Acquired resistance is best defined as:

• A. When a previously sensitive organism is no longer sensitive
• B. When a new organism arises
• C. When host immunity declines
• D. When resistance is inherited from parent strain

Answer: (A)

The main idea here is acquired resistance: an organism that was once sensitive to an antimicrobial becomes resistant after exposure. This happens when bacteria adapt under drug pressure by gaining or developing mechanisms such as enzymes that inactivate the drug, changes to the drug’s target so the medication no longer binds effectively, or increased efflux and reduced permeability that prevent the drug from reaching its target. As a result, the surviving population can continue to grow despite treatment.

In dental practice, this is why infections can persist or recur if bacteria acquire such resistance, underscoring the importance of appropriate antibiotic use and susceptibility testing to guide therapy. It is not about the sudden appearance of a new organism, nor about a decline in the host’s immune defenses, nor about resistance traits that are simply passed from parent to offspring without prior exposure.