A patient with large, penetrating vegetations on his mitral and aoic valves develops severe headaches. Funduscopic examination reveals papilledema. CT scan of the brain demonstrates a ring-enhancing lesion. Which of the following organisms is the most likely cause of the patient's disorder?

The cardiac lesions described are characteristic of acute bacterial endocarditis. The fact that the valvular vegetations are large and penetrating strongly suggests that they are due to a virulent, pyogenic pathogen. Among the answer choices, Staphylococcus aureus best fits this description, and is in fact the most common cause of acute bacterial endocarditis. Brain abscess, which produces ring-enhancing radiologic lesions, is a known complication of bacterial endocarditis, occurring when the vegetations fragment and release septic emboli. Herpes Virus can cause encephalitis, but is not a cause of vegetative endocarditis. Mycobacterium tuberculosis can cause brain masses, but does not usually cause endocarditis. Streptococcus pneumoniae can cause acute pyogenic meningitis, but does not usually cause endocarditis. Ref: Brooks G.F., Carroll K.C., Butel J.S., Morse S.A., Mietzner T.A. (2013). Chapter 14. The Streptococci, Enterococci, and Related Genera. In G.F. Brooks, K.C. Carroll, J.S. Butel, S.A. Morse, T.A. Mietzner (Eds), Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.