**Core Concept**
Tadalafil is a phosphodiesterase type 5 (PDE5) inhibitor, which plays a crucial role in the pathophysiology of erectile dysfunction (ED). PDE5 is an enzyme responsible for the degradation of cyclic guanosine monophosphate (cGMP) in the smooth muscle cells of the corpus cavernosum.

**Why the Correct Answer is Right**
Tadalafil works by inhibiting PDE5, thereby increasing the levels of cGMP in the smooth muscle cells of the corpus cavernosum. This leads to relaxation of the smooth muscle and increased blood flow to the penis, resulting in erection. The increased levels of cGMP also lead to vasodilation and increased blood flow to the penis, further contributing to the development of an erection.

**Why Each Wrong Option is Incorrect**
**Option A:** Tadalafil does not inhibit nitric oxide synthase (NOS), which is involved in the production of nitric oxide (NO) that stimulates the release of cGMP. Inhibiting NOS would actually decrease the levels of cGMP and reduce blood flow to the penis.

**Option B:** Tadalafil does not inhibit guanylyl cyclase, which is the enzyme responsible for the production of cGMP from guanosine triphosphate (GTP). Inhibiting guanylyl cyclase would actually decrease the levels of cGMP and reduce blood flow to the penis.

**Option C:** Tadalafil does not inhibit phosphodiesterase type 3 (PDE3), which is involved in the regulation of cardiac function and smooth muscle tone. Inhibiting PDE3 would actually lead to increased cardiac contractility and vasoconstriction, which is not relevant to the treatment of ED.

**Clinical Pearl / High-Yield Fact**
It's essential to note that PDE5 inhibitors like tadalafil are contraindicated in patients taking nitrates, as they can cause a significant drop in blood pressure. This is because nitrates also increase the levels of cGMP, and combining them with PDE5 inhibitors can lead to excessive vasodilation and hypotension.

**Correct Answer:** C.