**Core Concept**
The question is testing knowledge of drug interactions, specifically the contraindication of concomitant administration of a particular drug with organic nitrates due to a marked potentiation of vasodilatory action. This is a critical concept in pharmacology, as it can lead to severe hypotension and other adverse effects.

**Why the Correct Answer is Right**
The correct answer is a phosphodiesterase type 5 (PDE-5) inhibitor, such as sildenafil. This class of drugs works by inhibiting the enzyme PDE-5, which breaks down cyclic guanosine monophosphate (cGMP) in the smooth muscle cells of the corpus cavernosum. By increasing cGMP levels, PDE-5 inhibitors cause smooth muscle relaxation, leading to vasodilation and increased blood flow to the penis. When administered with organic nitrates, which also cause vasodilation by releasing nitric oxide, the vasodilatory effect is markedly potentiated, leading to a significant drop in blood pressure.

**Why Each Wrong Option is Incorrect**
* **Option A:** is incorrect because amyl nitrite is an organic nitrate, and its concomitant administration with PDE-5 inhibitors would be contraindicated.
* **Option B:** is incorrect because dobutamine is a beta-adrenergic agonist that increases heart rate and contractility, and its concomitant administration with PDE-5 inhibitors would not lead to potentiation of vasodilatory action.
* **Option C:** is incorrect because verapamil is a calcium channel blocker that causes vasodilation by blocking L-type calcium channels, but its concomitant administration with PDE-5 inhibitors would not lead to a marked potentiation of vasodilatory action.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that PDE-5 inhibitors, such as sildenafil, should not be administered with organic nitrates, as this can lead to severe hypotension. Additionally, patients taking PDE-5 inhibitors should be advised to seek medical attention if they experience symptoms of hypotension, such as dizziness or fainting.

**Correct Answer:** C. Verapamil is not the correct answer since it is actually a calcium channel blocker. However, since the question was provided with the correct answer as D, it must be that D is the correct answer.