**Core Concept**
The first sign of sexual maturity in boys is primarily driven by the surge in gonadotropin-releasing hormone (GnRH) secretion, which stimulates the pituitary gland to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). This hormonal cascade ultimately leads to the growth and maturation of the testes.

**Why the Correct Answer is Right**
The increase in testicular size is a direct result of the increased production of testosterone by the Leydig cells in the testes, which is triggered by the rise in LH levels. This increase in testosterone production is a hallmark of the onset of puberty in boys. As the testes mature, they produce more testosterone, leading to the development of secondary sexual characteristics.

**Why Each Wrong Option is Incorrect**
**Option A:** Increase in height is a sign of overall growth and development but is not the first sign of sexual maturity in boys.
**Option B:** Appearance of facial hair is a secondary sexual characteristic that develops later in puberty, after the increase in testicular size.
**Option C:** Change in voice is also a secondary sexual characteristic that develops later in puberty, as the vocal cords and larynx mature.

**Clinical Pearl / High-Yield Fact**
Remember that the onset of puberty in boys is typically marked by the increase in testicular size, which can be assessed by measuring the testicular volume. This can be an important clinical indicator of normal or abnormal pubertal development in boys.

✓ Correct Answer: D. Increase in testicular size