A 13-year-old boy with an unsteady gait was referred for evaluation. He had been well until 1 month prior, when he developed lower limb weakness after eating a vegetarian diet for 15 days. His medical and family histories were unremarkable. His walking had become unsteady, and he was less talkative. On examination, he scored 24 on a Mini-Mental State Examination, with impairments in temporal and spatial orientation, calculation, and short-term memory. Horizontal nystagmus was observed. His lower limb strength was 4 of 5 proximally and 5− of 5 distally, with increased muscle tone. Patellar reflexes were brisk. Ankle clonus and Babinski signs were present bilaterally. His gait was spastic (Video). Blood test results indicated normocytic anemia (hemoglobin level, 126 g/L; reference, >130 g/L [to convert to grams per deciliter, multiply by 0.1]), borderline folate elevation (2.15 ng/mL; reference, >2 μg/L [to convert nanomoles per liter, multiply by 2.266]), and an increased homocysteine level (8.85 mg/L; reference, <2.03 mg/L [to convert to micromoles per liter, multiply by 7.397]). Results of stool analysis; liver and kidney function tests; myocardial enzyme, electrolyte, ammonia, trace element, and B12 levels; thyroid function, arterial blood gas, antinuclear antibodies, and intrinsic factor antibody tests; and an infection panel test were unremarkable. Magnetic resonance imaging showed mild cerebral atrophy and a hyperintense T2-weighted signal in the cervical spinal cord. Electromyography results were normal. Electroencephalography showed diffuse background slowing with scattered epileptiform discharges. Urine organic acid analysis indicated methylmalonic acidemia. A serum amino acid analysis was nondiagnostic (Figure 1). Cerebrospinal fluid was normal.