A patient has painful, punched-out, ulcerated interdental papillae with a pseudomembrane and fetor oris, diagnosed as acute necrotizing ulcerative gingivitis (ANUG). If an antibiotic is indicated because of systemic involvement, the drug of choice is:

ANUG involves fusiform bacteria and spirochetes (anaerobes). Primary treatment is debridement and improved oral hygiene; when systemic signs (fever, malaise, lymphadenopathy) are present, metronidazole (effective against anaerobes) is the antibiotic of choice.Other options: Nystatin and fluconazole are antifungals; acyclovir is antiviral; none target the anaerobic bacteria of ANUG.Key exam takeaway: ANUG = anaerobic fusospirochetal infection; debridement plus metronidazole if there is systemic involvement.