Final answer:
The nurse should first monitor the client's blood pressure to assess the immediate impact of the medication error, before notifying the healthcare provider and documenting the incident.
Step-by-step explanation:
The nurse should prioritize patient safety and assessment in the event of a medication error. The first action the nurse should take is b) Monitor the client's blood pressure. Atenolol is a beta-blocker used for hypertension and could lower the patient's blood pressure, especially if not indicated for their condition. Monitoring the client's blood pressure provides immediate data on potential adverse effects the wrong medication might have on the client. After stabilizing and assessing the client, subsequent actions should include a) Notifying the healthcare provider immediately about the error, as well as d) Documenting the error in the medication administration record, and c) Assessing the client's heart rate and rhythm as a beta-blocker could also affect these. Documentation is essential for transparency and to improve the system to prevent future errors, but it comes after ensuring the client's immediate safety is addressed.