**Core Concept**
Nitroglycerin is a nitrate ester used in the management of angina pectoris. Its sublingual administration allows for rapid absorption and onset of action, providing quick relief from chest pain.

**Why the Correct Answer is Right**
The correct answer is because nitroglycerin is rapidly absorbed from the sublingual mucosa into the venous system, allowing it to exert its effects quickly. This is due to the high concentration of nitroglycerin in the sublingual tissue, which facilitates its rapid diffusion into the bloodstream. Once absorbed, nitroglycerin is metabolized by the enzyme aldehyde dehydrogenase to form nitric oxide (NO), a potent vasodilator that relaxes vascular smooth muscle and reduces myocardial oxygen demand.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because nitroglycerin is not effective through topical application due to its lack of absorption through intact skin.
* **Option B:** Incorrect because while nitroglycerin is metabolized by the liver, its rapid absorption from the sublingual mucosa is the primary reason for its effectiveness as a sublingual medication.
* **Option C:** Incorrect because nitroglycerin is not primarily excreted by the kidneys; its metabolism and excretion are not the key factors in its effectiveness as a sublingual medication.

**Clinical Pearl / High-Yield Fact**
The rapid absorption of nitroglycerin from the sublingual mucosa is due to the high concentration of blood vessels in this area, which allows for efficient diffusion of the drug into the bloodstream.

**Correct Answer:** C.