Final answer:
The symptoms suggest the patient likely suffers from appendicitis, most commonly caused by fecalith obstruction in children.
Step-by-step explanation:
The most common cause of the symptoms presented by the 11-year-old girl, which include severe abdominal pain that started near the middle of the abdomen and migrated to the right lower quadrant, nausea, loss of appetite, fever, and rebound tenderness in the right lower quadrant, is fecalith obstruction.
This presentation is typical of appendicitis, a condition frequently caused by obstruction due to a fecalith, or hardened stool. This obstruction leads to inflammation and potential infection of the appendix. The patient's symptoms, specifically the movement of pain to the right lower quadrant and the presentation with rebound tenderness and fever, suggest acute appendicitis as the most likely diagnosis. It's key to conduct a prompt examination and potentially apply imaging techniques like ultrasound or CT scanning to confirm the diagnosis and proceed with treatment, which usually involves surgical removal of the affected appendix.