Nephrotic Syndrome: This condition is characterized by significant proteinuria, hypoalbuminemia, and edema. It is a common cause of diffuse edema in children and adolescents, often presenting with sudden onset of swelling, particularly in the face, hands, and feet.

Allergic Reaction: An allergic reaction can cause diffuse edema, particularly if it involves anaphylaxis. This would be accompanied by other symptoms such as hives, itching, and potentially respiratory distress.
Insect Bite or Sting Reaction: Similar to an allergic reaction, a severe reaction to an insect bite or sting could lead to diffuse edema.
Menstrual Edema: In adolescent females, hormonal changes during the menstrual cycle can lead to fluid retention and edema.

Acute Kidney Injury: Any condition leading to acute kidney injury could result in diffuse edema due to fluid retention. This is critical to identify early as it can lead to severe complications if not promptly addressed.
Heart Failure: Although less common in healthy adolescents, new-onset heart failure can cause diffuse edema. It would typically be accompanied by other symptoms such as shortness of breath and fatigue.
Liver Disease (e.g., Cirrhosis): Liver dysfunction can lead to hypoalbuminemia and subsequently to edema. Identifying liver disease early is crucial for management and preventing further complications.

Lymphatic Obstruction: Conditions affecting the lymphatic system, such as lymphoma or congenital lymphatic dysplasia, can lead to edema. However, these are less common and typically present with other specific symptoms.
Thyroid Disorders: Both hypothyroidism and hyperthyroidism can cause edema, though this is less common in adolescents and usually accompanied by other thyroid-related symptoms.
Systemic Lupus Erythematosus (SLE): SLE is an autoimmune disease that can cause a wide range of symptoms, including edema. It is rare in adolescents and would typically be accompanied by other systemic symptoms.