Final answer:
The nurse should first check the patient's most recent potassium level to gather information about the possible cause of the prolonged QRS interval.
Step-by-step explanation:
The nurse should first check the patient's most recent potassium level. Abnormalities in the QRS interval on the electrocardiogram (ECG) can indicate an issue with the electrical conduction of the heart. Potassium imbalances, such as hyperkalemia, can affect the QRS interval and lead to changes in the ECG. By checking the patient's potassium level, the nurse can gather important information about the possible cause of the prolonged QRS interval.