Final answer:
A school nurse should recognize an enlarged spleen as the reason an adolescent child is excused from gym class following mononucleosis, due to the potential risk of spleen rupture during physical activity.
Step-by-step explanation:
The school nurse should identify an enlarged spleen as the reason for the adolescent child being excused from gym class after returning to school following a case of mononucleosis. An enlarged spleen is a common complication associated with mononucleosis, also known as mono, which is mostly caused by the Epstein-Barr virus (EBV). Engagement in physical activities, especially contact sports, can pose a risk of spleen rupture in individuals with an enlarged spleen due to mononucleosis. Therefore, the precaution to excuse the child from gym class is necessary to prevent potential injury.