**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 pregnant females suspected of having a fetus with CAH, the goal is to prevent virilization of the female fetus by inhibiting the conversion of 17-hydroxyprogesterone to androgens.

**Why the Correct Answer is Right**
The drug of choice is dexamethasone, a synthetic corticosteroid that crosses the placenta and suppresses fetal adrenal androgen production, thereby preventing virilization of the female fetus. This is a crucial intervention to preserve the female phenotype and prevent ambiguous genitalia. The mechanism of action involves inhibition of 17α-hydroxylase and 21-hydroxylase enzymes, which are deficient in CAH.

**Why Each Wrong Option is Incorrect**
**Option A:** Betamethasone is a corticosteroid used for fetal lung maturation in preterm labor, but it is not used for CAH prevention.
**Option B:** Hydrocortisone is a glucocorticoid used for replacement therapy in adrenal insufficiency, but it is not effective in crossing the placenta to suppress fetal adrenal androgen production.
**Option C:** Prednisolone is a corticosteroid with minimal placental transfer, making it ineffective for CAH prevention.

**Clinical Pearl / High-Yield Fact**
Dexamethasone therapy should be initiated early in pregnancy, ideally before 10 weeks of gestation, to prevent virilization of the female fetus. Regular monitoring of fetal growth and maternal side effects is essential.

**Correct Answer:** C. Dexamethasone.