A 40-year-old alcoholic develops cough with foul smelling sputum and fever. Chest x-ray shows an air-fluid level in the superior segment of the right lower lobe. The most likely etiologic agent is

(Braunwald, 15/e, p 1478.) Of the organisms listed, only anaerobic infection is likely to cause a necrotizing process. Type III pneumococci have been repoed to cause cavitary disease, but this is unusual. The location of the infiltrate suggests aspiration, also making anaerobic infection most likely. The superior segment of the right lower lobe is the one most likely to develop aspiration pneumonia.