## Core Concept
The head circumference is a critical anthropometric measure in infants, reflecting brain growth and development. At birth, the average head circumference is approximately 33-34 cm. During the first year of life, the brain grows rapidly, and this growth is reflected in an increase in head circumference.

## Why the Correct Answer is Right
The correct answer indicates that the increase in head circumference in the first year of life is about 50% of the head circumference at birth. Given that the average head circumference at birth is around 33-34 cm, a 50% increase would be approximately 16.5-17 cm. This increase is consistent with the expected growth of the infant's head during the first year, which typically results in a head circumference of about 46-47 cm by 12 months of age.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests a lower percentage increase. If the head circumference at birth is 33 cm, a 25% increase would only be 8.25 cm, which is less than expected for the first year of life.
- **Option B:** This option suggests a 75% increase, which would be 24.75 cm if the head circumference at birth is 33 cm. This is higher than the expected increase during the first year.
- **Option D:** This option suggests a 100% increase, which would double the head circumference at birth. This is far greater than the actual increase observed during the first year of life.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the head circumference of an infant is expected to increase by approximately 12-14 cm during the first year of life, and by about 2 cm during the second year. This rapid growth in early infancy reflects the critical period of brain development.

**Correct Answer:** .