Fibrous dysplasia is a bone disorder where normal bone is replaced with fibrous tissue, leading to weakened bones and deformities. It can affect one bone (monostotic) or multiple bones (polyostotic) and may cause pain, fractures, or changes in bone shape. The exact cause of fibrous dysplasia is a mutation in the GNAS gene, which affects bone development. This condition is often diagnosed through imaging techniques like X-rays or MRIs and may require treatment options such as medication or surgery to manage symptoms and complications.