**Core Concept**
The regulation of blood pressure is a complex process involving various physiological mechanisms, including the renin-angiotensin-aldosterone system (RAAS), the sympathetic nervous system, and the kidneys. The body maintains blood pressure homeostasis through a delicate balance of vasoconstriction and vasodilation, as well as fluid and electrolyte balance.

**Why the Correct Answer is Right**
The correct answer involves a decrease in blood pressure, which can be caused by various factors. One key mechanism is the release of vasodilatory substances, such as nitric oxide (NO) and prostacyclin (PGI2), which relax smooth muscle cells and cause blood vessels to dilate, leading to a decrease in peripheral resistance and a subsequent drop in blood pressure. Additionally, medications like nitroglycerin and hydralazine can also cause vasodilation and decrease blood pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is a common distractor, but it is not the correct answer. While certain antihypertensive medications like ACE inhibitors and ARBs can decrease blood pressure, they do so by blocking the RAAS system, which has a complex interplay with vasodilation and vasoconstriction.

**Option B:** This option is incorrect because certain antihypertensive medications like calcium channel blockers can actually decrease blood pressure by causing vasodilation, but they do so through a different mechanism than the correct answer.

**Option C:** This option is incorrect because certain medications like beta blockers can actually increase blood pressure in certain situations, such as in patients with pheochromocytoma.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that vasodilation is a primary mechanism of action for many antihypertensive medications, including nitroglycerin, hydralazine, and calcium channel blockers. Understanding the mechanisms of vasodilation and vasoconstriction is crucial for managing patients with hypertension and other cardiovascular conditions.

**Correct Answer: D. Nitroglycerin**