A 16-year-old female is brought in by her parents for "not eating." She has lost 15 pounds over 3 months and now weighs 98 lbs at 5'5". She describes herself as "fat" and exercises 2 hours daily. Her parents report she has become withdrawn and her grades have dropped. Vitals: HR 52, BP 90/58, temp 96.8°F. What is the FIRST assessment priority?