The first heart sound (S1) is created primarily by which of the following?

a. Closure of the semilunar valves
b. Opening of the semilunar valves
c. Opening of the atrioventricular valves
d. Closure of the atrioventricular valves

ANS: D
The first heart sound (S1) is produced by closure of the mitral and tricuspid (atrioventricular [AV]) valves during contraction of the ventricles.