A 2-year-old boy is referred for a second opinion because of abnormal laboratory results and maternal anxiety. His past history includes two hospital admissions for fever. On both occasions, findings on all investigations were unremarkable,and he was treated with antibiotics. The first hospitalization occurred 1 year ago, soon after the unexpected death of his 2-year-old sibling. The brother had been evaluated at the emergency department of a local hospital for fever and was discharged on antibiotics. He returned within hours and died. The autopsy reported bronchospasm and congestive heart failure as causes of death. Another sibling, who is 8 years old,suffers from epilepsy and had undergone a lobotomy. The child’s father had a brain aneurysm that was operated on recently.

The patient is a well-appearing Caucasian boy running around in the office. His physical examination yields only normal findings. Laboratory findings include platelet count,567 × 109/L (567 ×...

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