**Core Concept**
The gluteus maximus muscle is a major muscle of the buttocks responsible for extending and rotating the hip joint. It plays a crucial role in movements such as walking, running, and climbing stairs. The muscle's anatomical insertion is essential for its function.

**Why the Correct Answer is Right**
The gluteus maximus muscle is inserted into the iliotibial tract, a fibrous band that runs down the lateral aspect of the thigh. This insertion allows the muscle to extend the hip joint and assist in the abduction of the thigh. The iliotibial tract also serves as an attachment site for other muscles, including the tensor fasciae latae and the gluteus medius. The gluteus maximus muscle's insertion into the iliotibial tract provides a strong and stable attachment point for its contraction, enabling efficient movement of the hip joint.

**Why Each Wrong Option is Incorrect**
**Option A:** The lesser trochanter is actually the site of insertion for the psoas major muscle, which assists in hip flexion. This is in contrast to the gluteus maximus, which extends the hip joint.
**Option B:** The greater trochanter is a bony prominence on the femur, but it is not the site of insertion for the gluteus maximus muscle. Instead, the gluteus medius and gluteus minimus muscles insert into the greater trochanter.
**Option C:** The spiral line, also known as the linea aspera, is a fibrous band on the posterior aspect of the femur. While it does serve as an attachment site for various muscles, it is not the primary insertion site for the gluteus maximus.

**Clinical Pearl / High-Yield Fact**
A key anatomical relationship to remember is that the gluteus maximus muscle's insertion into the iliotibial tract is in close proximity to the sciatic nerve, which runs along the posterior aspect of the thigh. This relationship is important to consider when performing procedures such as hip surgery or when evaluating patients with neurological deficits.

**✓ Correct Answer: D. iliotibial tract**