**Core Concept**
Congenital adrenal hyperplasia (CAH) is a group of inherited disorders that affect the adrenal glands, leading to impaired production of cortisol and aldosterone. In CAH, the enzyme 21-hydroxylase is deficient, resulting in an accumulation of androgen precursors and a deficiency in glucocorticoids and mineralocorticoids.

**Why the Correct Answer is Right**
In a pregnancy with a history of CAH, the fetus is at risk of being affected by the same condition. Prenatal treatment with dexamethasone can suppress fetal adrenal steroidogenesis, reducing the risk of virilization in affected female fetuses. This treatment should be initiated early in pregnancy, ideally before 10 weeks of gestation, to be effective.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because it does not specify the correct timing or the appropriate steroid for prenatal treatment in CAH.
**Option B:** is incorrect because it does not address the specific concern of virilization in female fetuses with CAH.
**Option C:** is incorrect because it does not take into account the specific enzyme deficiency in CAH and the need for glucocorticoid treatment.

**Clinical Pearl / High-Yield Fact**
Prenatal treatment with dexamethasone in CAH is most effective when initiated before 10 weeks of gestation, as it can reduce the risk of virilization in affected female fetuses.

**Correct Answer:** D. Dexamethasone is the appropriate steroid for prenatal treatment in CAH, and it should be started early in pregnancy.