What would be the most appropriate next step in managing a 9-year-old girl with abdominal pain, vomiting, and signs of potential diabetes?

Administering a fingerstick blood glucose test is the most appropriate next step in this scenario because the patient is presenting with abdominal pain, vomiting, and signs suggestive of potential diabetes, which could indicate a hyperglycemic state such as diabetic ketoacidosis (DKA).

In cases where there are symptoms indicative of diabetes—such as unexplained abdominal pain and vomiting—it's critical to determine the patient's blood glucose level promptly. This helps in identifying whether the symptoms are indeed related to hyperglycemia or another issue. If the blood glucose level is significantly elevated, it confirms the suspicion of diabetes and necessitates urgent management to prevent complications, especially if DKA is present.

Taking immediate action to assess glucose levels is also key because the management plan will heavily depend on those results. If glucose levels are high, interventions like IV fluids to address dehydration and insulin administration would be necessary next steps. Thus, confirming the blood sugar level is foundational in guiding further care.