- The first heart sound (S1) is primarily caused by the closure of the atrioventricular (AV) valves, which include the mitral and tricuspid valves.
- During the cardiac cycle, after the atria contract and ventricular systole begins, the pressure within the ventricles rises sharply.
- This increase in pressure forces the AV valves to close, preventing the backflow of blood into the atria.

- The closure of these valves produces the characteristic “lub” sound, which is the first heart sound (S1).
- This sound marks the beginning of ventricular systole and the isovolumetric contraction phase.

In contrast:
- The closure of the semilunar valves produces the second heart sound (S2), not S1.
- The opening of the AV valves and semilunar valves is normally silent and does not produce heart sounds.

Therefore, the correct event responsible for producing the first heart sound (S1) is the closure of the atrioventricular valves.

Reference: Guyton and Hall Textbook of Medical Physiology, 13th Edition, Chapter 18: The Heart as a Pump and the Cardiac Cycle