When does spleen enlargement usually occur during infectious mononucleosis?

Spleen enlargement, or splenomegaly, is commonly associated with infectious mononucleosis, which is caused by the Epstein-Barr virus. This condition typically presents with a range of symptoms, including fever, sore throat, and fatigue. Spleen enlargement usually occurs during the second week of the illness as the body's immune response becomes increasingly activated to combat the viral infection.

During the first week, symptoms begin to manifest, but significant splenic enlargement is less common at that stage. By the second week, the immune system is more engaged, leading to an increase in lymphoid tissue and increased activity of the spleen, which is responsible for filtering blood and producing immune cells. Hence, the correct timing for noticeable spleen enlargement falls into this timeframe, as the symptoms and immune responses escalate.

In summary, the second week is when splenomegaly becomes most apparent due to the increased immune response, making it the correct choice in the context of this question.