Paget's disease is characterized by excessive breakdown of which of the following?

Paget's disease is primarily characterized by excessive breakdown and remodeling of bone tissue. This condition leads to disorganized bone formation, resulting in structurally abnormal bones that can become enlarged and weakened. The disease can cause pain, deformities, and an increased risk of fractures due to the compromised integrity of the bone.

The excessive activity results from an increase in osteoclastic activity, which is responsible for bone resorption. This is followed by an atypical increase in osteoblastic activity, leading to a chaotic and abnormal architecture of the newly formed bone. The disease usually affects the pelvis, skull, spine, and long bones, and the altered bone can become softer and more vascularized, leading to the characteristic symptoms associated with Paget's disease.

Other tissue types mentioned, such as muscle, joint cartilage, and nerve tissue, do not undergo the same pathological processes characteristic of Paget's disease. Thus, they are not relevant in the context of this condition, reinforcing why bone tissue is the correct answer.