Final answer:
A child with mononucleosis is excused from gym due to the risk of abdominal trauma that could lead to spleen rupture because of spleen enlargement.
Step-by-step explanation:
The reason a child with mononucleosis, commonly caused by the Epstein-Barr virus (EBV), should be excused from gym is due to the potential for sustaining abdominal trauma. Mononucleosis can cause the spleen to enlarge, which makes it more susceptible to rupture, especially with physical activity. Since the spleen lies in the abdominal cavity, any impact or strain could lead to serious complications. Spleen rupture requires immediate medical attention and can be life-threatening. Therefore, it is crucial to avoid physical activities that could increase this risk while the spleen is enlarged.