An observational study reports that communities with higher fermented food consumption have lower antibiotic-resistant infections. As a reviewer, what multi-step critique and follow-up would you recommend to strengthen causal inference?

• A. Advise immediate nationwide dietary policy changes promoting fermented foods because ecological associations always indicate a causal protective effect
• B. Suggest abandoning further research because community-level observational studies are definitive for infectious disease outcomes
• C. Recommend individual-level studies to control confounders (e.g., hygiene, healthcare access), propose a randomized trial or controlled feeding study where feasible to test causality, and include microbial outcome measures and blinded laboratory analyses to reduce bias
• D. Recommend repeating the same community-level correlation in another country without collecting individual-level data because replication at the ecological level is sufficient for causation

Answer: (C)