**Core Concept**
Magnesium sulphate is a calcium channel blocker and vasodilator that can potentiate the effects of other antihypertensive agents by enhancing the relaxation of smooth muscle and reducing peripheral resistance.

**Why the Correct Answer is Right**
Magnesium sulphate potentiates the hypotensive action of nifedipine by inhibiting the influx of calcium ions into vascular smooth muscle cells. This leads to a decrease in contractility and a subsequent decrease in blood pressure. Nifedipine, a dihydropyridine calcium channel blocker, also acts by inhibiting calcium influx into vascular smooth muscle cells, causing vasodilation. The combination of magnesium sulphate and nifedipine results in a synergistic effect, leading to a greater reduction in blood pressure than either agent alone.

**Why Each Wrong Option is Incorrect**
**Option A:** Methyldopa is an alpha-2 adrenergic agonist that reduces sympathetic outflow from the central nervous system, leading to a decrease in blood pressure. However, it does not have a synergistic effect with magnesium sulphate.
**Option C:** Enalapril is an angiotensin-converting enzyme (ACE) inhibitor that blocks the conversion of angiotensin I to angiotensin II, leading to vasodilation and a decrease in blood pressure. While it may have a synergistic effect with other antihypertensive agents, it does not have a specific interaction with magnesium sulphate.
**Option D:** Hydralazine is a direct vasodilator that acts on the smooth muscle of blood vessels, leading to a decrease in blood pressure. However, it does not have a synergistic effect with magnesium sulphate.

**Clinical Pearl / High-Yield Fact**
Magnesium sulphate can potentiate the effects of other antihypertensive agents, particularly those that act on the calcium channels, such as nifedipine. This is an important consideration in the management of hypertension, particularly in patients who are already on multiple antihypertensive medications.

**✓ Correct Answer: B. Nifedipine**