Viral Gastroenteritis: This is a common condition in adolescents, often presenting with nausea, vomiting, and decreased appetite due to inflammation of the stomach and intestines, usually caused by a viral infection.

Food Poisoning: Bacterial or viral infections from contaminated food can lead to nausea and decreased appetite.
Stress or Anxiety: Psychological factors can manifest physically, including nausea and loss of appetite, especially in adolescents dealing with school or social pressures.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic nausea and decreased appetite, among other symptoms.

Appendicitis: Although it typically presents with abdominal pain, nausea, and vomiting, decreased appetite can be an early symptom. Missing this diagnosis can lead to severe complications.
Diabetic Ketoacidosis (DKA): In undiagnosed diabetes, DKA can cause nausea, vomiting, and decreased appetite, and is a medical emergency.
Ingestion or Poisoning: Accidental or intentional ingestion of toxic substances can present with nausea and decreased appetite, requiring immediate medical attention.

Hepatitis: Inflammation of the liver can cause nausea, loss of appetite, and other systemic symptoms, though it's less common in otherwise healthy adolescents.
Gastric or Intestinal Obstruction: Though rare, obstructions can cause severe nausea, vomiting, and decreased appetite, and require urgent medical intervention.
Brain Tumors: Certain brain tumors, especially those affecting the hypothalamus or brainstem, can cause nausea and changes in appetite, though this would be extremely rare and likely accompanied by other neurological symptoms.