A 4-month-old boy was referred for evaluation of hypotonia. The infant was the product of an uneventful prenatal course and a spontaneous vaginal delivery at term. At age 1 month, the infant could lift his head and smile and startled appropriately. At a visit to the pediatrician at age 2 months, the mother expressed concern about a perceived slow development compared with his normal older siblings and poor head control. Results of examination at this time were reportedly normal, and the infant was treated for acute otitis media. Follow-up at age 3 months again revealed normal results. At age 4 months, hypotonia was noted by examination, and the infant was referred for care. Neurologic examination at this time revealed a floppy infant with decreased reflexes and tone. The weakness was more pronounced in the lower extremities. The elbows were floppy and held in flexion. Fasciculations were not definitely observed. Levels