A 3-dаy-оld full-term infаnt is brоught tо the newborn clinic for worsening jаundice. He is exclusively breastfed and has been feeding poorly. Physical examination is notable for jaundice extending to the lower extremities. Laboratory studies reveal: Total serum bilirubin: 24 mg/dL Direct bilirubin: 0.4 mg/dL Hemoglobin: normal Blood type: O+, Coombs negative While awaiting further treatment, the infant becomes lethargic and develops intermittent arching of the back. Which of the following is the most serious complication this infant is at risk for?