A patient consulted a dental surgeon about extraction of the roots of the 36 tooth. The tooth decayed long ago. Objectively: the crown of the 36 is destroyed by 2/3, percussion is painless, mucosa around the 36 tooth exhibits no evident pathological changes. X-ray picture of the alveolar process shows a roundish well-defined radiolucency near the apex of the medial root. What is the most likely diagnosis?