Birth length doubles by what age?
**Core Concept**
The question tests understanding of human growth and development, specifically the **physiological** changes that occur from birth through childhood. **Growth charts** are essential tools in pediatrics to monitor these changes.
**Why the Correct Answer is Right**
The correct answer is based on the principle that a child's birth length roughly doubles by the age of 4 years. This **growth pattern** is a result of the interplay between **hormones** such as growth hormone and thyroid hormone, **nutrition**, and **genetics**.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it suggests a doubling too early in life.
**Option B:** This option is also incorrect as it suggests the doubling happens later than it actually does.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that **growth velocity** is highest during the first year of life, but the doubling of birth length by age 4 is a crucial milestone.
**Correct Answer:** C. 4 years.