23. What causes an S3 heart sound?

A. Passive filling into a compliant left ventricle causing sudden tensing of the chordae tendinae

B. Active filling into a compliant left ventricle causing sudden tensing of the chordae tendinae

C. Passive filling into a non-compliant left ventricle

D. Active filling into a non-compliant left ventricle 

The third heart sound (S3), also known as the "ventricular gallop", occurs just after S2 when the mitral valve opens allowing passive filling of the left ventricle. The S3 sound is actually produced by the large amount of blood striking a very compliant left ventricle (LV).

S3