**Core Concept**
Systolic blood pressure (SBP) is the maximum pressure exerted by blood on the arterial walls during a cardiac cycle. It is influenced by several factors, including cardiac output, peripheral resistance, and arterial compliance.

**Why the Correct Answer is Right**
The correct answer is directly related to the concept of **afterload**, which is the resistance that the ventricles must overcome to eject blood into the aorta during systole. The higher the afterload, the greater the pressure required to eject blood, resulting in a higher systolic blood pressure. This is achieved through the **myogenic response**, where smooth muscle cells in the arterial walls contract in response to increased stretch, thereby increasing peripheral resistance and afterload.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not directly related to systolic blood pressure. Diastolic blood pressure is more closely related to peripheral resistance and arterial compliance.
* **Option B:** While cardiac output does affect systolic blood pressure, it is not the most direct factor. Cardiac output is influenced by factors such as heart rate, contractility, and preload.
* **Option D:** Not directly related to systolic blood pressure. Mean arterial pressure is a function of cardiac output, peripheral resistance, and arterial compliance.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that systolic blood pressure is a reflection of the **afterload**, which is influenced by factors such as peripheral resistance, arterial compliance, and cardiac output.

**Correct Answer: C. Afterload. Afterload is a major determinant of systolic blood pressure.**