**Core Concept**
Diastolic heart failure, also known as heart failure with preserved ejection fraction (HFpEF), is a condition where the left ventricle fails to relax and fill properly during diastole, leading to impaired filling and increased diastolic pressures. This is often associated with conditions such as hypertension, diabetes, and coronary artery disease.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of diastolic heart failure. In this condition, the left ventricle becomes stiff and non-compliant, making it difficult for the ventricle to relax and fill during diastole. This leads to increased diastolic pressures and impaired filling. The options that are least likely to occur in diastolic heart failure are those that are more associated with systolic dysfunction, where the left ventricle contracts poorly.

**Why Each Wrong Option is Incorrect**
**Option A:** Decreased ejection fraction is more characteristic of systolic heart failure, where the left ventricle contracts poorly, making it less likely to occur in diastolic heart failure.

**Option B:** Increased left ventricular end-diastolic pressure is a hallmark of diastolic heart failure, making it a likely occurrence.

**Option C:** Increased left ventricular end-systolic volume is more characteristic of systolic heart failure, where the left ventricle contracts poorly, making it less likely to occur in diastolic heart failure.

**Option D:** Increased left ventricular mass is a common finding in diastolic heart failure, making it a likely occurrence.

**Clinical Pearl / High-Yield Fact**
In diastolic heart failure, the left ventricle becomes stiff and non-compliant, making it difficult for the ventricle to relax and fill during diastole. This can be remembered using the mnemonic "STIFF" - Stiffness, Thickened walls, Increased pressures, Fibrosis, and Failure.

**Correct Answer: A. Decreased ejection fraction is more characteristic of systolic heart failure, where the left ventricle contracts poorly, making it less likely to occur in diastolic heart failure.**