The correct answer is A. Actinomyces israelii.
Actinomyces israelii is a filamentous, gram-positive, anaerobic, and non-acid-fast rod that typically presents as fluctuant nodules classically at the angle of the jaw (“lumpy jaw”) with fistulas and draining sinuses. Histologically, there are central basophilic “sulfur granules” with eosinophilic peripheries and surrounding granulomatous inflammation. IV Penicillin G is the initial treatment and should be used for 4-6 weeks. Oral Penicillin may be required afterwards for 2-12 months depending on the severity of infection.
Staphylococcal aureus can cause SSSS, TSS, impetigo, cellulitis, Botryomycosis, folliculitis, and a variety of other wound infections. It would appear with Gram positive cocci in clusters on direct Gram stain. Pseudomonas aeruginosa can cause Ecthyma gangrenosum, Botryomycosis, or hot tub folliculitis. It is a Gram-negative, aerobic rod on Gram stain. Mucor spp. typically presents in immunosuppressed patients with edema, erythema, or ulcerated plaques. Histologically, nonseptate broad ribbon-like hyphae with 90-degree branching are seen.
References: Reference: Bonnefond S, et al. Clinical features of actinomycosis: A retrospective, multicenter study of 28 cases of miscellaneous presentations. Medicine (Baltimore). 2016;95:e3923.