A teenager's jaw X-ray shows a diffuse 'ground glass' opacity blending into normal bone. What does this suggest? Fibrous dysplasia — normal bone is being replaced by fibro-osseous tissue in a diffuse, blending pattern (no capsule). It often stabilizes after puberty. Students contrast this with the well-defined lesions of tumors and the sequestra of osteomyelitis: the blending ground-glass margin is the diagnostic clue.