**Core Concept**
Sex differentiation in humans is the process by which the indifferent gonad develops into either ovaries or testes during embryonic development. This process is essential for the development of secondary sexual characteristics and reproductive organs.

**Why the Correct Answer is Right**
The best marker for sex differentiation is the presence of SRY (Sex-Determining Region Y) gene on the Y chromosome. The SRY gene encodes a transcription factor that initiates the development of testes from the indifferent gonad. The expression of SRY gene triggers a cascade of molecular events that ultimately lead to the development of testes and the secretion of anti-Müllerian hormone (AMH) and Müllerian inhibiting substance (MIS), which suppresses the development of female reproductive organs. The presence of SRY gene is a critical determinant of male sex differentiation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Kallmann syndrome is a genetic disorder characterized by delayed or absent puberty and anosmia (loss of smell). It is not related to sex differentiation.
* **Option B:** Androgen insensitivity syndrome is a condition where individuals have a mutation in the androgen receptor gene, leading to resistance to androgens (male hormones). This condition does not affect sex differentiation.
* **Option C:** Congenital adrenal hyperplasia (CAH) is a group of genetic disorders that affect the adrenal glands, leading to an overproduction of androgens. While CAH can affect sex differentiation, it is not the best marker for sex differentiation.

**Clinical Pearl / High-Yield Fact**
It's essential to note that sex differentiation is a complex process that involves multiple genetic and hormonal factors. A thorough understanding of the molecular mechanisms underlying sex differentiation is crucial for diagnosing and managing disorders related to sex development.

**Correct Answer:** C. SRY gene.