A12-month-old girl is brought to the ED after a seizurelike episode. On awakening from a nap, she screamed and then became stiff. Her eyes rolled back, and she began to shake. Both arms and legs were involved symmetrically. The episode lasted 2 minutes, after which she appeared tired. She has been well recently, with no fever or respiratory symptoms.

Her mother reports a history of similar episodes over the last 3 months, with increasing frequency in the last week (up to 10/d), but no medical attention has been sought until now. The child has had no serious illnesses, is receiving no medications, has no allergies, and has not been immunized (by parental choice). She walked at 10 months of age, but in the last week has become unsteady, stumbling when walking. The family history is noncontributory.

The physical examination reveals an alert child who has normal tone, strength, reflexes, and...

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