## Core Concept
The World Health Organization (WHO) defines low birth weight (LBW) based on the infant's weight at birth. This is a critical parameter for assessing newborn health and potential risks associated with low birth weight.

## Why the Correct Answer is Right
The correct definition of low birth weight, as per the WHO, is a birth weight of less than 2500 grams. This threshold is used globally to identify newborns who are at a higher risk of health problems. The classification of low birth weight is crucial for providing appropriate care and interventions to support the health and development of these infants.

## Why Each Wrong Option is Incorrect
* **Option A:** If the option is above 2500 grams, it does not align with the WHO's definition of low birth weight.
* **Option B:** Similarly, if this option is also above 2500 grams or not specifically defined as less than 2500 grams, it would be incorrect.
* **Option D:** If this option suggests a higher or lower threshold than 2500 grams, it would not be in line with the WHO's definition.

## Clinical Pearl / High-Yield Fact
A key point to remember is that low birth weight is a significant predictor of neonatal and infant mortality. Babies with low birth weight are at increased risk for health problems, including respiratory distress syndrome, infections, and developmental delays. The WHO's definition of less than 2500 grams for low birth weight is a universally accepted criterion used in clinical practice and research.

## Correct Answer: B. <2500 grams.