The chest x-ray shows a medially displaced gastric bubble, which is most likely caused by an enlarged spleen given the symptoms of upper abdominal pain.

The patient had a positive infectious mononucleosis screen. On an ultrasound, the diameter of the spleen was 18cm, which amounts to moderate splenomegaly.

This patient was advised to abstain from any vigorous exercises or participation in contact sports for at least 4-6 weeks due to the potential for splenic rupture.

Reference here.