Final answer:
Hyporeflexia is the indication of magnesium toxicity when a client is receiving magnesium sulfate for preterm labor. This condition occurs when serum magnesium levels are too high.
Step-by-step explanation:
The nurse should identify hyporeflexia as an indication of magnesium toxicity. Magnesium sulfate is used to treat preterm labor but can cause toxicity if levels become too high. The symptoms of magnesium toxicity include hyporeflexia (diminished reflexes), which occurs as the serum magnesium level exceeds recommended levels. Other findings associated with magnesium toxicity include respiratory depression (respirations less than 12/min), urinary output less than 30 mL/hr indicating possible renal failure, and a decrease in blood pressure. Tachycardia is not typically a sign of magnesium toxicity; bradycardia is more common. Therefore, option B, Hyporeflexia, is the correct answer.