Which condition is described as a disease of unknown cause that involves extensive bone destruction?

Paget's disease is characterized by abnormal and excessive bone remodeling, which can lead to weakened bones and deformities. The exact cause of Paget's disease is not fully understood, which aligns with the description of it being a disease of unknown cause. In this condition, bones may become enlarged and fragile, making them more susceptible to fractures.

Unlike osteoporosis, which primarily involves the loss of bone density due to age or hormonal changes, Paget's disease involves both the destruction of bone and the formation of new bone in a disorganized manner. It is distinct from rheumatoid arthritis, an autoimmune condition that primarily affects joints and not bone density. Osteomalacia, on the other hand, refers to a softening of the bones due to a deficiency in vitamin D or calcium, but does not exhibit the same extensive destructive changes seen in Paget's disease. Thus, it is clear why Paget's disease fits the criteria of a disease of unknown cause resulting in extensive bone destruction.