There are 4 major classes of antimicrobial resistance mechanisms employed by bacteria:

1. Drug target modification
2. Drug molecule modification
3. Altered permeability / efflux
4. Metabolic bypass

Nalidixic acid is an antibiotic that inhibits enzymes involved in the DNA replication process. Some strains of Pseudomonas aeruginosa are resistant to nalidixic acid and have been shown to have lower intracellular concentrations of the antibiotic when compared with sensitive strains. Which ONE of the following most likely explains the mechanism of resistance of P. aeruginosa to nalidixic acid?