## **Core Concept**
The **Body Mass Index (BMI)** is a widely used tool to categorize individuals into various weight categories, including underweight, normal weight, overweight, and obese. BMI is calculated by dividing a person's weight in kilograms by the square of their height in meters (kg/m^2). The World Health Organization (WHO) has established BMI categories to assess weight status.

## **Why the Correct Answer is Right**
The correct answer, , indicates that a person is considered underweight if their BMI is less than 18.5. This classification is based on the WHO's BMI categories, which define underweight as a BMI below 18.5, normal weight as 18.5-24.9, overweight as 25-29.9, and obese as 30 or greater. A BMI below 18.5 suggests that an individual may be at risk for malnutrition or other health issues related to being underweight.

## **Why Each Wrong Option is Incorrect**
- **Option A:** is incorrect because a BMI of 18.5-24.9 is considered normal weight according to the WHO's classification.
- **Option B:** is incorrect because a BMI of 25-29.9 falls into the overweight category.
- **Option D:** is incorrect because a BMI of 30 or greater is classified as obese.

## **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 the NEET PG, USMLE, AIIMS, and FMGE exams, understanding the WHO's BMI categories and their implications for health is crucial. A memorable point is that a BMI less than 18.5 (**underweight**) and greater than 30 (**obese**) are both associated with increased risks of various health problems.

## **Correct Answer:** . Less than 18.5.