What medical conditions put an athlete at risk for a ruptured spleen?

Enlargement. Mononucleosis portal hypertension leukemias with splenic enlargement essentially any enlargement increases risk, but in contact sports even a normal size spleen can be traumatized.
Mononucleosis. Any condition in which the spleen is enlarged can make it more susceptible to rupture when hit. In the typical adolescent/young adult who is involved in sports, by far the most common such condition is "mono." a variety of other blood disorders and lymphatic cancers can do this. Note that a perfectly normal spleen can rupture if there is a sharp blow to the left upper abdomen.