**Core Concept**
The concept being tested is the growth pattern of children, specifically the relationship between birth weight and weight at a later age. This is a fundamental aspect of pediatric growth and development.

**Why the Correct Answer is Right**
At 3 years of age, a child's weight is approximately 2 times their birth weight. This is because children typically experience a rapid growth phase during the first few years of life, with their weight increasing exponentially. This growth pattern is influenced by various factors, including genetics, nutrition, and overall health. The doubling of birth weight at 3 years is a commonly used benchmark in pediatric medicine to assess growth and development.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the doubling of birth weight typically occurs at 6 months, not 3 years.
* **Option B:** This option is incorrect because the tripling of birth weight is more characteristic of growth at 12 months, not 3 years.
* **Option C:** This option is incorrect because the quadrupling of birth weight is not a standard benchmark for growth at 3 years of age.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that growth patterns can vary significantly among children, and certain conditions, such as growth hormone deficiency or malnutrition, can affect growth rates. Therefore, it's crucial to plot growth charts regularly to monitor a child's growth and identify any potential issues.

**Correct Answer:** D. 2 times the birth weight.