## Core Concept
Preload refers to the initial stretching of the cardiac myocytes (muscle cells) prior to contraction. It is directly related to the **end-diastolic volume (EDV)**, which is the volume of blood in the ventricle at the end of diastole, just before the ventricle contracts. Preload is influenced by factors that affect venous return to the heart.

## Why the Correct Answer is Right
The correct answer, **D. Increased venous return**, directly increases the preload. When venous return increases, more blood flows back to the heart, filling the ventricles to a greater extent at the end of diastole. This increased volume stretches the cardiac myocytes, thereby increasing the preload. According to the **Frank-Starling law of the heart**, the greater the heart muscle is stretched during filling (up to a point), the greater the volume of blood that can be pumped out (stroke volume).

## Why Each Wrong Option is Incorrect
- **Option A:** A decrease in venous return would actually decrease preload, as less blood would be returning to the heart, leading to lower end-diastolic volumes.
- **Option B:** A decrease in blood volume (hypovolemia) would decrease venous return to the heart, thereby decreasing preload. This option does not increase preload.
- **Option C:** While not explicitly listed, typically, factors that decrease heart rate (negative chronotropy) or contractility (negative inotropy) might indirectly affect preload, but **increased afterload** (which is related to the resistance against which the heart must pump blood) primarily affects the heart's ability to eject blood during systole, not directly the preload.

## Clinical Pearl / High-Yield Fact
A key clinical scenario where preload is increased is in **fluid overload** states, such as heart failure, where administering IV fluids might seem counterintuitive but can sometimes be necessary. Conversely, conditions like **dehydration** or **hemorrhage** decrease preload. A simple way to remember factors that increase preload is to think of anything that increases **venous return** to the heart.

## Correct Answer Line
**Correct Answer: D. Increased venous return**