**Core Concept**
The development of masculine or feminine external genitalia in the fetus depends on the presence and activity of specific enzymes and hormones, particularly androgen hormones such as testosterone and dihydrotestosterone (DHT). This process is regulated by the sex chromosomes and involves the expression of specific genes.

**Why the Correct Answer is Right**
The presence of the Y chromosome, particularly the SRY gene, triggers the production of testes, which then produce androgens such as testosterone. The enzyme 5α-reductase converts testosterone into DHT, which is essential for the development of male external genitalia. Without sufficient androgen activity, female external genitalia develop. The Wolffian ducts, which are present in both males and females, are also influenced by androgens, leading to the development of male external genitalia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the relevant biological mechanisms involved in the development of masculine or feminine features.

**Option B:** This option is incorrect because the presence of a Y chromosome is necessary but not sufficient for the development of male external genitalia; the activity of androgens, particularly DHT, is also crucial.

**Option C:** This option is incorrect because the development of masculine or feminine features is not directly related to the presence or absence of a specific gene or protein outside of the sex chromosomes and androgen hormones.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the development of external genitalia is a complex process that involves the interplay of multiple hormones and enzymes, and abnormalities in this process can lead to disorders of sex development (DSD).

**Correct Answer: D. The presence and activity of androgen hormones, particularly dihydrotestosterone (DHT), most directly depend on the presence of a Y chromosome and the SRY gene.**