**Core Concept**
Very low birth weight (VLBW) is a term used to describe newborn babies who weigh less than 1,500 grams (approximately 3 pounds 5 ounces) at birth. This condition is a significant predictor of neonatal morbidity and mortality.

**Why the Correct Answer is Right**
VLBW is defined by the World Health Organization (WHO) as a birth weight of less than 1,500 grams. This weight threshold is used to identify newborns who are at increased risk of health complications, such as respiratory distress syndrome, hypothermia, and necrotizing enterocolitis. The exact weight threshold may vary slightly depending on the source, but 1,500 grams is the most commonly accepted definition.

**Why Each Wrong Option is Incorrect**
**Option A:** 500 grams - This is an extremely low birth weight, but it is not the definition of VLBW.
**Option B:** 2,000 grams - This is a low birth weight, but it is above the threshold for VLBW.
**Option C:** 1,000 grams - This is a very low birth weight, but it is not the exact definition used by WHO.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that VLBW is a critical determinant of neonatal outcomes, and newborns with this condition require close monitoring and specialized care to prevent complications.

**Correct Answer: D. 1,500 grams.**