**Core Concept**
Extremely low birth weight (ELBW) is a critical condition in neonatology, defined as a birth weight below a specific threshold. The World Health Organization and various pediatric organizations use this classification to identify infants at high risk for morbidity and mortality.

**Why the Correct Answer is Right**
Infants with a birth weight of less than 1000 grams (1 kg) are considered ELBW, which is associated with increased risks of respiratory distress syndrome, intraventricular hemorrhage, and necrotizing enterocolitis. This weight category is critical because it indicates a significant compromise in fetal growth, often due to preterm birth or placental insufficiency. The lower the birth weight, the higher the risk of complications and mortality.

**Why Each Wrong Option is Incorrect**
**Option B:** Infants with a birth weight of less than 1500 grams are considered very low birth weight (VLBW), which is still a high-risk category but not as severe as ELBW.
**Option C:** A birth weight of less than 2500 grams is the standard cutoff for low birth weight (LBW), which is a broader category that includes VLBW infants.
**Option D:** A birth weight of less than 2800 grams is not a commonly used classification in pediatrics.

**Clinical Pearl / High-Yield Fact**
Remember that ELBW infants are at an increased risk of long-term neurodevelopmental disabilities, including cerebral palsy, developmental delay, and cognitive impairments.

**✓ Correct Answer: A. <1000**