## **Core Concept**
The parasympathetic nervous system is one of the branches of the autonomic nervous system, which is responsible for promoting 'rest and digest' functions. It primarily uses **acetylcholine (ACh)** as its neurotransmitter. The postsynaptic parasympathetic nerve terminals and receptors are crucial for transmitting signals to various organs.

## **Why the Correct Answer is Right**
The postsynaptic parasympathetic nerve terminals and receptors primarily release and respond to **acetylcholine (ACh)**. ACh acts on muscarinic receptors in the target organs. The correct answer, **norepinephrine**, is a neurotransmitter primarily associated with the sympathetic nervous system, not the parasympathetic nervous system. Norepinephrine acts on various adrenergic receptors.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Acetylcholine* is the primary neurotransmitter released by postsynaptic parasympathetic nerve terminals.
- **Option B:** *Vasopressin* is not typically associated with parasympathetic neurotransmission but is more related to the regulation of water balance in the body.
- **Option C:** *Substance P* is involved in transmitting pain signals and some autonomic functions but is not a primary neurotransmitter of the parasympathetic nervous system.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the parasympathetic nervous system uses **acetylcholine** as its primary neurotransmitter for both preganglionic and postganglionic neurons. This is in contrast to the sympathetic nervous system, which uses **norepinephrine** as its primary postganglionic neurotransmitter.

## **Correct Answer:** D. Norepinephrine.