**Core Concept**
The question is testing the understanding of the sensitization of pain receptors, specifically the role of substances that can enhance the sensitivity of nociceptors without directly exciting them. This concept is crucial in the understanding of pain modulation and the potential therapeutic targets for pain management.

**Why the Correct Answer is Right**
Substances like bradykinin and histamine can sensitize nociceptors by opening cation channels and increasing the excitability of the pain receptors. This sensitization is achieved through the activation of proteinase-activated receptors (PARs) and bradykinin receptors, which increases the release of neurotransmitters and enhances the excitability of the nociceptors. This process does not directly excite the pain receptors but rather makes them more sensitive to stimuli.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a substance that enhances the sensitivity of pain receptors. A correct answer would be a substance like bradykinin or histamine.
* **Option B:** This option might be appealing but is incorrect because it directly excites the pain receptors. Substances like acetylcholine or substance P directly activate the nociceptors, whereas the correct answer enhances sensitivity.
* **Option C:** This option is incorrect because it is a substance that directly excites the pain receptors. Glutamate is an excitatory neurotransmitter that directly activates the nociceptors.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that pain sensitization can occur through various mechanisms, including the release of pro-inflammatory mediators, which can enhance the sensitivity of nociceptors. This knowledge is crucial in understanding the pathophysiology of chronic pain and developing effective pain management strategies.

**Correct Answer: B. Histamine.**