Final answer:
The most appropriate nursing diagnosis for a 13-year-old with anorexia nervosa would be 'Imbalanced Nutrition: Less Than Body Requirements', with additional considerations for 'Disturbed Body Image' or 'Anxiety'.
Step-by-step explanation:
The nursing diagnosis most appropriate for a 13-year-old with anorexia nervosa who is worried that the i.v. fluids will cause weight gain would likely be 'Imbalanced Nutrition: Less Than Body Requirements' due to the client's refusal or inability to meet the nutritional requirements necessary for health. Moreover, considering the psychological aspects, 'Disturbed Body Image' or 'Anxiety' related to the fear of weight gain could also be pertinent nursing diagnoses. It is essential to provide supportive care, address misconceptions around weight gain with the i.v. fluids, and engage the client in a comprehensive treatment plan involving mental and nutritional therapy to treat the eating disorder.