## **Core Concept**
The question pertains to the growth and development of infants, specifically focusing on the doubling of birth length. This is a key milestone in pediatric growth assessment.

## **Why the Correct Answer is Right**
The birth length of a child typically doubles by the age of 4 years. This is a general growth milestone used in pediatrics to assess the overall growth and development of a child. At birth, the average length of a baby is about 50 cm, and it roughly doubles to around 100 cm by the age of 4.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 2 years - This is too early for the birth length to double. By 2 years, a child's height is usually around 86-88 cm, which is less than double the average birth length.
- **Option B:** 6 years - This is later than necessary. While children continue to grow, their birth length typically doubles by 4 years, not 6.
- **Option D:** 8 years - This is significantly later than when the birth length doubles. By 8 years, children have grown considerably more than double their birth length.

## **Clinical Pearl / High-Yield Fact**
A useful clinical pearl is that a child's height at 2 years of age is expected to be approximately 85-90 cm, and it roughly equals the child's birth length in centimeters plus 25 cm (or more simply, to remember that a child's length at birth doubles at about 4 years of age). This helps in assessing growth patterns.

## **Correct Answer:** C. 4 years.