In boys, what is an early sign of precocious puberty?

In boys, an early sign of precocious puberty is testicular enlargement. This occurs when there is a premature activation of the hypothalamic-pituitary-gonadal axis, leading to earlier than normal development of secondary sexual characteristics. Testicular enlargement signifies the onset of sexual maturation, as it indicates that the testes are beginning to produce testosterone and other hormones associated with puberty.

The other options represent characteristics or signs usually associated with later stages of puberty or are not applicable to boys. For example, voice deepening and facial hair growth are signs that typically occur later in puberty as testosterone levels rise and secondary sexual characteristics develop. Menstrual cramps are specific to females and do not relate to male puberty at all. Thus, testicular enlargement is the most reliable and early indicator of precocious puberty in boys.