Doctors use a stethoscope to hear the sounds produced during each cardiac cycle. The second sound is heard when:

Explanation

The second heart sound (S2) is produced by the closure of the semilunar valves (aortic and pulmonary valves) after the ventricles have ejected blood into the aorta and pulmonary artery. This prevents backflow into the ventricles. The other options involve processes that do not produce the second heart sound: the AV node's signal reception is an electrical event without an associated sound; the opening of the AV valves produces the first heart sound (S1); and ventricular wall vibrations, though occurring, are not responsible for the distinct second heart sound.