A well-defined radiolucent lesion is present at the apex of a non-vital tooth. What is it most likely to be?

Correct answer: B. Periapical (radicular) cyst

• A. Condensing osteitis is radiopaque, not radiolucent — a sclerotic bone response to low-grade pulpal inflammation.
• B. Correct. A radiolucency at the apex of a pulpally necrotic tooth is inflammatory in origin. A well-defined, larger lesion favours a radicular cyst, which arises from proliferation of the epithelial rests of Malassez.
• C. Periapical cemento-osseous dysplasia occurs at vital teeth, most often the mandibular anteriors in middle-aged Black women, and passes through radiolucent, mixed and radiopaque stages.
• D. Idiopathic osteosclerosis is also radiopaque, and by definition occurs at a tooth with a normal, vital pulp.

Note: Radiographs alone cannot separate a granuloma from a cyst — that requires histology. Vitality testing is what distinguishes an inflammatory lesion from a fibro-osseous one.