**Core Concept**
Very Low Birth Weight (VLBW) is a term used to describe newborn babies who are born weighing less than a specific threshold. This concept is crucial in pediatric medicine as it influences the baby's health outcomes, growth, and development.

**Why the Correct Answer is Right**
The World Health Organization (WHO) defines Very Low Birth Weight as a baby born weighing less than 1500 grams. This definition is based on the baby's weight at birth, which is a critical factor in determining their overall health and development. Babies born with low birth weight are at a higher risk of various health complications, including respiratory distress, infections, and developmental delays.

**Why Each Wrong Option is Incorrect**
* **Option A:** 1000 grams - This is actually the threshold for Extremely Low Birth Weight (ELBW), which is a more severe category of low birth weight.
* **Option B:** 2000 grams - This is not a recognized threshold for VLBW, and babies born at this weight are generally considered to be of normal weight.
* **Option C:** 2500 grams - This is actually the threshold for a normal birth weight, and babies born at this weight or above are generally considered to be of normal weight.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the definition of VLBW can vary slightly depending on the source, but 1500 grams is the most commonly accepted threshold. This knowledge is crucial for healthcare providers to accurately identify and manage babies with VLBW.

**Correct Answer: D. 1500 grams**