**Question:** Mechanism of action of sildenafil -

A. Increases blood flow to the penile tissue
B. Blocks the action of phosphodiesterase type 5 (PDE5)
C. Inhibits the release of norepinephrine
D. Enhances endothelium-dependent relaxation of corpus cavernosum

**Correct Answer:** B. Blocks the action of phosphodiesterase type 5 (PDE5)

**Core Concept:**
Phosphodiesterase (PDE) enzymes are key regulators of cyclic guanosine monophosphate (cGMP) levels in various tissues. In the penile tissue, cGMP plays a crucial role in maintaining the adequate erectile response by relaxing smooth muscles and allowing increased blood flow.

**Why the Correct Answer is Right:**
Sildenafil (Viagra) is a selective inhibitor of phosphodiesterase type 5 (PDE5). By blocking PDE5, sildenafil increases the intracellular cGMP levels, which leads to relaxation of smooth muscles and dilation of blood vessels in the penile tissue. This results in enhanced erectile response and improved erectile function.

**Why Each Wrong Option is Incorrect:**

A. Increasing blood flow to the penile tissue is only a partial description of the drug's effect and does not fully explain its mechanism of action.

C. Inhibiting the release of norepinephrine is unrelated to the mechanism of action of sildenafil, as it does not directly target noradrenergic neurons.

D. Enhancing endothelium-dependent relaxation is a part of the mechanism but does not fully explain the drug's effect, as it only describes the beneficial effect of sildenafil on penile tissue and does not address the inhibition of PDE5.

**Clinical Pearl:**
Understanding the mechanism of action of sildenafil helps to appreciate the importance of PDE5 inhibitors in the treatment of erectile dysfunction due to various causes, such as diabetes, hypertension, and neurological disorders. Knowledge of PDE5 inhibitors can also be useful in understanding the clinical effects of drugs like sildenafil on other organ systems, such as the cardiovascular system, where it can potentially interact with nitrates.