**Core Concept**
The 5-phosphodiesterase (PDE) inhibitors are a class of vasodilators used primarily in the treatment of erectile dysfunction (ED). These medications work by inhibiting the enzyme PDE5, which is responsible for the degradation of cyclic guanosine monophosphate (cGMP) in the corpus cavernosum, thereby increasing blood flow to the penis and facilitating erection.

**Why the Correct Answer is Right**
Tadalafil, a 5-PDE inhibitor, has the longest duration of action among the given options. This is due to its longer half-life of approximately 17.5 hours, which allows for a more sustained effect. As a result, tadalafil can remain effective for up to 36 hours after administration, making it a popular choice for patients who require a longer duration of action. The mechanism of action of tadalafil involves the inhibition of PDE5, which leads to the accumulation of cGMP and subsequent smooth muscle relaxation in the corpus cavernosum.

**Why Each Wrong Option is Incorrect**
**Option A:** Sildenafil has a half-life of approximately 3-5 hours, which is significantly shorter than tadalafil's half-life. Therefore, sildenafil has a shorter duration of action compared to tadalafil.
**Option B:** Vardenafil has a half-life of approximately 4-6 hours, which is also shorter than tadalafil's half-life. Vardenafil is known for its rapid onset of action, but its duration of action is not as long as tadalafil's.
**Option D:** Udenafil has a half-life of approximately 11-13 hours, which is shorter than tadalafil's half-life. Although udenafil has a longer duration of action compared to sildenafil and vardenafil, it is still not as long-lasting as tadalafil.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the duration of action of 5-PDE inhibitors can vary depending on individual patient factors, such as age, renal function, and concurrent medications. Therefore, patients should be advised to follow the recommended dosage and administration instructions to maximize the efficacy and minimize the risk of adverse effects.

**✓ Correct Answer: C. Tadalafil**