This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Central Nervous system, Subject name: Pharmacology
[{'content': 'Drug of choice for controlling severe pain in cancer patients is: \nSelect and explain. \nA. Morphine, B. Diclofenac, C. Ibuprofen, D. Codiene', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Morphine is a potent analgesic and relieves pain without loss of consciousness. Dull aching visceral pain is relieved better than sharp aching pain. Ref KD Tripathi 8th ed.', 'role': 'assistant'}]