Which statement best describes tadalafil's pharmacologic action?

• A. Selective PDE5 inhibition leading to increased cGMP and vasodilation
• B. Inhibits PDE2 leading to increased cGMP
• C. Inhibits PDE4 reducing inflammation
• D. Activates guanylate cyclase

Answer: (A)

Tadalafil works by selectively inhibiting PDE5, the enzyme that breaks down cGMP in penile smooth muscle. During sexual arousal, nitric oxide triggers production of cGMP, and blocking its breakdown with tadalafil lets cGMP accumulate, activating downstream pathways that relax smooth muscle and dilate blood vessels in the erectile tissue. This combination—selective PDE5 inhibition plus the NO-triggered rise in cGMP—best explains the drug’s action and why it facilitates erection. The other options describe different enzymes or direct activations that aren’t how tadalafil works (PDE2 or PDE4 inhibition or direct activation of guanylate cyclase).