Final answer:
The priority nursing diagnosis for a patient with ulcerative colitis is Fluid volume deficit due to frequent diarrhea and the potential for dehydration.
Step-by-step explanation:
In planning care for the patient with ulcerative colitis, the nurse would identify Fluid volume deficit as a priority nursing diagnosis. Ulcerative colitis is characterized by frequent diarrhea, which can lead to dehydration and fluid imbalance. Assessing the patient's fluid status and implementing measures to maintain or restore fluid balance would be a crucial aspect of their care.