## **Core Concept**
Attributable risk (AR), also known as the risk difference, is a measure used in epidemiology to quantify the difference in risk of a particular disease or outcome between an exposed group and an unexposed group. It essentially estimates the risk of disease attributable to a specific risk factor. The formula for attributable risk is: AR = (Risk in exposed) - (Risk in unexposed).

## **Why the Correct Answer is Right**
To calculate the attributable risk, we first need to calculate the risk of lung cancer in both the exposed and unexposed groups. The risk in the exposed group is calculated as the number of cases among the exposed divided by the total number of exposed individuals: Risk in exposed = 20 / 2000 = 0.01. The risk in the unexposed group is calculated similarly: Risk in unexposed = 8 / 8000 = 0.001. Then, the attributable risk is: AR = 0.01 - 0.001 = 0.009 or 9 per 1,000 population. To express it as a percentage, we use the formula: (Risk in exposed - Risk in unexposed) / Risk in exposed * 100 = (0.01 - 0.001) / 0.01 * 100 = 90%. However, attributable risk is often presented in terms of per population, hence the focus is on the actual difference.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it doesn't match our calculated attributable risk.
- **Option B:** Similarly, this doesn't align with our calculations.
- **Option D:** This option is also incorrect based on our step-by-step calculation of attributable risk.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that attributable risk helps in understanding the public health impact of a risk factor. It not only tells us about the additional risk but also guides in planning preventive measures. For instance, eliminating smoking could potentially reduce lung cancer incidence by the attributable risk proportion.

## **Correct Answer:** B.