Final answer:
The older child with exercise intolerance and inability to keep up in PE class could be experiencing symptoms of infectious mononucleosis (mono), caused by the Epstein-Barr virus. Further evaluation by a healthcare professional is recommended for confirmation.
Step-by-step explanation:
The symptoms described in the question point to a possible diagnosis of infectious mononucleosis, also known as mono. Mono is caused by the Epstein-Barr virus and is commonly seen in teenagers and young adults. It presents with symptoms such as excessive fatigue, sore throat, swollen lymph nodes, and exercise intolerance.
In this case, the older child with exercise intolerance and inability to keep up in PE class could be experiencing these symptoms due to mono. It is important for them to visit a healthcare professional for further evaluation and testing to confirm the diagnosis.