Which one of the following drugs is used for fetal therapy of congenital adrenal hyperplasia?
**Core Concept**
Congenital adrenal hyperplasia (CAH) is a group of inherited disorders that affect the adrenal glands, leading to impaired cortisol production and an overproduction of androgens. Fetal therapy involves administering medications to the mother to alleviate symptoms or prevent complications in the fetus.
**Why the Correct Answer is Right**
Dexamethasone is a synthetic glucocorticoid that can cross the placenta and suppress fetal adrenal androgen production, thereby reducing virilization of female fetuses with CAH. This is essential for preventing ambiguous genitalia in affected females. The exact mechanism involves the inhibition of 21-hydroxylase, the enzyme responsible for cortisol production, thereby reducing the substrate available for androgen production in the fetal adrenal glands.
**Why Each Wrong Option is Incorrect**
**Option A:** Hydrocortisone is a glucocorticoid used to replace cortisol in patients with CAH, but it is not used for fetal therapy due to its inability to cross the placenta effectively.
**Option B:** Fludrocortisone is a mineralocorticoid used to replace aldosterone in patients with CAH, but it is not used for fetal therapy as it does not address the issue of fetal androgen production.
**Option C:** Prednisolone is a glucocorticoid that is not effective in crossing the placenta, making it unsuitable for fetal therapy in CAH.
**Clinical Pearl / High-Yield Fact**
Fetal therapy with dexamethasone in CAH requires precise timing and dosing to minimize the risk of maternal side effects while maximizing the benefits to the fetus.
**Correct Answer:** C. Dexamethasone.