Bilateral tonsillectomy is performed on an otherwise healthy 11-year-old female with recurrent upper respiratory tract infections. On pathologcal examination of the tonsils, numerous small, yellow granules are noted. A granule crushed between two slides have a dense, gram-positive center and numerous branching filaments at the periphery. The granules are most likely composed of which of the following organisms?

Actinomyces are normal inhabitants of the gastrointestinal tract that grow under anaerobic and microaerophilic conditions. Although they are gram-positive rods, they grow as branching filaments and have been confused with fungi. The yellow colonies (sulfur granules) are found in low-oxygen niches like the tonsils and in actinomycotic abscesses. Aspergillus fumigatus may be present in the respiratory tract as an oppounistic pathogen; however, fungus balls are generally seen only in pre-existing cavities (e.g., bronchiectasis, TB), not in the tonsils. Blastomyces dermatitidis is a respiratory pathogen that is seen as thick-walled yeasts within granulomas. Candida albicans, also a normal inhabitant of the oral cavity, would present as whitish plaques and would appear microscopically as budding yeasts.