You are given a graph showing weekly counts: Weeks 1–8: 2–4 cases per week (stable baseline); Weeks 9–10: a spike to 40 cases in one city; Weeks 11–18: citywide and neighboring regions show rising counts to 600 weekly. What analytic steps would you take to determine whether the Week 9–10 spike was the origin of the later epidemic, and which conclusion is most plausible?

• A. Steps: ignore temporal relationships and only compare raw cumulative counts; conclusion: initial spike and later epidemic are always unrelated by definition, so no further investigation is needed.
• B. Steps: assume that the initial spike reduced susceptibility so no seeding could occur; conclusion: the later epidemic must arise from a different pathogen and no genomic or contact tracing is worthwhile.
• C. Steps: immediately declare the spike a pandemic without examining geographic spread or travel patterns; conclusion: the spike proves global dissemination despite absence of cases outside the city.
• D. Steps: review spatial links and travel/contact patterns from the spike area to affected regions, compare case genotypes if available, and assess timing; plausible conclusion: the Week 9–10 localized outbreak likely seeded wider transmission leading to the epidemic if linkage and timing are consistent.

Answer: (D)