**Core Concept**
The longest muscle in the human body is a fascial structure that runs from the base of the skull to the heels, playing a crucial role in maintaining posture and facilitating movement. This muscle is a component of the muscular fascia that envelops the body, providing support and stability to the musculoskeletal system.

**Why the Correct Answer is Right**
The correct answer is the **Tensor Fasciae Latae muscle**, which is a part of the iliotibial tract. However, the longest fascial structure in the body is actually the **Iliotibial Tract (IT Tract)**, not the Tensor Fasciae Latae. The Iliotibial Tract is a long, fibrous band that extends from the iliac crest to the lateral condyle of the tibia, providing support to the lateral aspect of the thigh and knee joint.

**Why Each Wrong Option is Incorrect**
* **Option A:** The Sartorius muscle is the longest muscle of the thigh, but it is not the longest muscle in the body.
* **Option B:** The Pectoralis major muscle is a large muscle in the chest, but it is not the longest muscle in the body.
* **Option C:** The Biceps brachii muscle is a major muscle of the upper arm, but it is not the longest muscle in the body.

**Clinical Pearl / High-Yield Fact**
The Iliotibial Tract is prone to friction and irritation, particularly in runners and cyclists, leading to conditions such as iliotibial band syndrome.

**Correct Answer: D. Iliotibial Tract**