Final answer:
The nurse must immediately notify the attending physician if the serum magnesium level of the patient receiving magnesium sulfate for severe preeclampsia is at 5 mg/dl.
Step-by-step explanation:
The nurse must immediately notify the attending physician if the serum magnesium level of the patient receiving magnesium sulfate for severe preeclampsia is at 5 mg/dl. Monitoring the serum magnesium level is important because magnesium sulfate can cause toxicity at high levels, leading to serious complications such as respiratory depression, cardiac arrhythmias, and neuromuscular blockade. Therefore, an elevated serum magnesium level requires immediate attention and possible intervention from the physician.