In the periapical abscess case, what is the PRIMARY source of the infection?

Correct answer: A. Necrotic pulp tissue in the root canal system

• A. CORRECT. A periapical abscess results from infection originating in the NECROTIC PULP. When the pulp dies (from caries, trauma, or periodontal disease), bacteria colonize the avascular root canal space and eventually escape through the apex, infecting the periapical tissues. The necrotic root canal is the source; the periapical abscess is the consequence.
• B. External contamination from recent procedures could cause infection but would be an atypical and uncommon cause of periapical abscess.
• C. Periodontal bacteria cause periodontal abscesses (which originate from the gingival sulcus/pocket). A PERIAPICAL abscess originates from the pulp/root apex, not the sulcus — though combined endo-perio lesions can occur.
• D. Hematogenous infection of the periapex is extremely rare. The vast majority of periapical abscesses are caused by direct extension from an infected root canal.