The third heart sound (S3), heard as a soft low-frequency sound early in diastole, is generated by which mechanism?
- A Rapid passive filling of the ventricle causing vibration of its relaxed walls ✓
- B Closure of the mitral valve against a stiff left atrium
- C Turbulent flow across a stenosed aortic valve
- D Atrial contraction against a non-compliant ventricle
Correct answer: A. Rapid passive filling of the ventricle causing vibration of its relaxed walls
Explanation
S3 occurs during the rapid filling phase of early diastole when blood rushes into a compliant, relaxed ventricle and sets its walls and blood column into vibration. It is physiological in children and young adults but indicates ventricular volume overload or failure in older patients. The description in option D, atrial contraction against a stiff ventricle, defines the fourth heart sound, which is the classic tempting distractor because both sounds occur in diastole.
Reference: Ganong Review of Medical Physiology, 26th ed.
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