Final answer:
The indication for additional endodontic intervention after nonsurgical endodontic treatment that is NOT valid is the development of a periradicular radiolucency, as this may be part of the natural healing process if the patient is asymptomatic, and the lesion's size is stable.
Step-by-step explanation:
After a tooth has had nonsurgical endodontic treatment, the one option that is NOT an indication for additional endodontic intervention is the development of a periradicular radiolucency. This is because the development of a new periradicular radiolucency after a nonsurgical endodontic procedure may suggest a new or persistent infection which requires further treatment. However, if a patient is asymptomatic and the lesion is monitored without changes, this may be part of the healing process and thus not immediately indicative of a need for further intervention.
Indications for additional endodontic intervention include the presence of clinical signs and symptoms, presence of a periradicular radiolucency, or the enlargement of the original periradicular radiolucency, which all suggest failed healing or reinfection requiring additional treatment.