## Core Concept
Nitrates are a class of drugs used primarily for the treatment of angina pectoris. They work by releasing nitric oxide (NO) in the body, which then causes relaxation of vascular smooth muscle, leading to vasodilation. This vasodilation decreases the preload (the initial stretching of the cardiac myocytes prior to contraction) and, to a lesser extent, afterload, thereby reducing the myocardial oxygen demand.

## Why the Correct Answer is Right
The correct answer, **D. Increasing heart rate**, is right because nitrates actually decrease heart rate as a reflex response to the decrease in blood pressure caused by vasodilation, although this effect might not always be prominent. The primary mechanisms by which nitrates decrease myocardial oxygen consumption include:
- Decreasing preload: By dilating veins, nitrates decrease the venous return to the heart, which reduces the end-diastolic volume and therefore the wall tension during contraction (according to Laplace's law). This reduction in wall tension decreases the myocardial oxygen demand.
- Reducing afterload: Although less significant with nitrates, the dilation of arterioles can decrease the resistance against which the heart must pump, thereby reducing afterload and myocardial oxygen demand.
- Possibly a direct effect on myocardial cells: Nitrates may have a direct effect on myocardial cells, although this is less clear.

## Why Each Wrong Option is Incorrect
**Option A: Decreasing preload** - This is a correct mechanism by which nitrates decrease myocardial oxygen consumption. By reducing the volume of blood in the ventricles at the end of diastole, nitrates decrease the stretch on the cardiac myocytes, which in turn decreases the force of contraction required and thus the oxygen demand.

**Option B: Decreasing afterload** - This is also a mechanism through which nitrates can decrease myocardial oxygen consumption, although it's more related to the arterial dilation effect which is less pronounced with nitrates compared to their venous dilation effect.

**Option C: Decreasing contractility** - Nitrates can indirectly decrease contractility by reducing preload (through decreased venous return), which subsequently decreases the force of contraction due to the Frank-Starling mechanism. This reduction in contractility contributes to a decrease in myocardial oxygen demand.

## Clinical Pearl / High-Yield Fact
A key point to remember is that nitrates are most effective in treating angina caused by coronary artery disease, particularly when the angina is precipitated by exertion or emotional stress. They are not suitable for variant (Prinzmetal's) angina caused by coronary artery spasm, as they can potentially worsen the spasm by causing reflex vasoconstriction.

## Correct Answer: D. Increasing heart rate