## **Core Concept**
The 21 Cities Study by Dean is related to **epidemiology**, specifically focusing on the relationship between **water fluoride levels** and **dental caries** (tooth decay) prevalence in communities.

## **Why the Correct Answer is Right**
The 21 Cities Study, conducted by H. Trendley Dean, was pivotal in establishing the **optimal level of fluoride** in drinking water for the prevention of dental caries. Dean's research involved analyzing the dental health of populations in 21 cities in the United States with varying levels of naturally occurring fluoride in their water supplies. The study found a significant correlation between the fluoride levels in drinking water and the prevalence of dental caries, leading to the recommendation of **0.7-1.2 mg/L** of fluoride in drinking water as optimal for dental health.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately represent the focus of the 21 Cities Study.
- **Option B:** This option is incorrect as it does not pertain to the study's findings or purpose.
- **Option C:** This option is incorrect because, although it might relate to public health or dental studies, it does not specifically address the 21 Cities Study's focus on fluoride levels and dental caries.
- **Option D:** This option is incorrect as it does not accurately describe the study's objective or outcome.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **optimal fluoride level** in drinking water, as suggested by Dean's study, is around **0.7-1.2 mg/L**. This range is crucial for preventing dental caries without causing **dental fluorosis**, a condition characterized by discoloration and pitting of the teeth due to excessive fluoride intake.

## **Correct Answer:** . Water Fluoridation Level