Mutant prevention concentration is the drug level that blocks growth of first-step resistant mutants. Its distance from the MIC sets the width of the mutant selection window. Across a 100-isolate Gram-negative panel, median MPC/MIC ratios ran from 4 to 16 depending on the molecule. That spread has direct consequences for how a fluoroquinolone such as norfloxacin gets dosed. The numbers below come from a hospital isolate collection and are used here only as class-level read-across. The pharmacology of the selection window does not change with the host species. What changes is the achievable exposure, and that is where a veterinary formulator or a prescribing veterinarian has to do the arithmetic. Why the selection window matters more than the MIC Routine susceptibility testing reports one number: the MIC, the lowest concentration that stops visible growth of the bulk population. That number describes the majority. It says nothing about the small subpopulation already carrying a first-st...