Final answer:
Calcium gluconate must be available when administering magnesium sulfate to treat severe preeclampsia, as it is the antidote for magnesium sulfate toxicity.
Step-by-step explanation:
The nurse should have calcium gluconate readily available for a client who is receiving magnesium sulfate to treat severe preeclampsia. Calcium gluconate is the antidote for magnesium sulfate toxicity, which can occur during treatment. It is important for healthcare providers to monitor patients closely for signs of magnesium toxicity and to have calcium gluconate accessible to counteract these effects if necessary.