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<P>This 17-year-old boy was transferred for evaluation of fever, sore throat, and headache. He was well until 3 months prior to admission when he had developed fever and fatigue. Despite five courses of antibiotics, these symptoms continued intermittently for the next 2 months to the point that he was unable to play sports because of fatigue, having reportedly lost 25 pounds. Five days prior to admission, he started to have intermittent non-bilious emesis prompting another course of antibiotics. Three days prior to admission, he developed jaundice and right upper quadrant pain. On admission to an outside hospital, computed tomography (CT) of the abdomen revealed hepatosplenomegaly and mesenteric lymphadenopathy, and he was transferred. His past medical history was remarkable for meningococcemia at 11 years, from which he recovered without sequelae. The family history is unremarkable. There is no significant travel history. There are a dog and a cat at home.</P> <H4>Key Learning Points</H4> <OL> <LI>The major predisposing factor for recurrent invasive Neisseria infections is a deficiency of terminal complement.</LI> <LI>The heterophile antibody test is 90% sensitive for detecting EBV infection in adolescents; the sensitivity is much lower in children younger than 4 years old. The false-positive rate is very low.</LI> <LI> The diagnosis of hemophagocytic lymphohistiocytosis (HLH) is definite if a known genetic mutation is identified. A clinical diagnosis can be made if 5 of 8 criteria are found: 1) fever, 2) splenomegaly, 3) cytopenias affecting 2 of 3 cell lines, 4) hypertriglyceridemia or hypofibrinogenemia, 5) hemophagocytosis in a tissue without evidence of malignancy, 6) low natural killer cell activity, 7) serum ferritin greater than 500 ng/mL, and 8) soluble CD25 <u>></u>2400 U/mL.</LI> <LI>The diagnosis of X-linked lymphoproliferative disorder should be suspected in any boy with severe EBV-associated HLH.</LI> </OL> <H4>ABOUT THE AUTHOR</H4> <P>Dr. Listernick is Professor of Pediatrics at Feinberg School of Medicine, Northwestern University, and Director of the Diagnostic and Consultation Service, Division of General Academic Pediatrics, Children’s Memorial Hospital, Chicago, IL.</P>