Final answer:
Calcium gluconate is the medication used to treat magnesium toxicity, as it acts as the antidote by counteracting the effects of excess magnesium.
Step-by-step explanation:
If magnesium toxicity is suspected, the medication that a nurse should prepare to administer is calcium gluconate. Calcium gluconate is the antidote to magnesium toxicity and works by counteracting the effects of excess magnesium in the body. It can quickly restore cardiac and neuromuscular function impacted by high levels of magnesium. Unlike potassium chloride, which is used to treat hypokalemia, calcium gluconate specifically addresses the issues of magnesium overload. Administration of magnesium sulfate would be inappropriate as it would exacerbate the toxicity.