Genetics of resistance — plasmid mediation: Which antimicrobial listed commonly exhibits plasmid-mediated resistance in clinical isolates?

Introduction / Context:
Resistance genes can reside on chromosomes or mobile elements (plasmids, transposons). Plasmid-mediated resistance spreads rapidly across species and settings, shaping empirical therapy choices.

Given Data / Assumptions:

• The question seeks a drug with well-documented plasmid-mediated resistance.
• β-lactamases encoded on plasmids are prevalent in Enterobacterales and other pathogens.
• Several distractor drugs typically acquire resistance via chromosomal mutations.

Concept / Approach:
Ampicillin resistance is classically plasmid-mediated through transferable β-lactamases (e.g., TEM-1). In contrast, nalidixic acid (a quinolone) and rifampicin often involve chromosomal mutations in gyrA/parC and rpoB, respectively. Methicillin resistance in S. aureus is mecA-mediated (SCCmec element), not a simple plasmid; vancomycin resistance can be operon-based on transposons (e.g., vanA on Tn1546) but the best single canonical answer is ampicillin.

Step-by-Step Solution:
Identify plasmid-encoded β-lactamase → ampicillin.Exclude drugs usually linked to chromosomal target mutations.Select ‘‘Ampicillin’’.

Verification / Alternative check:
Widespread plasmids (IncF, IncI, etc.) carrying bla genes causing ampicillin resistance confirm this mechanism.

Why Other Options Are Wrong:

• Nalidixic acid/Rifampicin: Mainly chromosomal target mutations.
• Methicillin: Resistance via mecA on SCCmec, not a typical plasmid.
• Vancomycin: Complex operons (van genes) often on transposons; not the classic single best answer here.

Common Pitfalls:
Equating any mobile element with plasmids; transposons and chromosomal cassettes differ from plasmid carriage.

Final Answer:
Ampicillin.