Campbell Biology 11th Edition by Lisa Urry,Michael Cain,Steven Wasserman,Peter Minorsky,Jane Reece

Exercise 5

Within six months of effectively using methicillin to treat S. aureus infections in a community, all new S. aureus infections were caused by MRSA. How can this best be explained
A) A patient must have become infected with MRSA from another community.
B) In response to the drug, S. aureus began making drug-resistant versions of the protein targeted by the drug.
C) Some drug-resistant bacteria were present at the start of treatment, and natural selection increased their frequency.
D) S. aureus evolved to resist vaccines.