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
Which substance enhances the sensitivity of pain receptors but does not directly excite them? 
Select and explain. 
A. Bradykinin, B. Serotonin, C. Potassium ions, D. Prostaglandins
D: Prostaglandins
Answer: D: Prostaglandins 
Explanation: Prostaglandins & substance P are believed to enhance the sensitivity of pain receptors but do not actually excite them.