**Core Concept**
The question is testing the knowledge of neurotransmitters involved in Obsessive-Compulsive Disorder (OCD). OCD is a psychiatric disorder characterized by recurrent, intrusive thoughts (obsessions) and repetitive behaviors (compulsions). The underlying neurochemical mechanism involves an imbalance of certain neurotransmitters in the brain.

**Why the Correct Answer is Right**
Serotonin (5-HT) is the primary neurotransmitter involved in OCD. It plays a crucial role in regulating mood, anxiety, and obsessive-compulsive behaviors. The serotonin hypothesis of OCD suggests that decreased serotonin levels or impaired serotonin signaling contribute to the development of OCD symptoms. This is supported by the efficacy of serotonin reuptake inhibitors (SRIs), such as fluoxetine and sertraline, in treating OCD.

**Why Each Wrong Option is Incorrect**
**Option A:** Dopamine is primarily associated with reward, motivation, and movement regulation. While dopamine dysregulation may contribute to various psychiatric disorders, it is not the primary neurotransmitter involved in OCD.
**Option B:** GABA (gamma-aminobutyric acid) is an inhibitory neurotransmitter that helps regulate anxiety and stress responses. Although GABA dysregulation may be involved in anxiety disorders, it is not directly linked to OCD.
**Option C:** Glutamate is an excitatory neurotransmitter that plays a crucial role in learning and memory. While glutamate dysregulation has been implicated in various psychiatric disorders, it is not the primary neurotransmitter involved in OCD.

**Clinical Pearl / High-Yield Fact**
The serotonin hypothesis of OCD is supported by the efficacy of SRIs in treating OCD symptoms. However, it's essential to note that not all patients with OCD respond to SRI treatment, and other treatment options, such as cognitive-behavioral therapy (CBT), may be necessary.

**Correct Answer:** C. Serotonin is the primary neurotransmitter involved in OCD.