Final answer:
Endodontic therapy is contraindicated for a single rooted tooth that has a vertical fracture of the root and an associated deep periodontal defect, as it compromises the tooth's stability and allows bacteria to re-infect the tooth.
Step-by-step explanation:
The question is related to when endodontic therapy, or root canal treatment, is contraindicated for a single rooted tooth. Endodontic therapy may be contraindicated for various reasons, often related to the extent or nature of damage to the tooth and surrounding tissues. For instance, a tooth that has discoloration may not necessarily be contraindicated for the procedure, as discoloration can have different causes and does not always indicate a severe underlying problem. An acute apical abscess or a chronic abscess with a draining sinus tract may still be treated with endodontic therapy to remove the infection and save the tooth. However, a tooth with a horizontal fracture of the apical third of the root might be considered for this therapy if the rest of the tooth structure is sound enough to warrant it. Conversely, a tooth with a vertical fracture of the root and an associated deep periodontal defect is typically considered a poor candidate for endodontic therapy because the vertical fracture can allow bacteria to re-infect the tooth after treatment, and the periodontal defect may compromise the tooth's stability.