Final answer:
In caring for a patient who has undergone surgery to create an ileal conduit, the nurse must measure the circumference of the stoma to determine the appropriate size for the appliance.
Step-by-step explanation:
To determine the size of the appliance needed for a patient with an ileal conduit, the nurse should measure the circumference of the stoma (Option A). This is important to ensure a proper fit for the appliance, minimizing the risk of irritation or leakage. When caring for a patient with an ileostomy, accuracy in sizing the appliance is critical for the patient's comfort and to prevent complications.