Final answer:
The finding that requires immediate follow-up in the scenario presented is the adolescent's high casual blood glucose level of 250 mg/dL, which is a concern for a patient with Type 1 diabetes. Other findings such as high white blood cell count and the physical signs of infection also warrant attention.
Step-by-step explanation:
The adolescent's medical record in the emergency department shows several findings that require attention, but the finding that requires immediate follow-up by the nurse is the casual blood glucose level of 250 mg/dL. This value is significantly elevated, indicating poor glucose control, which can be particularly dangerous for an individual with Type 1 diabetes mellitus. As high blood glucose can lead to diabetic ketoacidosis, a potentially life-threatening condition, it's essential to address this abnormality promptly. Additionally, fever, redness, and pustules around the injury site also point to the possibility of infection, which can exacerbate diabetes-related complications.