## **Core Concept**
The Pearl index, also known as the Pearl rate, is a commonly used measure to evaluate the effectiveness of a contraceptive method. It represents the number of failures (unintended pregnancies) per 100 woman-years (HWY) of exposure. The formula to calculate the Pearl index is: Pearl Index = (Number of failures / Number of woman-years of exposure) * 100.

## **Why the Correct Answer is Right**
The correct formula for the Pearl index is given by: Pearl Index = (Number of failures / Number of woman-years of exposure) * 100. This directly translates to the failure rate per 100 woman-years. Therefore, the correct answer representing this calculation is .

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option does not match the standard formula for the Pearl index, which involves a multiplication by 100 to express the rate per 100 woman-years.
- **Option B:** Similar to option A, this does not align with the conventional Pearl index formula.
- **Option C:** This option also does not represent the correct calculation for the Pearl index.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the Pearl index is a useful comparative measure of contraceptive efficacy but does not account for factors like user compliance. Different contraceptive methods have different Pearl index values, for example, the pill has a Pearl index around 0.1-0.5, while an IUD might have a value around 0.8-1.0.

## **Correct Answer Line**
**Correct Answer: D. **