**Core Concept**
Magnesium sulfate (MgSO4) is a commonly used medication in the management of pre-eclampsia and eclampsia due to its ability to act as a cerebral vasodilator and anticonvulsant. Its mechanism of action involves the inhibition of neuronal calcium channels, leading to a decrease in excitatory neurotransmitter release and ultimately, a reduction in seizure activity.

**Why the Correct Answer is Right**
Pulmonary edema is a well-documented side effect of magnesium sulfate therapy, particularly when used in high doses or in patients with pre-existing cardiac or renal dysfunction. This occurs due to the vasodilatory effects of magnesium, which can lead to a decrease in systemic vascular resistance and an increase in pulmonary capillary hydrostatic pressure, resulting in fluid accumulation in the lungs. The risk of pulmonary edema is further increased in patients with underlying heart or kidney disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypotension is a potential side effect of magnesium sulfate, but it is not the most severe or commonly associated side effect. Hypotension typically occurs due to vasodilation, but it is often manageable with dose adjustments or supportive care.

**Option B:** Polyuria is not a direct side effect of magnesium sulfate. While magnesium can affect renal function, it is not typically associated with increased urine production.

**Option C:** Coma is not a direct side effect of magnesium sulfate. While magnesium toxicity can lead to central nervous system depression, coma is a rare and severe manifestation that occurs at very high serum levels.

**Clinical Pearl / High-Yield Fact**
It is essential to monitor patients receiving magnesium sulfate closely for signs of pulmonary edema, including respiratory distress, cough, or decreased oxygen saturation. Early recognition and treatment of pulmonary edema can significantly improve patient outcomes.

**Correct Answer:**
✓ Correct Answer: D. Pulmonary edema