Psychiatryniche
A 22-year-old woman is admitted after three weeks of increasingly odd behaviour. Her mother says she had a flu-like illness a month ago, then became convinced her housemates were poisoning her food. Over the past week she has developed intermittent grimacing and lip-smacking movements she cannot suppress, her pulse has swung between 48 and 130/minute, and she had a witnessed generalised seizure this morning. She is drowsy between episodes. Temperature is 37.4°C. Urine toxicology and computed tomography of the brain are normal. Which one of the following investigations is most likely to establish the diagnosis?