What type of investigation was conducted in the Colorado Brown Stain study that led to the discovery of fluoride's effects on teeth?

Correct answer: D. Assessment (epidemiologic investigation)

• A. Incorrect. 'Assurance' is not a standard research design category.
• B. Incorrect. The Colorado Brown Stain investigation was observational, not a randomized controlled trial.
• C. Incorrect. It was not a case-control study — it was a community-level epidemiologic survey/assessment.
• D. Correct. Dr. Frederick McKay and Dr. G.V. Black conducted an EPIDEMIOLOGIC ASSESSMENT (observational survey) of the Colorado Brown Stain phenomenon in the early 1900s. They systematically ASSESSED the prevalence and distribution of the mottled brown staining among Colorado communities, eventually correlating it with high natural fluoride concentrations in water. This epidemiologic assessment led to the discovery of both dental fluorosis and fluoride's caries-protective effects.