You are answering a USMLE-style medical question. Question: A 65-year-old man with metastatic lung cancer has been experiencing severe, unremitting pain. He has required escalating doses of oral morphine, but is now having dose limiting side-effects. His pain management team recommends using a medication that can reduce his opioid need through interaction with the NMDA-receptor. Which of the following was the most likely recommended agent? Options: A) Propofol B) Ketamine C) Fentanyl D) Midazolam Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.