**Core Concept**
Synaptic transmission between pain fibers from the skin and spinal cord neurons involves the release of neurotransmitters that transmit pain signals. This process is crucial for the perception and processing of pain in the central nervous system.

**Why the Correct Answer is Right**
Substance P is a neuropeptide that plays a key role in transmitting pain signals from nociceptors (pain-sensing neurons) in the skin to the spinal cord. It is released from the terminals of primary afferent neurons in response to painful stimuli and binds to its receptor, neurokinin 1 (NK1), on the terminals of secondary afferent neurons in the spinal cord, facilitating the transmission of pain signals. Substance P is a member of the tachykinin family and is involved in the transmission of pain, itching, and other sensations.

**Why Each Wrong Option is Incorrect**
**Option A:** Acetylcholine is a neurotransmitter involved in transmitting signals from the central nervous system to the peripheral nervous system and in the autonomic nervous system, but it is not primarily involved in transmitting pain signals from the skin to the spinal cord.

**Option C:** Endorphins are natural pain-relieving chemicals produced by the body that bind to opioid receptors in the brain and spinal cord, reducing the perception of pain, but they are not involved in transmitting pain signals from the skin to the spinal cord.

**Option D:** Somatostatin is a hormone that regulates the release of other hormones and has various functions in the body, including inhibiting the release of other neurotransmitters, but it is not primarily involved in transmitting pain signals from the skin to the spinal cord.

**Clinical Pearl / High-Yield Fact**
Substance P is often referred to as the "pain molecule" due to its key role in transmitting pain signals from nociceptors to the spinal cord. It is also involved in other physiological processes, including inflammation and mood regulation.

**✓ Correct Answer: B. Substance P**