Tadalafil is categorized pharmacologically as a(n):

• A. PDE1 inhibitor
• B. Adenylate cyclase activator
• C. Guanylate cyclase inhibitor
• D. PDE5 inhibitor

Answer: (D)

This question tests understanding of how tadalafil works at the molecular level. Tadalafil is a PDE5 inhibitor. In the erectile tissue, nitric oxide stimulates guanylate cyclase to raise cGMP, which causes smooth muscle relaxation and increased blood flow. PDE5 normally breaks down cGMP; by inhibiting PDE5, tadalafil keeps cGMP levels high, sustaining vasodilation and aiding erection. An adenylate cyclase activator would raise cAMP, not the same pathway. A guanylate cyclase inhibitor would lower cGMP and counteract the effect. A PDE1 inhibitor targets a different phosphodiesterase family with a different role, not the primary enzyme controlling cGMP in penile tissue. Thus, the correct category is PDE5 inhibitor.