6. A 21-year-old woman went to her dentist because of facial asymmetry. This asymmetry had developed gradually over 3 years. The patient had no symptoms. A diffusely opaque lesion was found in her right maxilla. All laboratory tests (complete blood count, alkaline phosphatase, calcium) were within normal limits. A biopsy specimen was interpreted as a fibro-osseous lesion. This patient most likely has ______.