Final answer:
The nurse should first document the results in the client's chart.
Step-by-step explanation:
The nurse should first document the results in the client's chart. Decreased deep tendon reflexes can be a potential side effect of magnesium sulfate IV administration. It is important for the nurse to document this finding to monitor the client's response to the medication and communicate it to the healthcare team.
The other actions listed - placing the call light within reach of the client, dimming the lights, and discontinuing the IV infusion - may be appropriate interventions depending on the overall assessment of the client's condition and the healthcare provider's orders. However, the priority in this situation is to document the finding.