnormalities.

Think about the position of the heart in the chest cavity. Isabelle's VSD is in the interventricular septum between the mitral and tricuspid valves.

At what anatomical location would the nurse practitioner best hear Isabelle's murmur?