**Core Concept**
Neurofibromatosis type 1 (NF1) is a genetic disorder characterized by the development of multiple neurofibromas, which are benign tumors that arise from the nerve sheath. Orthopaedic manifestations are common in NF1 and can significantly impact the quality of life.

**Why the Correct Answer is Right**
The most common orthopaedic manifestation of NF1 is **scoliosis**. Scoliosis in NF1 is often more severe and more likely to be progressive than in the general population. This is due to the abnormal development of the spine and the surrounding soft tissues, which can lead to an increased risk of curvature and deformity. The pathogenesis of scoliosis in NF1 is thought to be related to the genetic mutation that affects the development and function of the spine and the surrounding muscles.

**Why Each Wrong Option is Incorrect**
**Option A:** While osteosarcoma is a common malignancy associated with NF1, it is not the most common orthopaedic manifestation of the disease.
**Option B:** Bone fractures are a common complication of NF1, but they are not the most common orthopaedic manifestation.
**Option C:** Joint hypermobility is often seen in NF1, but it is not the most common orthopaedic manifestation of the disease.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that scoliosis in NF1 can be more severe and progressive than in the general population, and therefore, patients with NF1 should be closely monitored for spinal deformity.

**Correct Answer:** C. Scoliosis