Viagra improves erectile function by ________.

A) increasing blood pressure in the penis
B) causing contractions of the penile muscles
C) stimulating the muscles of the pelvic floor
D) expanding the erectile tissues in the penis

Viagra works by inhibiting an enzyme called phosphodiesterase 5 (PDE5), which breaks down a chemical called cyclic guanosine monophosphate (cGMP). When cGMP levels are increased, the smooth muscles in the erectile tissues of the penis relax and blood flow is increased, leading to an erection. Therefore, Viagra expands the erectile tissues in the penis, improving erectile function.