S4 heart sound is best described as which of the following?

• A. Early-diastolic reverberation caused by a dilated left ventricle
• B. Late-diastolic reverberation caused by a stiff left ventricle
• C. Opening sound of the AV valves
• D. Atrial contraction

Answer: (B)

S4 is an atrial gallop produced when the atria contract against a noncompliant, stiff ventricle, so the sound occurs late in diastole just before S1. This late diastolic reverberation best describes the mechanism: atrial contraction against a ventricle with reduced compliance, typically seen with LV hypertrophy or diastolic dysfunction.

In contrast, an S3 is an early diastolic sound from rapid ventricular filling into a dilated ventricle, not from a stiff ventricle. An opening sound of the AV valves relates to valve opening events (such as mitral stenosis) rather than the late diastolic atrial kick. While S4 involves atrial contraction, the defining feature is the late-diastolic sound caused by a noncompliant ventricle, making the described option the correct fit.