**Core Concept**
Decreased levels of serotonin have been linked to increased suicidal behavior in patients with mood disorders. Serotonin plays a crucial role in regulating mood, anxiety, and impulsivity.

**Why the Correct Answer is Right**
Serotonin acts as a neurotransmitter in the brain and is involved in various physiological processes, including mood regulation. The serotonin hypothesis of depression suggests that decreased serotonin levels contribute to the development of depressive symptoms, including suicidal ideation. Studies have consistently shown that patients with depression and low serotonin levels are at a higher risk of suicide. This is thought to be due to the inhibitory effect of serotonin on impulsivity and aggression.

**Why Each Wrong Option is Incorrect**
**Option A:** Noradrenaline (norepinephrine) is associated with arousal and stress responses, but it is not directly linked to suicide risk. While noradrenaline dysregulation can contribute to depressive symptoms, its role in suicidal behavior is less clear.

**Option C:** GABA (gamma-aminobutyric acid) is an inhibitory neurotransmitter that plays a crucial role in regulating anxiety and stress responses. While GABA dysregulation can contribute to anxiety disorders, it is not directly associated with suicide risk.

**Option D:** Dopamine is involved in reward processing and motivation, but it is not directly linked to suicide risk. While dopamine dysregulation can contribute to mood disorders, its role in suicidal behavior is less clear.

**Clinical Pearl / High-Yield Fact**
The serotonin hypothesis of depression highlights the importance of serotonin in mood regulation. Clinicians should consider assessing serotonin levels and function in patients with depression, particularly those with a history of suicidal behavior.

**✓ Correct Answer: B. Serotonin**