**Core Concept**
Cerebral blood flow (CBF) is a critical aspect of brain function, and its regulation is essential for maintaining proper brain function. The major determinant of CBF is the resistance to blood flow in the cerebral vasculature, which is influenced by various factors including metabolic demands, blood pressure, and autoregulatory mechanisms.

**Why the Correct Answer is Right**
The correct answer is **C. Arterial PO2**, as cerebral blood flow is directly proportional to the partial pressure of oxygen in the arterial blood (PaO2). This is because the brain has a high demand for oxygen, and a decrease in PaO2 triggers a vasodilatory response to increase CBF. This is mediated by the release of vasodilatory substances such as adenosine, which acts on adenosine receptors to increase blood flow. The cerebral vasculature is also sensitive to changes in carbon dioxide levels (PaCO2), but the primary determinant of CBF is the PaO2.

**Why Each Wrong Option is Incorrect**
* **Option A:** While blood pressure is an important factor in determining CBF, it is not the major determinant. Cerebral autoregulation maintains a relatively constant CBF across a wide range of blood pressures.
* **Option B:** The carbon dioxide levels in the blood do influence CBF, but they are not the primary determinant. An increase in PaCO2 causes vasodilation, but a decrease in PaO2 has a more significant effect on CBF.
* **Option D:** The myogenic response, which is the contraction of smooth muscle in response to increased pressure, is an important mechanism for regulating CBF, but it is not the major determinant.

**Clinical Pearl / High-Yield Fact**
The cerebral vasculature is highly sensitive to changes in PaO2 and PaCO2 levels, and an imbalance in these levels can lead to significant changes in CBF, potentially resulting in cerebral ischemia or hyperemia.

**Correct Answer: C. Arterial PO2**