**Core Concept**
The question is testing knowledge of neurotransmitters, specifically peptides. Neurotransmitters are chemical messengers that transmit signals between neurons. Peptide neurotransmitters are a subclass of neurotransmitters composed of short chains of amino acids, similar to proteins.

**Why the Correct Answer is Right**
The correct answer is **Dopamine**. Dopamine is a biogenic amine neurotransmitter, not a peptide. It is synthesized from the amino acid tyrosine through a series of enzymatic reactions involving tyrosine hydroxylase, aromatic L-amino acid decarboxylase, and DOPA decarboxylase. Dopamine plays a crucial role in motor control, reward, and motivation.

**Why Each Wrong Option is Incorrect**
**Option A: Substance P** is a peptide neurotransmitter involved in pain transmission and mood regulation. It is a member of the tachykinin family and is synthesized from the precursor protein preprotachykinin A.

**Option B: Vasopressin** is a peptide hormone and neurotransmitter involved in water reabsorption in the kidneys and social behavior. It is synthesized in the hypothalamus and released from the posterior pituitary gland.

**Option C: Angiotensin II** is a peptide hormone and neurotransmitter involved in blood pressure regulation. It is synthesized from angiotensin I through the action of angiotensin-converting enzyme (ACE) and plays a crucial role in the renin-angiotensin-aldosterone system.

**Clinical Pearl / High-Yield Fact**
Remember that peptide neurotransmitters are involved in a wide range of physiological processes, including pain modulation, mood regulation, and social behavior. Understanding the differences between peptide and biogenic amine neurotransmitters is crucial for diagnosing and treating neurological and psychiatric disorders.

**Correct Answer: D. Dopamine**