**Core Concept**
The **blood-nerve barrier** is a specialized barrier that restricts the passage of substances from the bloodstream to the nervous system, similar to the blood-brain barrier. This barrier is crucial for maintaining the internal environment of the nerve. The **perineurium**, a protective sheath surrounding nerve fibers, plays a key role in this process.

**Why the Correct Answer is Right**
The correct answer is related to the structure that forms this barrier. The **perineurium** is composed of layers of flattened cells that encircle the nerve fibers, creating a tight junction that prevents the free diffusion of molecules. This unique structure is essential for the selective permeability of the blood-nerve barrier.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not specifically relate to the formation of the blood-nerve barrier.
**Option B:** Incorrect as it is not directly involved in forming the barrier.
**Option D:** Incorrect because, although important for nerve function, it is not the primary component of the blood-nerve barrier.

**Clinical Pearl / High-Yield Fact**
The blood-nerve barrier is crucial for nerve function and protection. Damage to this barrier can lead to nerve dysfunction and disease.

**Correct Answer:** D. Perineurium