Final answer:
An adolescent with an upper respiratory tract infection and abdominal pain is likely experiencing inflammation of the mesenteric lymph nodes, a symptom usually associated with infections like pharyngitis.
Step-by-step explanation:
The likely reason an adolescent patient with an upper respiratory tract infection complains of abdominal pain could be inflammation of the mesenteric lymph nodes. Typically, infections like pharyngitis and other respiratory ailments can lead to swollen lymph nodes, which can present as abdominal pain. While gastroenteritis, appendicitis, and other gastrointestinal conditions also cause abdominal pain, they are generally accompanied by more specific gastrointestinal symptoms such as vomiting, diarrhea, and localized tenderness, which were not mentioned as being present in this case.