What hormone is deficient due to 21-Hydroxylase deficiency in Congenital Adrenal Hyperplasia?

In the context of Congenital Adrenal Hyperplasia (CAH) due to 21-Hydroxylase deficiency, the hormone that is primarily deficient is cortisol. The enzyme 21-hydroxylase is crucial in the adrenal cortex for the production of cortisol and aldosterone from progesterone and 17-hydroxyprogesterone. When this enzyme is deficient, the conversion of pregnenolone to progesterone and then through several steps to cortisol is disrupted.

As a result, individuals with this condition often present with low levels of cortisol, which can lead to an increase in adrenocorticotropic hormone (ACTH) due to the body's feedback mechanism. This causes adrenal hyperplasia and an overproduction of androgens, which can lead to symptoms such as virilization.

While aldosterone levels are also often affected in severe cases, it is the deficiency in cortisol that is the hallmark of 21-Hydroxylase deficiency. The body's inability to produce adequate cortisol and aldosterone highlights the critical hormonal imbalances in CAH. The resultant lack of aldosterone can lead to salt-wasting crises, but it is primarily cortisol that is deficient due to the enzymatic block.

Thus, the correct answer focuses on