A 55-year-old woman had a root canal. Several weeks later, she presents with a draining sinus on the same side of the face. The pus contains yellow granules, which turn out to be clumps of organisms in a calcium phosphate matrix. The most likely organism is A) a gram-negative rod B) a gram-positive, branching, beaded rod C) a gram-positive coccus, which grows in short chains D) a partially acid-fast, branching rod E) a slender, gram-negative rod with pointed ends The answer is B. Actinomyces israelii is a branching, beaded, gram-positive rod. It is normal flora of the oral pharynx and gastrointestinal tract. When it is displaced from its normal niche, it causes destructive abscesses that expand by forming sinus tracts in tissue without regard to tissue planes. The pus from the abscess contains â€œsulfurâ€ granules that are composed of organisms in a calcium phosphate matrix. Prevotella melaninogenicus is an anaerobic, gram-negative rod (choice A). It forms black-pigmented colonies on a special blood agar. It is normal flora of the mouth, and can cause oral or pulmonary abscesses when displaced. Peptostreptococcus are gram-positive cocci that grow in short chains (choice C). They are normal flora of the mouth, intestinal tract, vagina, and skin. They are often isolated from infections with other anaerobic or aerobic bacteria. Nocardia asteroides is a branching, beaded, gram-positive rod that inhabits soil. It is an aerobic organism that can cause localized or disseminated infection, especially in the immunocompromised. On Gram stain, it resembles A. israelii. They can be easily distinguished because N. asteroides stains positive using a modified acid-fast stain (choice D). Fusobacterium nucleatum is a pale-staining, slender, gram-negative rod with pointed ends (choice E). It is normal flora of the oral cavity and can cause oral, dental,and pleuropulmonary infections when displaced from its normal environment.