Antimicrobials and Resistance · USMLE Step 1

A patient treated with vancomycin for MRSA develops increasing MIC values over time. Which resistance mechanism is most likely?

  1. Altered penicillin-binding proteins
  2. Increased efflux pump activity
  3. D-Ala-D-Ala replaced with D-Ala-D-Lac
  4. Increased β-lactamase secretion
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Correct answer: D-Ala-D-Ala replaced with D-Ala-D-Lac

Vancomycin resistance occurs when bacteria replace D-Ala-D-Ala with D-Ala-D-Lac, reducing drug binding affinity.

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