A 21-year-old woman came to the hospital with fever, jaundice, and abdominal pain that persisted despite antibiotics. Although her monospot test for mononucleosis came back negative, blood tests for a virus called Epstein-Barr Virus (EBV) confirmed she had infectious mononucleosis. Imaging scans revealed multiple areas of dead tissue (infarcts) in her spleen caused by blocked blood vessels. She recovered well with rest and supportive care, highlighting how mononucleosis can sometimes cause serious complications even when the initial screening test is negative.