S1 (lubb) corresponds to which event?

• A. It marks the end of diastole and is heard at the base
• B. It marks the beginning of diastole and is heard best at the apex
• C. It marks the beginning of systole and results from the closure of the AV valves; best heard toward the apex
• D. It marks the closure of the semilunar valves

Answer: (C)

S1 is the sound produced when the atrioventricular valves (mitral and tricuspid) close as the ventricles begin to contract, marking the start of ventricular systole. It is heard best at the apex (left 5th intercostal space, midclavicular line) with the stethoscope. The other statements mix up timing or valve involvement: the end of diastole and opening of systole is not S1, and the closure of the semilunar valves creates S2 heard best at the base. Thus, the onset of systole due to AV valve closure at the apex best matches the described event.