You are answering a USMLE-style medical question. Question: A chronic opioid abuser undergoes emergency surgery. Following the operation, the man is started on a patient controlled analgesia (PCA) thought to be dosed adequately to control his pain in the face of his opioid tolerance. He reports intense pain 6 hours after the conclusion of the surgery. Which agent could have been given intraoperatively to reduce this patient's risk of developing postoperative hyperalgesia? Options: A) Ketamine B) Midazolam C) Propofol D) Etomidate Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.