**Core Concept**
Perthe's disease, also known as Legg-Calve-Perthes disease, is a condition characterized by transient osteochondritis of the femoral head. The disease affects the blood supply to the femoral head, leading to necrosis of the subchondral bone and eventual deformity of the joint.

**Why the Correct Answer is Right**
In Perthe's disease, the movement that is primarily restricted is **internal rotation** of the hip. This is due to the involvement of the posterior aspect of the femoral head, which is responsible for the rotational movements of the hip joint. The disease leads to a reduction in the range of motion, particularly in internal rotation, as a result of the formation of a subchondral fracture and subsequent collapse of the femoral head.

**Why Each Wrong Option is Incorrect**
* **Option A:** Flexion of the hip is not typically restricted in Perthe's disease, as the anterior aspect of the femoral head is less affected by the disease process.
* **Option B:** Extension of the hip is also not typically restricted, as the disease primarily affects the posterior aspect of the femoral head.
* **Option D:** Abduction of the hip is not specifically restricted in Perthe's disease, as the disease process does not primarily affect the gluteal muscles or the surrounding soft tissues.

**Clinical Pearl / High-Yield Fact**
Perthe's disease often presents with a limp and pain in the hip, and it typically affects children between the ages of 4 and 8. Early diagnosis and treatment are crucial to prevent long-term complications and deformity of the hip joint.

**Correct Answer:** C.