Which of the following is an adverse effect associated with magnesium sulfate therapy in pregnancy?

• A. Flushing
• B. Depressed respirations
• C. Extreme muscle weakness
• D. Hyperactive deep tendon reflexes

Answer: (C)

Magnesium sulfate acts as a CNS depressant and neuromuscular blocker, so when levels rise, the most telling adverse effect is progressive weakness of the skeletal muscles. Extreme muscle weakness signals magnesium toxicity affecting the neuromuscular system and can progress to respiratory compromise if the diaphragm or other muscles are involved. That's why this option is the best choice: it directly reflects the drug’s neuromuscular toxicity. Flushing can occur but is usually mild and not the definitive toxicity sign, and depressed respirations are a serious toxicity sign but occur after weakness has already indicated toxicity; hyperactive reflexes would not be expected with magnesium therapy—reflexes tend to diminish. If toxicity is suspected, stop the infusion and have calcium gluconate available as an antidote.