A nurse is planning to administer a controlled substance to a client who is experiencing pain. Which of the following actions should the nurse plan to take first?

The correct answer is B: Identify the client using two identifiers. This is the first step the nurse should take to ensure the medication is administered to the correct patient, following the "five rights" of medication administration. By confirming the client's identity using two identifiers (such as name and date of birth), the nurse minimizes the risk of medication errors. Documenting the administration (choice A) should come after the medication is given. Removing the medication from the cabinet (choice C) and comparing inventory records (choice D) are important but should not be the first step in ensuring safe medication administration.