**Core Concept**
The first pubertal sign in males is a reflection of the onset of puberty, which is triggered by the surge in gonadotropin-releasing hormone (GnRH) from the hypothalamus. This leads to an increase in luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion from the anterior pituitary gland, stimulating the testes to produce testosterone.

**Why the Correct Answer is Right**
The first pubertal sign in males is usually the enlargement of the testes, which is a result of the increased production of testosterone by the Leydig cells in the testes. This increase in testosterone levels stimulates the growth and development of the male reproductive organs, including the penis and scrotum. The enlargement of the testes is a sensitive indicator of the onset of puberty in males.

**Why Each Wrong Option is Incorrect**
**Option A:** Enlargement of the penis is not typically the first pubertal sign in males, as it occurs later in puberty.
**Option B:** Voice deepening is a secondary pubertal change that occurs later in puberty, after the enlargement of the testes.
**Option C:** Acne can occur at any age and is not a specific indicator of the onset of puberty in males.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that the Tanner stages, which describe the physical development of adolescents, are a useful tool for assessing pubertal development in children and adolescents.

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