Final answer:
The nurse should secure the call bell within easy reach before leaving the patient's room after a complete bed bath.
Step-by-step explanation:
The most important thing for the nurse to do before leaving the patient's room after finishing a complete bed bath is to secure the call bell within easy reach. This ensures that the patient can easily call for assistance if needed. Pulling all the side rails up on both sides of the patient's bed is also important for safety, but securing the call bell is the most crucial task.