A hospital uses a particular broad‑spectrum antibiotic extensively for ten years. Over time clinicians observe breakthrough infections by a formerly susceptible bacterial species. Which evolutionary mechanism most plausibly explains this phenotypic change and what immediate stewardship action would be most appropriate?

• A. Natural selection favoring resistant mutants already present at low frequency; immediately implement antibiotic stewardship to reduce use of that antibiotic and rotate to targeted therapy guided by susceptibility testing.
• B. Horizontal gene transfer of human immune genes into bacteria in the hospital environment; immediately start universal antiviral prophylaxis for all patients to prevent further transfers.
• C. Spontaneous development of multicellularity in the bacterium producing biofilms identical to eukaryotic tissues; immediate hospital closure is the only remedy until the bacterium returns to unicellularity.
• D. Global warming causing bacteria to photosynthesize and thereby evade antibiotics; immediate installation of cooling systems will reverse the phenotype within days.

Answer: (A)