## Core Concept
The question revolves around the concept of the blood-brain barrier (BBB) and its composition. The BBB is a highly selective semipermeable border that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS). It is primarily composed of endothelial cells that line the brain's capillaries.

## Why the Correct Answer is Right
The correct answer, **endothelial cells**, is right because the blood-brain barrier is primarily formed by the tight junctions between the endothelial cells lining the capillaries in the brain. These tight junctions restrict the passage of solutes, cells, and proteins into the brain, thereby protecting the CNS. The endothelial cells play a crucial role in maintaining the integrity and function of the BBB.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because, although astrocytes provide support to endothelial cells and can influence the BBB's function by their end-feet covering the capillaries, they are not the primary cell type responsible for the barrier's structure.
- **Option B:** This option is incorrect as it does not directly relate to a specific cell type; moreover, pericytes, which are sometimes implied here, do play a role in the BBB but are not the primary cell type forming the barrier.
- **Option C:** This option is incorrect because microglia, the resident immune cells of the brain, do not form the structural barrier but are involved in immune surveillance and phagocytic functions within the CNS.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the blood-brain barrier restricts the entry of many antibiotics into the brain, which is why some infections within the CNS require specific antibiotics that can cross this barrier. This is clinically relevant for treating conditions like meningitis or brain abscesses.

## Correct Answer Line
**Correct Answer: D. Endothelial cells.**