## **Core Concept**
The question pertains to a type of glial cell in the central nervous system (CNS) that plays a crucial role in immune surveillance and responds to infection or inflammation by becoming phagocytic. These cells are key components of the CNS's defense mechanism.

## **Why the Correct Answer is Right**
The correct answer, **Microglia**, are a type of glial cell located throughout the brain and spinal cord. They act as the first and main form of active immune defense in the CNS. Microglia are derived from monocyte/macrophage lineage and serve as the resident macrophages of the CNS. In response to inflammation or infection, microglia can become activated and exhibit phagocytic behavior, helping to clear pathogens and debris from the CNS.

## **Why Each Wrong Option is Incorrect**
- **Option A: Astrocytes** are a subtype of glia that provide many functions, including biochemical support of endothelial cells that form the blood–brain barrier, provision of nutrients to the nervous tissue, and maintenance of extracellular ion balance. While they play critical roles in the CNS, they are not primarily phagocytic in response to inflammation.
- **Option B: Oligodendrocytes** are responsible for myelination of neurons in the CNS, which is crucial for the rapid transmission of neuronal signals. They do not have a significant role in immune responses or phagocytosis.
- **Option D: Ependymal cells** line the cerebrospinal fluid (CSF) spaces in the CNS, including the ventricles and central canal of the spinal cord. They are involved in the production and circulation of CSF but are not primarily involved in immune surveillance or phagocytosis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that microglia are the resident immune cells of the CNS and play a critical role in neuroinflammation and neurodegenerative diseases. Their activation can have both protective and detrimental effects on the CNS, depending on the context and the specific signals they receive.

## **Correct Answer: C. Microglia**