**Core Concept**
A very low birth weight (VLBW) baby is defined by a birth weight that falls below a certain threshold, which is critical for determining the infant's risk of complications and the need for specialized care. The World Health Organization (WHO) and various medical societies use this classification to guide clinical management.

**Why the Correct Answer is Right**
The correct answer is based on the definition of VLBW, which is typically considered to be less than 1500 grams (or 1.5 kg). This threshold is chosen because it corresponds to a gestational age of approximately 28 weeks, at which point the infant's lungs and other vital organs are not yet fully developed, increasing the risk of respiratory distress syndrome (RDS), intraventricular hemorrhage (IVH), and other complications. The lower the birth weight, the higher the risk of these conditions.

**Why Each Wrong Option is Incorrect**
* **Option B:** This option is incorrect because a birth weight of 2000 grams is still considered low, but not very low, and would not necessitate the same level of specialized care as a VLBW infant.
* **Option C:** This option is incorrect because a birth weight of 2500 grams is actually within the normal range for a term infant, and the baby would not be considered VLBW.
* **Option D:** This option is incorrect because there is no widely accepted definition of VLBW that corresponds to a birth weight of 3000 grams. In fact, this would be considered a high birth weight.

**Clinical Pearl / High-Yield Fact**
To remember the definition of VLBW, recall that 1500 grams corresponds to a gestational age of approximately 28 weeks, at which point the infant's lungs are not yet fully developed, increasing the risk of respiratory distress syndrome (RDS).

**Correct Answer:** A. 1.5 kg