The drug Viagra promotes blood vessel dilation by prolonging signaling through nitric oxide (NO). How does Viagra boost NO? a. It inhibits guanylyl cyclase in smooth muscle cells.
b. It prevents acetylcholine from binding to its receptors.
c. It stimulates guanylyl cyclase in smooth muscle cells.
d. It blocks the enzyme that degrades cyclic GMP.
e. It binds to acetylcholine receptors on endothelial cells.

The correct answer is option d, that is, it prevents the enzyme, which degrades cyclic GMP.

Explanation:

Oral therapy for erectile dysfunction is Viagra, which is the citrate salt of sildenafil, it works by preventing a specific phosphodiesterase type 5 or PDE5, which is the inhibitor of cGMP. During sexual stimulation, nitrous oxide is released in the corpus cavernosum causing the erection of the penis in males. The nitrous oxide then stimulates the enzyme guanylate cyclase that further makes the concentration of cGMP to elevate, this generates smooth muscle relaxation in the corpus cavernosum and permitting blood the inward flow of blood.

Viagra works by increasing the influence of nitric acid by preventing PDE5, which is accountable for the deprivation of cGMP in the corpus cavernosum. At the time of sexual stimulation, the discharging of nitrous oxide takes place, the viagra inhibits PDE5 causing the levels of cGMP to elevate in the corpus cavernosum leading to smooth muscle relaxation and continuous flow of blood to the corpus cavernosum.