Final answer:
When encountering resistance in the ankle during passive ROM, the nurse should first assess the ankle for swelling to determine if further movement could cause harm to the patient.
Step-by-step explanation:
When a nurse encounters resistance in the ankle while providing passive Range of Motion (ROM) to a patient's left lower extremity, the first thing they should do is assess the ankle for swelling. Swelling could indicate an injury or underlying issue that needs attention before continuing with the ROM activity. By assessing the ankle, the nurse can determine if further movement could cause harm to the patient.