**Core Concept**
A sclerotic lesion at the diaphysis in a 20-year-old patient suggests a bone-forming process within the shaft of a long bone. This type of lesion is characterized by increased bone density due to an abnormal proliferation of osteoblasts, the cells responsible for bone formation.

**Why the Correct Answer is Right**
The correct diagnosis is a **Simple Bone Cyst (SBC)**, also known as a unicameral bone cyst. This type of cyst is a benign, fluid-filled lesion that typically occurs in the metaphysis or diaphysis of long bones in young individuals. The cyst is thought to result from a failure of bone formation during growth, leading to a region of decreased bone density surrounded by a sclerotic (dense) border. As the cyst grows, it may cause bone thinning and an increased risk of fracture.

**Why Each Wrong Option is Incorrect**
* **Option A:** Fibrous Cortical Defect (FCD) is a benign lesion that can occur in the long bones of children and adolescents, but it is typically located in the metaphysis and is characterized by a thin layer of cortical bone surrounding a fibrous tissue-filled cavity, not a sclerotic lesion.
* **Option B:** Osteosarcoma is a malignant bone tumor that can occur at any age, but it is more common in older individuals and typically presents with a mix of lytic (bone-destroying) and sclerotic lesions.
* **Option C:** Ewing's Sarcoma is a malignant bone tumor that typically occurs in children and adolescents, but it is characterized by a lytic lesion with a "moth-eaten" appearance and a soft tissue mass, not a sclerotic lesion.

**Clinical Pearl / High-Yield Fact**
When considering a sclerotic lesion at the diaphysis in a young patient, remember that a Simple Bone Cyst (SBC) is a common cause, and imaging studies such as X-rays or CT scans can help confirm the diagnosis.

**Correct Answer:** C. Osteosarcoma