**Core Concept**
Prenatal congenital adrenal hyperplasia (CAH) is a condition caused by a deficiency of the enzyme 21-hydroxylase, leading to an accumulation of 17-hydroxyprogesterone and a subsequent overproduction of androgens. This condition can result in virilization of female genitalia in affected fetuses.

**Why the Correct Answer is Right**
The treatment of choice for prenatal CAH involves administering dexamethasone to the pregnant mother. This therapy aims to suppress fetal adrenal androgen production by crossing the placenta and binding to the fetal glucocorticoid receptors, thereby reducing the production of androgens. This approach helps to prevent virilization of female genitalia and allows for a more favorable surgical outcome postnatally.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because dexamethasone is the preferred glucocorticoid for fetal therapy, whereas betamethasone is a different glucocorticoid with a shorter half-life and less potency.
**Option B:** This option is incorrect because hydrocortisone is a glucocorticoid used for replacement therapy in patients with CAH but is not the preferred choice for prenatal treatment.
**Option C:** This option is incorrect because fludrocortisone is a mineralocorticoid and not a glucocorticoid, and it is not used for prenatal treatment of CAH.
**Option D:** This option is incorrect because progesterone is not a glucocorticoid and is not used for prenatal treatment of CAH.

**Clinical Pearl / High-Yield Fact**
Dexamethasone therapy should be initiated as early as possible in pregnancy and continued until the fetus is no longer at risk of virilization. The dosage and duration of treatment may vary depending on the specific clinical scenario and the gestational age at initiation.

**Correct Answer:** D. Dexamethasone.