Final answer:
Vancomycin is effective against gram-positive bacteria like S. aureus but not gram-negative bacteria, penicillin is preferred for MSSA, and vancomycin is usually given intravenously and can be dialyzed in renal failure cases. Therefore, the correct option is A.
Step-by-step explanation:
Treating endocarditis with Vancomycin targets gram-positive bacteria such as Staphylococcus aureus, including methicillin-resistant strains (MRSA), but it is not effective against gram-negative bacteria. When methicillin-sensitive Staphylococcus aureus (MSSA) is the causative organism, penicillin or nafcillin is preferred due to better effectiveness and tolerance. Vancomycin is typically administered intravenously due to poor oral absorption, making it unsuitable for oral outpatient therapy. While vancomycin can be dialyzed out in patients with renal failure, monitoring levels is important to maintain therapeutic efficacy.