Final answer:
The nurse should monitor the client for dehydration as the priority assessment.
Step-by-step explanation:
The nurse should monitor the client for dehydration as the priority assessment. Bacterial gastroenteritis can lead to excessive fluid loss through diarrhea, vomiting, and sweating, which can quickly lead to dehydration. It is important for the nurse to monitor the client's fluid intake and output, as well as vital signs such as blood pressure and heart rate, to ensure prompt intervention if dehydration occurs.