**Core Concept**
In Tetralogy of Fallot (TOF), a congenital heart defect characterized by four key features, a systolic murmur is present due to the abnormal anatomy of the heart. The murmur is primarily caused by the obstruction of blood flow from the right ventricle to the pulmonary artery.

**Why the Correct Answer is Right**
The systolic murmur in TOF is due to the narrowing of the pulmonary valve or the right ventricular outflow tract, which leads to turbulent blood flow. This obstruction results in a harsh, ejection-type systolic murmur. The murmur is best heard at the left upper sternal border. The pathophysiology involves the right ventricular hypertrophy and the subsequent narrowing of the outflow tract, which increases the resistance to blood flow.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the cause of the systolic murmur in TOF. While it may be a feature of other cardiac conditions, it is not relevant to the murmur in TOF.
**Option B:** This option is incorrect because it refers to a different type of cardiac murmur, which is not characteristic of TOF.
**Option C:** This option is incorrect because it does not accurately describe the pathophysiology of the systolic murmur in TOF.

**Clinical Pearl / High-Yield Fact**
The systolic murmur in TOF is often described as a harsh, ejection-type murmur best heard at the left upper sternal border. It is an important diagnostic clue for this congenital heart defect.

**Correct Answer: D. Right ventricular outflow tract obstruction.**