## **Core Concept**
Substance P is a neuropeptide belonging to the tachykinin peptide family. It is involved in various physiological processes, including pain transmission and modulation of neurotransmission. Substance P is primarily found in the nervous system.

## **Why the Correct Answer is Right**
The correct answer, **D. Sensory nerve terminals**, is right because substance P is predominantly localized in the terminals of sensory neurons, particularly those involved in the transmission of pain. It is released from the terminals of nociceptive neurons and plays a significant role in pain perception and neurogenic inflammation.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although substance P can interact with various immune cells, it is not primarily stored in immune cells in peripheral tissues.
- **Option B:** This option is incorrect as substance P is not primarily associated with postganglionic sympathetic neurons; its main role is related to sensory neurons.
- **Option C:** This option is incorrect because substance P is not primarily stored in parasympathetic nerve terminals; its localization and function are more closely associated with sensory neurons.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that substance P is involved in pain transmission. It acts on neurokinin-1 (NK1) receptors, which are targets for certain antiemetic and analgesic drugs. Understanding the role of substance P in pain and inflammation can help in managing conditions like neuropathic pain and chemotherapy-induced nausea.

## **Correct Answer:** D. Sensory nerve terminals.