**Core Concept:** The connective tissue layer surrounding the muscle fibers, also known as the epimysium, perimysium, and endomysium, plays a crucial role in muscle function and protection.

**Why the Correct Answer is Right:**

The correct answer, **D. Endomysium**, refers to the thin layer of connective tissue that surrounds individual muscle fibers within the muscle. The epimysium is the outer layer, connecting adjacent muscle fascicles. The perimysium is the layer between the fascicles, while the endomysium surrounds each individual muscle fiber.

**Why Each Wrong Option is Incorrect:**

A. **Epimysium:** This layer connects adjacent muscle fascicles, but not individual muscle fibers.
B. **Perimysium:** This layer separates muscle fascicles, not surrounds individual muscle fibers.
C. **Endomysium:** This layer surrounds each muscle fiber, not just the ones in the endomysium.

**Clinical Pearl / High-Yield Fact:** Understanding the different layers of connective tissue surrounding muscle fibers is essential for understanding muscle structure, function, and pathology. In diseases like dermatomyositis and polymyositis, the endomysium becomes thickened and inflamed, leading to muscle dysfunction and pain.

**Correct Answer:** D. Endomysium