Sildenafil acts by inhibiting?
**Core Concept**
Sildenafil is a phosphodiesterase type 5 (PDE5) inhibitor used in the treatment of erectile dysfunction. It works by increasing the levels of cyclic guanosine monophosphate (cGMP) in the smooth muscle cells of the corpus cavernosum.
**Why the Correct Answer is Right**
Sildenafil inhibits PDE5, an enzyme responsible for breaking down cGMP. By blocking PDE5, sildenafil allows cGMP to accumulate, leading to smooth muscle relaxation and increased blood flow to the penis. This results in improved erectile function. The mechanism of action of sildenafil is closely related to the nitric oxide-cGMP pathway, where nitric oxide (NO) stimulates guanylate cyclase to convert guanosine triphosphate (GTP) to cGMP.
**Why Each Wrong Option is Incorrect**
**Option A:** Phosphodiesterase type 2 (PDE2) - Sildenafil specifically targets PDE5, not PDE2, which is involved in the regulation of cAMP and cGMP levels in the brain.
**Option B:** Adenylate cyclase - Adenylate cyclase is an enzyme involved in the production of cAMP, not the breakdown of cGMP, which is the target of sildenafil.
**Option C:** Phosphodiesterase type 3 (PDE3) - PDE3 is involved in the regulation of cAMP levels in the heart and smooth muscle, and is not the target of sildenafil.
**Clinical Pearl / High-Yield Fact**
Sildenafil is also used to treat pulmonary arterial hypertension (PAH) due to its ability to reduce pulmonary vascular resistance by increasing cGMP levels in pulmonary vascular smooth muscle cells.
**Correct Answer: C. Phosphodiesterase type 5 (PDE5)**