A periapical radiograph shows a combined lateral-and-apical radiolucency (rarefaction) along a tooth's root. What is the most likely cause?

A combined apical-and-lateral pattern of bony rarefaction along a root is a classic (though not universally present) radiographic sign of a vertical root fracture, sometimes described as a 'J-shaped' or halo-type radiolucency — though the fracture line itself is often not directly visible due to its orientation relative to the X-ray beam.