A nurse is caring for four clients. After administering morning medications, she realizes that the nifedipine prescribed for one client was inadvertently administered to another client. Which of the following actions should the nurse take first?

A. Notify the client's provider. – While the provider must be notified, the first priority is to assess the client for any adverse effects from the medication error. B. Administer the medication to the correct client. – This can be done later, but the immediate concern is the safety of the client who received the wrong medication. C. Fill out an occurrence form. – This is an important later step in reporting the medication error, but not the first action. D. Check the client's vital signs. – Nifedipine is a calcium channel blocker that can lower blood pressure and heart rate. The priority action is to assess the client’s condition, especially vital signs, to detect and respond to any adverse effects promptly.