Which antibiotic is NOT used to treat anthrax?

Vancomycin is primarily used for treating infections caused by gram-positive bacteria, particularly those that are resistant to other antibiotics such as methicillin-resistant Staphylococcus aureus (MRSA). It is not effective against anthrax, which is caused by the bacterium Bacillus anthracis.

In contrast, ciprofloxacin and penicillin are both effective treatments for anthrax, particularly in cases of inhalational anthrax. Ciprofloxacin is a fluoroquinolone antibiotic that works well against Bacillus anthracis, while penicillin targets the bacterium’s cell wall synthesis. Streptomycin, an aminoglycoside antibiotic, may be used in certain cases of anthrax as well, particularly in combination with other antibiotics for more severe infections.

Thus, vancomycin's lack of effectiveness against Bacillus anthracis is what makes it the correct answer in this scenario.