A 6-year-old boy had been in his normal state of good health until a few hours prior to presentation to the emergency department. His mother reports that he began to have difficulty walking, and she noticed that he was falling and unable to maintain his balance. Which of the following is the most likely to allow diagnosis of his condition?

Cerebellar ataxia in childhood can occur in association with infection, metabolic abnormalities, ingestion of toxins, hydrocephalus, cerebellar lesions, multiple sclerosis, labyrinthitis, polyradiculopathy, and neuroblastoma. Although a muscle biopsy might help identify muscular dystrophy, the finding of a triplet repeat expansion on chromosome 9 might identify Friedreich's ataxia, and finding skin telangiectasia might help identify ataxia telangiectasia, none of these would be expected to cause acute ataxia but rather more chronic symptoms. Ingestion (intentional or accidental) of barbiturates, phenytoin, alcohol, and other drugs also must be considered. Agenesis of the corpus callosum is usually diagnosed by imaging studies; however, it does not cause acute ataxia.