## Core Concept
Nitroglycerin is a nitrate used primarily for the treatment of angina pectoris. It works by releasing nitric oxide (NO) in the body, which then activates guanylate cyclase, increasing cyclic guanosine monophosphate (cGMP) levels. This results in smooth muscle relaxation, particularly in blood vessels.

## Why the Correct Answer is Right
The correct answer, reflex tachycardia, is a known effect of nitroglycerin. When nitroglycerin causes vasodilation, it decreases blood pressure. The body compensates for this decrease in blood pressure through the baroreceptor reflex, leading to an increase in heart rate, known as reflex tachycardia.

## Why Each Wrong Option is Incorrect
- **Option A:** Decreased myocardial oxygen demand. This is a correct effect of nitroglycerin. By reducing preload (the initial stretching of the cardiac myocytes prior to contraction), nitroglycerin decreases the heart's workload, which in turn reduces myocardial oxygen demand.
- **Option B:** Vasodilation. This is also a correct effect of nitroglycerin. Nitroglycerin causes the dilation of blood vessels, particularly venous vessels, but also arterial vessels to some extent. This vasodilation contributes to its therapeutic effects.
- **Option C:** Reduced preload. This is another correct effect of nitroglycerin. By dilating veins, nitroglycerin decreases the venous return to the heart, which reduces the preload.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that while nitroglycerin can cause reflex tachycardia, beta-blockers can mitigate this effect but are often used concomitantly with nitrates in the setting of acute myocardial infarction to reduce the risk of worsening ischemia due to tachycardia.

## Correct Answer: D. Reflex tachycardia.