A laboratory performs a double-disk diffusion test (D test) on a Staphylococcus aureus isolate that appears erythromycin resistant but clindamycin susceptible. Flattening of the clindamycin zone adjacent to the erythromycin disk is reported. What does this finding indicate?

• A Constitutive MLSb resistance, so clindamycin should be avoided
• B Efflux-mediated macrolide resistance only, so clindamycin remains reliable
• C Inducible MLSb resistance, so clindamycin carries a risk of treatment failure ✓
• D Beta-lactamase hyperproduction unrelated to clindamycin susceptibility

Explanation

Zone flattening toward the erythromycin disk indicates inducible erm-mediated resistance: erythromycin acts as an inducer, and mutants with constitutive expression arise during clindamycin monotherapy, risking failure even though the initial MIC is low. Constitutive resistance would show clindamycin resistance on routine testing without needing a D test, and efflux (msr/mef) spares clindamycin entirely, giving a true positive D test result.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

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