Which condition is characterized by the abnormal fusion of joints?

The condition characterized by the abnormal fusion of joints is ankylosis. This condition involves the stiffening or immobility of a joint due to the abnormal growth of bone or the presence of fibrous tissue that results in bony fusion, often following a period of inflammation. Ankylosis can severely limit the movement of the affected joint and is commonly associated with various types of arthritis, particularly inflammatory conditions.

In contrast, achondroplasia is a genetic disorder affecting bone growth that leads to dwarfism, and it does not involve the fusion of joints. Arthritis refers to inflammation of the joints, which can cause pain and swelling but does not necessarily involve the fusion or immobility of joints unless it progresses to ankylosis. Arthrochondritis is the inflammation of the cartilage associated with a joint but does not denote the fusion of joints. Thus, ankylosis distinctly describes the condition where joints become abnormally fused, making it the correct answer.