This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Physiology of Pain, Subject name: Physiology
[{'content': 'Which substance enhances the sensitivity of pain receptors but does not directly excite them? \nSelect and explain. \nA. Bradykinin, B. Serotonin, C. Potassium ions, D. Prostaglandins', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Prostaglandins & substance P are believed to enhance the sensitivity of pain receptors but do not actually excite them.', 'role': 'assistant'}]