**Core Concept**
The development of sexual maturity in boys is a complex process involving the activation of the hypothalamic-pituitary-gonadal axis. This process is triggered by the pulsatile release of gonadotropin-releasing hormone (GnRH) from the hypothalamus, which stimulates the anterior pituitary gland to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The increase in LH levels triggers the Leydig cells in the testes to produce testosterone, leading to the development of secondary sexual characteristics.

**Why the Correct Answer is Right**
The first sign of sexual maturity in boys is the enlargement of the testes, which occurs due to the increase in testosterone levels. This is a result of the activation of the hypothalamic-pituitary-gonadal axis, where GnRH stimulates the release of LH from the anterior pituitary gland. LH then acts on the Leydig cells in the testes to increase testosterone production. The increase in testosterone levels leads to the growth and development of the testes, which is the first visible sign of sexual maturity in boys.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the enlargement of the testes is the first visible sign of sexual maturity in boys, not the development of pubic hair or the growth of the penis.
**Option B:** This option is incorrect because the growth of the penis occurs later in the process of sexual maturity, after the enlargement of the testes.
**Option C:** This option is incorrect because the development of pubic hair also occurs later in the process of sexual maturity, after the enlargement of the testes.
**Option D:** This option is incorrect because the development of facial hair is a sign of puberty in boys, but it is not the first sign of sexual maturity.

**Clinical Pearl / High-Yield Fact**
The Tanner stages of puberty provide a widely used classification system for the development of secondary sexual characteristics in boys. The Tanner stages range from Stage 1 (prepubertal) to Stage 5 (adult), and the enlargement of the testes is typically the first sign of sexual maturity in boys, corresponding to Tanner Stage 2.

**Correct Answer:** A. Enlargement of the testes