What action should the nurse take first when a client has a serum potassium level of 5.9 mEq/L before administering losartan?

When a serum potassium level is 5.9 mEq/L, this indicates hyperkalemia, which is a condition that can lead to serious cardiac complications. Losartan is an angiotensin II receptor blocker that can increase potassium levels. Therefore, it is essential for the nurse to prioritize the safety of the client before proceeding with medication administration.

Withholding the scheduled dose of losartan is the most appropriate first action in this scenario, as administering the medication could further elevate the potassium level and increase the risk of adverse effects, especially cardiac issues. This proactive measure aims to prevent potential complications associated with high potassium levels.

The other actions, while potentially relevant, do not address the most immediate concern. Checking the client's apical pulse might help monitor for any cardiac effects of hyperkalemia but does not directly address the risk involved in administering losartan. Notifying the healthcare provider and repeating the serum potassium level are also crucial steps, but they should occur after ensuring the patient is not put at further risk by administering a medication that could exacerbate the situation. Thus, the correct action is to withhold the medication until further evaluation and intervention can be made regarding the elevated potassium level.