**Final Answer:**
The external genitalia's development is determined by the presence or absence of the hormone dihydrotestosterone (DHT).
**Explanation:**
The differentiation of external genitalia is regulated by hormones during fetal development. The precursor for male external genitalia, in the absence of DHT, develops into the female genitalia. DHT, derived from testosterone through the action of the enzyme 5-alpha reductase, is crucial in masculinizing the external genitalia. In the absence of this hormone, the default pathway leads to the development of female genitalia.
The process begins with the presence of testosterone secreted by the fetal testes. Testosterone gets converted into DHT by the enzyme 5-alpha reductase. DHT binds to androgen receptors, triggering a signaling cascade that promotes the development of male external genitalia. If this hormone is absent due to a deficiency in the enzyme or receptors, the default pathway prevails, resulting in the formation of female external genitalia.
This hormonal influence demonstrates the critical role of DHT in the determination of male external genitalia. Understanding the interplay between hormones and their receptors sheds light on the developmental processes underlying the formation of external genitalia and emphasizes the significance of hormonal balance in sexual differentiation during fetal development.