## **Core Concept**
The Body Mass Index (BMI) is a simple index of weight-for-height that is commonly used to classify underweight, normal weight, overweight, and obesity in adults. It is calculated by dividing a person's weight in kilograms by the square of their height in meters (kg/m^2). A BMI of 20 falls within the normal weight range.

## **Why the Correct Answer is Right**
A BMI of 20 is considered normal weight because it falls within the BMI range of 18.5 to 24.9. This range is associated with a lower risk of health problems compared to being underweight, overweight, or obese. The World Health Organization (WHO) defines the BMI categories as follows: underweight (<18.5), normal weight (18.5-24.9), overweight (25-29.9), and obese (≥30). ## **Why Each Wrong Option is Incorrect** - **Option A:** Underweight is incorrect because a BMI of 20 does not fall below 18.5. - **Option B:** Overweight is incorrect because a BMI of 20 does not fall within the 25-29.9 range. - **Option C:** Obese is incorrect because a BMI of 20 is far below the threshold of 30. - **Option D:** Normal weight is the correct classification for a BMI of 20, making the other options incorrect. ## **Clinical Pearl / High-Yield Fact** A key point to remember is that BMI is not a perfect measure as it does not differentiate between muscle and fat mass. However, for the purpose of clinical classification and guidelines, BMI remains a widely used and practical tool. ## **Correct Answer:** D. Normal weight.