**Question:** BMI of underweight individual: March 2008

**Core Concept:** Body Mass Index (BMI) is a measure of body fat based on height and weight that applies to adult humans, defined as weight in kilograms divided by the square of height in meters (kg/m^2). A person's BMI is determined by their weight and height, which are both essential components of the calculation.

**Why the Correct Answer is Right:** In this question, we are dealing with the BMI of an underweight individual. The correct answer should be lower than the normal BMI range (18.5 to 24.9). Underweight is defined as a BMI below 18.5.

**Why Each Wrong Option is Incorrect:**

A. **18.5 - 24.9:** This option represents the normal BMI range, which is incorrect for an underweight individual.
B. **25 - 29.9:** This range represents the overweight category, which is also incorrect for an underweight individual.
C. **30 - 34.9:** This range corresponds to the obese category, again incorrect for an underweight individual.
D. **35 or more:** This range represents the extremely obese category, which is highly unlikely for an underweight individual.

**Why the Correct Answer is Right:** The correct answer, **18.5**, falls within the underweight category according to the World Health Organization classification system. An underweight individual has a BMI below 18.5, indicating lower than average body fat percentage and lean muscle mass.

**Core Concept:** Underweight is a clinical term used to describe a person's weight status when their BMI falls below 18.5. A lower BMI indicates lower body fat percentage and lean muscle mass.

**Why Each Wrong Option is Incorrect:**

A. A BMI between 18.5 and 24.9 indicates normal weight, which is not applicable for an underweight individual.
B. BMI ranges of 25 to 29.9 represent overweight, another incorrect classification for an underweight person.
C. BMI ranges of 30 to 34.9 denote obesity, which is also not suitable for an underweight individual.
D. BMI ranges of 35 or more represent extreme obesity, irrelevant for an underweight person.

**Clinical Pearls:** Understanding BMI ranges is crucial for accurately diagnosing and treating patients' weight status. These ranges help healthcare professionals make informed decisions regarding patient care, nutrition guidance, and potential interventions to improve health outcomes.