**Core Concept**
Panic disorder is a complex condition involving multiple neurotransmitter systems, including **serotonin**, **gamma-aminobutyric acid (GABA)**, and **norepinephrine**. The pathophysiology of panic disorder also involves the **hypothalamic-pituitary-adrenal (HPA) axis**.

**Why the Correct Answer is Right**
Since the correct answer is not provided, let's discuss the general involvement of substances in panic disorder. **Serotonin** plays a crucial role in regulating mood and anxiety, with **selective serotonin reuptake inhibitors (SSRIs)** being a common treatment. **GABA** is an inhibitory neurotransmitter that helps reduce anxiety. **Norepinephrine** and the **HPA axis** are also involved in the body's stress response.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific substance, it's challenging to provide a detailed explanation. However, if a substance like **serotonin** were listed, it would be incorrect to say it's not involved in panic disorder because **SSRIs** are used to treat it.
**Option B:** Similarly, if **GABA** were listed, it would be incorrect because **GABA** helps regulate anxiety.
**Option C:** If **norepinephrine** were listed, it would be incorrect because **norepinephrine** is involved in the body's stress response.
**Option D:** Without the specific substance, it's challenging to provide a detailed explanation.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **SSRIs** are considered a first-line treatment for panic disorder, highlighting the importance of **serotonin** in its pathophysiology.

**Correct Answer:** D. Dopamine