an 8-year-old girl is rushed to the emergency department by her parents because she has become delirious. the child was diagnosed with influenza three days prior. her parents say that she had begun vomiting yesterday, almost nonstop, and has not been able to hold down fluids. they also note that she has been breathing rapidly. your exam reveals a tachypneic, disoriented female with hyperreflexia, a positive babinski reflex, and liver enlargement. csf analysis reveals a normal protein and cell count. what is the most likely diagnosis?