**Core Concept**
Phosphodiesterase type 5 (PDE5) inhibitors are a class of vasodilatory medications used to treat erectile dysfunction (ED) by increasing nitric oxide-mediated relaxation of smooth muscle in the corpus cavernosum.

**Why the Correct Answer is Right**
Viagra (sildenafil) works by inhibiting the action of PDE5, an enzyme that breaks down cyclic guanosine monophosphate (cGMP). This leads to increased levels of cGMP, causing relaxation of smooth muscle in the corpus cavernosum and facilitating erection. This mechanism is similar to the natural process of erection, which is mediated by the release of nitric oxide (NO) from non-adrenergic, non-cholinergic (NANC) neurons in the penis.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because PDE5 inhibitors are not used to treat hypertension. Although they do cause vasodilation, their primary indication is ED.
**Option B:** Incorrect because beta-blockers, such as propranolol, can actually exacerbate erectile dysfunction.
**Option C:** Incorrect because nitrates, such as nitroglycerin, can interact with PDE5 inhibitors and cause a dangerous drop in blood pressure.

**Clinical Pearl / High-Yield Fact**
PDE5 inhibitors are contraindicated in patients taking nitrates due to the risk of hypotension. This is a classic exam trap, so remember to always check for interactions with nitrates when prescribing PDE5 inhibitors.

**Correct Answer: D. Erectile Dysfunction**