A 60-year-old woman with a past medical history of rheumatoid arthritis and a prosthetic aortic valve, was brought by her family after found her extremely lethargic and confused at home. She had been complaining of fevers, chills, headache, and neck pain

In the case presented, the 60-year-old woman has a history of rheumatoid arthritis and a prosthetic aortic valve, and her symptoms include lethargy, confusion, fever, chills, headache, and neck pain. Given her history of a prosthetic valve, she is at increased risk for infective endocarditis. The most likely causative organism associated with infective endocarditis in individuals with prosthetic valves is Staphylococcus epidermidis, which is a coagulase-negative staphylococcus and part of the normal skin flora. It is a common cause of infections in patients with prosthetic devices, particularly vascular prosthetic materials like heart valves. The other organisms listed are less consistent with the given clinical presentation: - Coxsackie virus A generally causes viral illnesses and is less likely to cause endocarditis. - Adenovirus serotype 12 is also a viral pathogen not typically associated with endocarditis. - Streptococcus viridans is associated with endocarditis, particularly in patients with pre-existing heart conditions, but is less common in cases involving prosthetic valves compared to Staphylococcus epidermidis. Therefore, the most probable causative organism in this scenario is: **Staphylococcus epidermidis**.