The first sign of sexual development in males is typically the:

Rationale: A. The appearance of downy pubic hair occurs after the initial signs of puberty and typically represents Tanner stage 2–3 development. B. Enlargement and lengthening of the penis usually occur after testicular enlargement, often a few months later, as puberty progresses. C. Axillary and facial hair develop in later stages of puberty and are not initial indicators of sexual maturation in males. D. Enlargement of the testicles and scrotum is the first and most reliable sign of puberty in males, generally occurring between ages 9 and 14. Testicular growth reflects activation of the hypothalamic-pituitary-gonadal axis, leading to increased testosterone production and the subsequent development of secondary sexual characteristics. Monitoring testicular size is therefore a key component in assessing the onset and progression of male puberty.