What is the most common cause of ambiguous genitalia in fetuses?

The most common cause of ambiguous genitalia in fetuses is Congenital Adrenal Hyperplasia (CAH). This condition is primarily associated with an enzyme deficiency, most commonly 21-hydroxylase deficiency, which leads to an accumulation of adrenal androgens and subsequently causes virilization of the external genitalia in genetically female fetuses.

In cases of CAH, the overproduction of these androgens can lead to the development of ambiguous genitalia due to the influence of testosterone, even in individuals with XX chromosomes, resulting in structures that may be atypical or not clearly defined as male or female. This makes CAH a prevalent reason for cases of ambiguous genitalia encountered in clinical practice.

In contrast, 17-Hydroxylase deficiency does cause sexual development abnormalities, but it typically results in individuals with XY chromosomes having female or ambiguous external genitalia, owing to a lack of androgen production. Other choices like Campomelic Dysplasia and Cystic Fibrosis are not primarily associated with the formation of ambiguous genitalia, as their clinical presentations are related to skeletal abnormalities and respiratory issues, respectively.