Acquired resistance occurs when an organism that was previously susceptible becomes resistant. Which of the following best describes this concept?

• A. Intrinsic resistance
• B. Cross-resistance
• C. Acquired resistance
• D. Inducible resistance

Answer: (C)

The idea being tested is how bacteria gain resistance after they have already been susceptible. Acquired resistance occurs when an organism that could be inhibited by a drug develops the ability to resist it, usually through mutation or by obtaining resistance genes from other organisms. This change is not present in the original population and arises under selective pressure from antibiotic exposure, via mechanisms like altering the drug target, producing enzymes that inactivate the drug, increasing efflux pumps, or reducing drug entry. This is different from intrinsic resistance, which is built-in and seen in all strains of a species without prior exposure. Cross-resistance involves one resistance mechanism providing protection against several drugs, not necessarily because of a recent change in the organism. Inducible resistance refers to resistance that is turned on only in the presence of the drug, rather than being continuously expressed. Therefore, the description of becoming resistant after being susceptible aligns with acquired resistance.