**Core Concept**
Bone growth in children is a complex process involving the coordinated action of multiple cell types, including osteoblasts, osteoclasts, and chondrocytes. The growth plates, composed of cartilage, are responsible for the longitudinal growth of bones.

**Why the Correct Answer is Right**
The increase in height during the first year of life is primarily due to the rapid growth of the growth plates, which are composed of cartilaginous tissue. This growth is fueled by the production of growth hormone, which stimulates the production of insulin-like growth factor-1 (IGF-1). IGF-1, in turn, promotes the proliferation and differentiation of chondrocytes in the growth plates, leading to the elongation of bones.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because growth in the first year of life is not primarily due to the growth of long bones, but rather the growth of the growth plates.

**Option B:** This option is incorrect because while growth hormone does play a role in growth during the first year of life, it is not the sole factor responsible for the increase in height.

**Option C:** This option is incorrect because while the growth plates are responsible for longitudinal growth, they are not the only factor contributing to growth in the first year of life.

**Clinical Pearl / High-Yield Fact**
The growth spurt during the first year of life is a critical period of development, and any disruptions to this process can have long-term consequences for growth and development.

**Correct Answer: D. Growth in the first year of life is primarily due to the growth of the growth plates, which are composed of cartilaginous tissue.**