**Core Concept**
The onset of male puberty is characterized by a series of physiological changes triggered by the surge of 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, ultimately stimulating the testes to produce testosterone.

**Why the Correct Answer is Right**
The earliest sign of male puberty is the enlargement of the testes, also known as testicular enlargement, which occurs due to the increased production of testosterone by the Leydig cells in the testes. This increase in testosterone levels triggers a cascade of events, including the growth of pubic hair, the development of axillary hair, and the deepening of the voice. The production of testosterone is mediated by the LH receptor on the Leydig cells, which responds to the increase in LH secretion from the anterior pituitary gland.

**Why Each Wrong Option is Incorrect**
**Option A:** Enlargement of the penis is not the earliest sign of male puberty, as it occurs later in the pubertal process.
**Option B:** The development of axillary hair is a secondary sign of male puberty, following the enlargement of the testes.
**Option C:** The deepening of the voice is a late sign of male puberty, occurring after the testicular enlargement and the growth of pubic hair.

**Clinical Pearl / High-Yield Fact**
Remember the sequence of pubertal changes in males: testicular enlargement, growth of pubic hair, development of axillary hair, and finally, deepening of the voice.

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