**Core Concept:** Histological examination of tumors is essential to differentiate between enchondroma and chondrosarcoma, as they have distinct characteristics that can be evaluated under a microscope.

**Why the Correct Answer is Right:** Enchondroma is a benign cartilaginous tumor originating from chondrocytes, usually found in the epiphyses of long bones. It is characterized by a monomorphic appearance, low cellularity, and no mitotic activity. On the other hand, chondrosarcoma is a malignant cartilaginous tumor originating from chondrocytes. It shows high cellularity, pleomorphism, and increased mitotic activity.

**Why Each Wrong Option is Incorrect:**

A. **Mitotic activity:** Enchondroma exhibits minimal mitotic activity, while chondrosarcoma demonstrates significant mitotic activity, making this feature less helpful in differentiating the two tumors.

B. **Cellularity:** While both tumors can exhibit varying degrees of cellularity, chondrosarcoma generally has a higher degree of cellularity than enchondroma due to its malignant nature.

C. **Mitotic index:** Mitotic index refers to the number of mitotic figures per unit area. Enchondroma has a low mitotic index, while chondrosarcoma has a higher mitotic index, making this feature less helpful in differentiating the two tumors.

D. **Microscopic appearance:** Both enchondroma and chondrosarcoma can exhibit similar microscopic appearances due to their cartilaginous origin. The focus should be on histological features beyond microscopic appearance.

**Clinical Pearl:** Histological examination is crucial in distinguishing between enchondroma and chondrosarcoma, as the presence of pleomorphism, mitotic activity, and cellularity help differentiate these two entities.

**Correct Answer:** **D. Pleomorphism:** Pleomorphism refers to the presence of cells with diverse shapes and sizes within a tumor. Enchondroma typically demonstrates uniform cells, while chondrosarcoma exhibits pleomorphic cells, making this feature more helpful in differentiating the two tumors.