**Core Concept**
The Blood-Brain Barrier (BBB) is a highly selective permeable barrier that separates the circulating blood from the brain's extracellular fluid in the central nervous system (CNS). It is composed of specialized cells that regulate the passage of molecules between the blood and the brain, maintaining a stable environment for neuronal function.

**Why the Correct Answer is Right**
The BBB is formed by a unique combination of astrocytes and endothelial cells. Astrocytes, a type of glial cell, play a crucial role in forming the BBB by extending their foot processes around the endothelial cells, which line the brain's capillaries. This tight junction between the astrocytes and endothelial cells creates a selective barrier that restricts the passage of most molecules, including ions, proteins, and pathogens, into the brain. The endothelial cells also express tight junction proteins, such as occludin and claudin, which further reinforce the barrier. This selective permeability is essential for maintaining the brain's internal environment and preventing the entry of harmful substances.

**Why Each Wrong Option is Incorrect**
**Option B:** Choroidal cells are part of the choroid plexus, which produces cerebrospinal fluid (CSF), but they are not directly involved in forming the BBB.
**Option C:** Oligodendrocytes are a type of glial cell that myelinate axons, but they do not contribute to the formation of the BBB.
**Option D:** Endothelial cells alone do not form the BBB; they require the presence of astrocytes to create the tight junctions necessary for selective permeability.

**Clinical Pearl / High-Yield Fact**
The BBB is a critical structure that is disrupted in various neurological conditions, such as multiple sclerosis, stroke, and neurodegenerative diseases. Understanding the components and function of the BBB is essential for developing effective treatments for these conditions.

**✓ Correct Answer: A. Astrocytes and endothelial cells**