Results of complete blood cell count, urea and electrolyte measurements, liver function test, serum copper and ceruloplasmin levels, erythrocyte sedimentation rate, vasculitic screen, porphyria screen, thyroid function test and thyroid autoantibodies, and cerebrospinal fluid examination (including polymerase chain reaction for herpes simplex virus type 1) were unremarkable. Brain MRI showed bilaterally symmetrical areas of hyperintensity on T2-weighted and diffusion-weighted images in the insular cortex and cingulate gyrus (Figure 2A). Magnetic resonance spectroscopy (1.5-T single-voxel point-resolved spectroscopy; echo time, 35 milliseconds; relaxation time, 1500 milliseconds; 8 cm3 voxel over the insular cortex) was performed, and revealed prominently elevated Glx and lactate levels; N-acetylaspartate, myoinositol, and choline levels were decreased (Figure 2B). The diagnosis of adult-onset citrullinemia was considered on the basis of characteristic MRI and MRS findings, and confirmed with elevated levels of plasma ammonia (1147 μg/dL), plasma citrulline (6 mg/dL [to convert to micromoles per liter, multiply by 57.081]), and urine citrulline (129 mmol/mol; reference range, 0-4 mmol/mol). The plasma glutamine (4 mg/dL) level was not increased. Oral sodium benzoate treatment and hemodialysis were started, but the patient developed status epilepticus and became comatose. Despite treatment, his condition deteriorated rapidly and he died 12 days after diagnosis.