A 50-year-old woman seen in the emergency department for an acute onset of confusion. was preceded by abdominal cramps and diarrhea. She has no medical problems and does not take any medications. Her Temp 38 C. BP 160/80, pulse98/min. She is contused and disoriented to place.. Hemoglobin 8.5 g/dl Leukocyte count 8000 Platelets 30.000/ul Creatinine 2 mg/dl LDH 800 units/L Coombs test negative Which of the following is the single most appropriate investigation?