Which condition is most likely to cause sex reversal in XY fetuses?

The condition most likely to cause sex reversal in XY fetuses is an abnormality in the SOX9 gene. The SOX9 gene plays a critical role in the development of testes and male sexual differentiation. It is essential for the formation of Sertoli cells, which are pivotal for testicular development. Mutations or abnormalities in the SOX9 gene can disrupt normal testicular development, potentially leading to XY individuals developing as phenotypically female or exhibiting ambiguous genitalia. This is because the presence or functionality of SOX9 is crucial for establishing male characteristics during fetal development.

In contrast, the other options do not primarily contribute to sex reversal in XY fetuses. Absent scapula, for instance, relates more to skeletal abnormalities and does not influence gonadal differentiation. 21-Hydroxylase deficiency is a form of congenital adrenal hyperplasia and may lead to androgen excess, but it typically occurs in 46,XX individuals rather than causing sex reversal in XY individuals directly. Increased levels of DHEA, a precursor to sex steroids, may alter the hormonal environment, but this would not directly impact the genetic regulation of sex determination in XY fetuses affected by an SOX9 gene issue.