**Core Concept**
Precocious puberty refers to the early onset of pubertal changes, typically before the age of 8 in girls and 9 in boys. This condition is characterized by the premature activation of the hypothalamic-pituitary-gonadal axis, leading to the development of secondary sexual characteristics.

**Why the Correct Answer is Right**
The correct answer, A. Sexual maturity is attained early, is accurate because precocious puberty involves the premature activation of the hypothalamic-pituitary-gonadal axis, leading to the early development of secondary sexual characteristics. This is due to the premature secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus, which stimulates the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary gland. These hormones, in turn, stimulate the gonads to produce sex hormones, such as estrogen and testosterone, leading to the development of secondary sexual characteristics.

**Why Each Wrong Option is Incorrect**
**Option B:** Mental function is increased - This statement is incorrect because precocious puberty is not associated with an increase in mental function. While the premature development of secondary sexual characteristics may lead to social and emotional changes, it does not affect cognitive function.

**Option C:** No reproductive function - This statement is incorrect because children with precocious puberty do develop reproductive function. In fact, early sexual maturity can lead to early menarche and increased risk of pregnancy.

**Option D:** Body proportions are enlarged - This statement is incorrect because precocious puberty does not typically involve an increase in body proportions. While children with precocious puberty may develop secondary sexual characteristics, their body proportions remain normal.

**Clinical Pearl / High-Yield Fact**
It is essential to note that precocious puberty can be a sign of underlying pathology, such as a tumor or genetic disorder. Therefore, a thorough evaluation, including imaging studies and hormonal assays, is necessary to determine the underlying cause and guide appropriate management.

**✓ Correct Answer: A. Sexual maturity is attained early**