Final answer:
The spleen commonly enlarges during the second or third week of infectious mononucleosis. This condition is mostly caused by the Epstein-Barr Virus and can lead to serious complications if not monitored properly.
Step-by-step explanation:
During infectious mononucleosis (IM), caused mainly by the Epstein-Barr Virus (EBV), an individual may experience enlargement of the spleen and sometimes the liver. This enlargement tends to occur in the second or third week of infection. The initial clinical symptoms often include pharyngitis, fever, fatigue, and lymph node swelling. After about a month, the disease is generally self-limiting, although extreme fatigue may persist for several months. It is important to monitor for complications such as spleen enlargement which can lead to a potential risk of rupture.