Question

An 18-month-old boy is brought to the pediatric neurology clinic by his parents due to a two-week history of progressive unsteadiness. The parents are highly anxious and state, "He only just started running at 17 months, and now he can barely walk! His development has always been terribly slow." They also express concern regarding his past medical history, noting that during his first week of life, he had a blotchy red rash with tiny yellow pustules on his chest that resolved on its own. They fear this was an untreated viral infection that is now damaging his brain. Furthermore, three weeks ago, he had a mild respiratory illness characterized by a low-grade fever, rhinorrhea, and diffuse wheezing, which resolved over a week with supportive care. Vital signs are within normal limits for his age. Physical examination reveals an irritable child with a broad-based, unsteady gait. While sitting quietly, he exhibits rapid, chaotic, multidirectional conjugate eye movements without a predictable pattern. Brief, shock-like contractions of his proximal arm and leg muscles are also noted. Deep tendon reflexes are 2+ bilaterally, and muscle tone is normal. Which of the following is the most appropriate initial diagnostic or management step for this patient?