## Core Concept
The classification of birth weight in newborns is crucial for assessing their health and development. Birth weight is categorized into several groups, including low birth weight (LBW), very low birth weight (VLBW), and extremely low birth weight (ELBW). These categories help in identifying babies who are at higher risk and may require special care.

## Why the Correct Answer is Right
Extremely low birth weight (ELBW) infants are those who weigh less than 1000 grams at birth. This classification is critical because ELBW infants are at a significantly higher risk for complications, including respiratory distress syndrome, intraventricular hemorrhage, and long-term developmental delays. The correct answer, therefore, reflects the threshold used to define this high-risk group.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests a higher threshold than 1000 grams, which does not accurately reflect the definition of ELBW.
- **Option B:** Similarly, this option also suggests a higher threshold than 1000 grams, making it incorrect for the definition of ELBW.
- **Option D:** This option suggests a lower threshold than 1000 grams, which, although indicating a very low birth weight, does not correctly represent the definition of ELBW.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the risk of complications and the need for intensive care increase as birth weight decreases. ELBW infants often require care in a neonatal intensive care unit (NICU) and may have long-term health and developmental issues. Remember, the threshold for ELBW is **less than 1000 grams**.

## Correct Answer: C. 1000 gm.