**Core Concept**
Auto-regulation of cerebral blood flow is a vital mechanism that ensures a constant supply of blood and oxygen to the brain despite changes in systemic blood pressure. This is achieved through the intrinsic ability of cerebral blood vessels to dilate or constrict in response to alterations in blood pressure.

**Why the Correct Answer is Right**
The auto-regulatory mechanism primarily involves the myogenic response, metabolic vasodilation, and the tubuloglomerular feedback mechanism. When systemic blood pressure increases, the myogenic response causes cerebral blood vessels to constrict, reducing blood flow. Conversely, when blood pressure decreases, the vessels dilate to maintain a constant blood flow. This is mediated by the endothelium-derived relaxing factor, nitric oxide (NO), which activates guanylate cyclase, leading to increased cyclic guanosine monophosphate (cGMP) levels and subsequent smooth muscle relaxation.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect as it does not specify the correct mechanism or location of auto-regulation.
**Option B:** This is incorrect as it does not accurately describe the underlying physiological response.
**Option C:** This is incorrect as it is a different mechanism altogether and not related to auto-regulation of cerebral blood flow.

**Clinical Pearl / High-Yield Fact**
Auto-regulation of cerebral blood flow is a critical mechanism that helps maintain cerebral perfusion pressure and prevent ischemia, even in the face of significant fluctuations in systemic blood pressure. Understanding this mechanism is essential for managing various neurological conditions, such as stroke or traumatic brain injury.

**Correct Answer:** A. Cerebral blood vessels (arterioles) are the primary site of auto-regulation, allowing them to adapt to changes in systemic blood pressure.