**Core Concept**
Quintiles are a type of quantile used to divide a dataset into five equal parts, each representing a specific percentage of the data. This concept is essential in statistics and data analysis, particularly in medical research and clinical trials.

**Why the Correct Answer is Right**
Quintiles are used to categorize data into five groups, with each group representing 20% of the data. This method is employed to compare and analyze continuous variables, such as age, blood pressure, or cholesterol levels. By dividing the data into quintiles, researchers can identify trends and patterns in the data, which can inform clinical decisions.

**Why Each Wrong Option is Incorrect**
**Option A:** Quartiles, not quintiles, divide a dataset into four equal parts.
**Option B:** Deciles divide a dataset into ten equal parts, not five.
**Option C:** Percentiles divide a dataset into 100 equal parts, not five.

**Clinical Pearl / High-Yield Fact**
When analyzing continuous variables, quintiles can provide a more nuanced understanding of the data than simple mean or median values. This can be particularly useful in identifying correlations between variables and predicting patient outcomes.

**Correct Answer:** C. 5