**Core Concept**
The growth velocity in infants is influenced by various physiological changes, including the maturation of the hypothalamic-pituitary-adrenal (HPA) axis and the increase in growth hormone secretion. The first year of life is characterized by rapid growth and development, with an average weight gain of 700-1000 grams per month in the first six months.

**Why the Correct Answer is Right**
The growth velocity is maximum in the first year of life due to the high levels of growth hormone and insulin-like growth factor-1 (IGF-1) secreted by the pituitary gland. This period is also marked by a significant increase in muscle mass and bone density, which contributes to the rapid growth and development of the infant. Additionally, the high caloric intake and nutrient availability during this period support the growth and development of the infant. The growth velocity in the first year of life is approximately 25-30% of the total growth that occurs from birth to adulthood.

**Why Each Wrong Option is Incorrect**
**Option B:** The second year of life is a period of continued growth and development, but the growth velocity is slower compared to the first year. The infant's growth rate begins to slow down as they start to become more active and their energy expenditure increases.
**Option C:** The seventh year of life is a period of steady growth, but it is not characterized by a high growth velocity. Children typically experience a growth spurt during puberty, but this occurs much later in childhood.
**Option D:** Adolescence is a period of rapid growth and development, but it is not the time of maximum growth velocity in infancy. The growth spurt during puberty is a result of the increase in sex hormones, particularly testosterone and estrogen, which stimulate the growth plates in the bones.

**Clinical Pearl / High-Yield Fact**
Infants grow rapidly in the first year of life, with an average weight gain of 700-1000 grams per month in the first six months. This period is characterized by a high growth velocity, and it is essential for pediatricians to monitor the growth and development of infants to identify any potential growth or developmental delays.

**✓ Correct Answer: A. In the first year of life**