**Core Concept**
Tadalafil is a phosphodiesterase type 5 (PDE5) inhibitor used to treat erectile dysfunction. PDE5 is an enzyme found in the smooth muscle of the corpus cavernosum, which is responsible for the relaxation of smooth muscle and subsequent blood flow to the penis.

**Why the Correct Answer is Right**
When PDE5 is inhibited, cGMP (cyclic guanosine monophosphate) levels increase, leading to smooth muscle relaxation and increased blood flow to the penis. As the corpus cavernosum becomes engorged with blood, venous blood is trapped within the penis, preventing the normal venous outflow. The blood then enters the **deep dorsal vein of the penis**, which is responsible for draining the penis.

**Why Each Wrong Option is Incorrect**
**Option A:** There is no such vein as the "superficial dorsal vein of the penis" in the context of erectile function. The superficial dorsal vein of the penis is not involved in the venous drainage of the erect penis.
**Option B:** The **internal pudendal veins** are involved in the venous drainage of the penis, but they are not the primary vessel that receives blood exiting the penis during erection.
**Option C:** The **femoral vein** is a major vein in the lower limb, but it is not directly involved in the venous drainage of the penis.

**Clinical Pearl / High-Yield Fact**
In patients with erectile dysfunction, the use of PDE5 inhibitors like tadalafil can lead to a significant increase in blood flow to the penis, resulting in improved erectile function.

**Correct Answer:** C.