Which antibiotic is NOT commonly used to treat endocarditis?

Ciprofloxacin is not commonly used to treat endocarditis because it is a fluoroquinolone antibiotic that primarily targets gram-negative bacteria and is not effective against the common organisms associated with endocarditis, such as Staphylococcus aureus and Streptococcus species. Endocarditis typically requires antibiotics that are effective against these specific pathogens.

Nafcillin, which is a penicillinase-resistant penicillin, is often used to treat infections caused by methicillin-sensitive Staphylococcus aureus. Vancomycin is utilized for covering methicillin-resistant Staphylococcus aureus (MRSA) and other gram-positive bacteria. Penicillin G is traditionally effective against streptococcal infections and is used for certain types of endocarditis, particularly those involving Streptococcus species.

In summary, the primary reason ciprofloxacin is the correct answer is that it does not meet the required spectrum of activity for the pathogens commonly associated with endocarditis, making it an inappropriate choice for treating this condition.