All of the following organisms are associated with acute necrotizing ulcerative gingivitis (ANUG) EXCEPT one. Which is the EXCEPTION?

Correct answer: C. Streptococcus viridans

• A. Fusobacterium nucleatum is a bridging species and key ANUG pathogen — it is part of the fusospirochetal complex originally described in ANUG.
• B. Treponema vincentii (spirochetes) are KEY pathogens in ANUG — they are found in the deep advancing zone of ANUG lesions and give the disease its alternative name 'Vincent's angina' or 'trench mouth.'
• C. CORRECT. Streptococcus viridans (alpha-hemolytic streptococci) are NORMAL ORAL FLORA and are NOT specific pathogens of ANUG. They are associated with endocarditis prophylaxis scenarios, not necrotizing periodontal diseases. ANUG is caused by a FUSOSPIROCHETAL complex of anaerobes.
• D. Prevotella intermedia is consistently elevated in ANUG — it has been linked to the hormonal changes that exacerbate ANUG.