What is the primary physical cause of the first heart sound (S1, 'lub') heard on cardiac auscultation?
Answer and explanation
Correct answer: A. Closure of atrioventricular valves
- Step 1: At the onset of ventricular isovolumetric contraction, ventricular pressure exceeds atrial pressure. Step 2. Sudden closure and tensing of mitral and tricuspid valves along with turbulent deceleration of blood produces the S1 sound. Answer: Closure of the atrioventricular (mitral and tricuspid) valves. Why not the others: Closure of semilunar valves produces the second heart sound S2 ('dub'); rapid filling can produce physiological S3; atrial contraction produces S4.
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