**Core Concept**
The epiphysis is the rounded end of a long bone that is initially separated from the main bone by a layer of cartilage, but eventually fuses with the metaphysis in adulthood. Certain bone tumors have a predilection for involving the epiphysis, making it an important consideration for diagnosis and management.

**Why the Correct Answer is Right**
The epiphysis is commonly involved in chondroblastoma, a rare type of bone tumor that primarily affects the epiphyses of long bones. This tumor is characterized by the presence of chondroblasts and cartilaginous matrix, which are typically found in the epiphysis. The epiphysis is also a common site for other bone tumors such as giant cell tumor of bone, but chondroblastoma is particularly known for its epiphyseal involvement.

**Why Each Wrong Option is Incorrect**
**Option A:** Osteosarcoma is a malignant bone tumor that typically affects the metaphysis of long bones, not the epiphysis.
**Option B:** Ewing's sarcoma is a malignant bone tumor that can affect any part of the bone, but it is not specifically known for epiphyseal involvement.
**Option C:** Chordoma is a malignant bone tumor that arises from the notochord, typically affecting the spine or base of the skull, and is not characteristically epiphyseal.

**Clinical Pearl / High-Yield Fact**
Chondroblastoma is the most common primary malignant bone tumor to affect the epiphysis, and it often presents in young adults with pain and swelling in the affected bone.

**Correct Answer:** C. Chondroblastoma