**Core Concept**
Hydralazine is a vasodilator used to treat hypertension in pregnancy, particularly in cases of severe preeclampsia or eclampsia. It is one of the few antihypertensive medications that are considered safe for use during pregnancy.

**Why the Correct Answer is Right**
Hydralazine works by directly relaxing the smooth muscle of the arterioles, leading to vasodilation and a subsequent decrease in blood pressure. This effect is particularly beneficial in pregnant women with preeclampsia, as it can help alleviate symptoms such as headache, visual disturbances, and proteinuria. Hydralazine is also a good choice because it has a rapid onset of action and can be administered intravenously, which is often necessary in emergency situations.

**Why Each Wrong Option is Incorrect**
**Option A:** Sodium nitroprusside is a potent vasodilator, but it is contraindicated in pregnancy due to its potential to cause cyanide toxicity, which can be particularly harmful to the fetus.
**Option C:** ACE inhibitors, such as lisinopril or enalapril, are contraindicated in pregnancy due to the risk of fetal renal damage and other complications.
**Option D:** Indapamide is a thiazide-like diuretic that is not typically used to treat hypertension in pregnancy due to its potential to cause fetal growth restriction and other adverse effects.

**Clinical Pearl / High-Yield Fact**
When managing hypertension in pregnancy, it's essential to choose medications that have a proven safety record for both the mother and the fetus. In general, vasodilators like hydralazine are preferred over diuretics or ACE inhibitors, which can pose a risk to fetal development.

✓ Correct Answer: B. Hydralazine