**Core Concept**
Increased preload refers to an increase in the volume of blood within the ventricles of the heart at the end of diastole, which can lead to increased ventricular wall tension and potentially result in heart failure. This concept is crucial in understanding the Frank-Starling mechanism of the heart.

**Why the Correct Answer is Right**
The Frank-Starling mechanism states that the stroke volume of the heart increases in response to an increase in the volume of blood filling the heart (preload) when all other factors remain constant. This is achieved through increased length of the cardiac myocytes, which results in a greater force of contraction. The key factor in this mechanism is the length-tension relationship of cardiac muscle, where increased length results in increased force of contraction.

**Why Each Wrong Option is Incorrect**
**Option A:**
- This option is incorrect because anemia, characterized by a decrease in red blood cell mass or hemoglobin concentration, typically leads to a decrease in oxygen delivery to tissues and can result in increased heart rate and contractility to compensate for the decrease in oxygen delivery, but it does not directly increase preload.

**Option B:**
- This option is incorrect because heart failure with preserved ejection fraction (HFpEF) is a condition where the heart's ability to relax and fill with blood is impaired, leading to increased diastolic pressure and potential increases in preload, but it does not directly increase preload.

**Option C:**
- This option is incorrect because increased intrathoracic pressure, such as that seen in obesity or during positive pressure ventilation, can lead to reduced venous return and decreased preload.

**Option D:**
- This option is incorrect because hypovolemia, characterized by a decrease in blood volume, typically leads to decreased preload and a decrease in stroke volume.

**Clinical Pearl / High-Yield Fact**
In patients with heart failure with preserved ejection fraction (HFpEF), diuretics are often used to reduce fluid overload and decrease diastolic pressure, but they can also lead to decreased cardiac output due to decreased preload.

**Correct Answer:** D.