**Core Concept**
Microglia are a type of glial cell located throughout the brain and spinal cord, acting as the main form of active immune defense in the central nervous system (CNS). They play a crucial role in surveillance, phagocytosis, and antigen presentation within the CNS.

**Why the Correct Answer is Right**
Microglia are derived from the yolk sac and migrate to the brain during embryonic development. Upon activation, they undergo morphological changes, becoming amoeboid in shape and extending processes to survey their surroundings. Their primary function is to phagocytose debris, dead cells, and foreign particles, thereby maintaining CNS homeostasis and preventing inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** Astrocytes are another type of glial cell involved in CNS maintenance, but they are primarily responsible for supporting neurons through nutrient and waste exchange, and are not directly involved in immune surveillance.
**Option B:** Oligodendrocytes are responsible for myelination of neurons, enabling faster signal transmission, but they do not participate in immune defense mechanisms.
**Option C:** Ependymal cells line the ventricles and central canal, producing cerebrospinal fluid, but they are not involved in immune responses.

**Clinical Pearl / High-Yield Fact**
Microglia activation is a hallmark of neuroinflammatory disorders, such as multiple sclerosis and Alzheimer's disease, and can contribute to disease progression and pathology.

**Correct Answer:** C. Microglia