A periapical radiograph shows a thin radiolucent line running between teeth #21 and #22. What is it?

Correct answer: C. Nutrient canal

• A. A lateral periodontal cyst is a well-defined round or teardrop radiolucency, not a thin line.
• B. A root fracture line would lie within the root itself, not in the interdental bone between two teeth.
• C. Correct. Nutrient canals are normal anatomic channels carrying neurovascular bundles through the alveolar bone. They are most visible in the mandibular anterior region, especially in thin bone and in patients with hypertension or diabetes.
• D. The PDL space follows the root outline; it does not run vertically through interdental bone.