Which heart sound is produced by the closure of the atrioventricular (AV) valves and signals the beginning of systole?

correct answer is: b) S1. Shutterstock Reasoning: Choice A reason: The S2 heart sound, often described as "dub," is produced by the closure of the semilunar valves (aortic and pulmonic) at the end of ventricular systole. It marks the beginning of diastole, which is the period when the ventricles relax and fill with blood before the next contraction. Choice B reason: The S1 heart sound, or "lub," is created by the simultaneous closure of the mitral and tricuspid valves. This occurs when ventricular pressure exceeds atrial pressure at the start of ventricular contraction, effectively signaling the onset of systole and the pumping of blood into the systemic and pulmonary circuits. Choice C reason: S4 is an abnormal diastolic sound, also known as an atrial gallop, that occurs just before S1. It is caused by the atria contracting and pushing blood into a stiff or non-compliant ventricle, which is often a sign of long-standing hypertension or left ventricular hypertrophy. Choice D reason: S3 is a diastolic sound, often called a ventricular gallop, that occurs early in the filling phase. It results from a large volume of blood hitting a compliant or dilated ventricle, which is a classic clinical finding in patients with fluid volume overload or congestive heart failure.