A clinician treats a mixed bacterial infection with an antibiotic and afterwards the remaining population is resistant. Which explanation best describes what occurred?

The antibiotic directly caused the bacteria to mutate into resistant strains in response to drug pressure in the majority of cells, creating resistance during treatment.

Resistant cells were present before treatment, and the antibiotic preferentially killed susceptible cells so resistant genotypes became more common by selection.

Antibiotics converted nonbacterial nucleic acids in the sample into resistant plasmids that spontaneously integrated into unrelated cells, so resistance was synthetically introduced by the drug chemistry.

The host immune response turned off after antibiotic exposure, allowing previously dormant environmental bacteria to colonize and be mistaken for treatment-induced resistance when in fact they were newly acquired species.