**Core Concept**
Excessive anxiety and fear accompanied by intrusive thoughts and compulsions to perform rituals to alleviate distress suggest a diagnosis of Obsessive-Compulsive Disorder (OCD), a psychiatric condition characterized by abnormalities in the brain's serotonin system.

**Why the Correct Answer is Right**
The first-line pharmacological treatment for OCD primarily involves selective serotonin reuptake inhibitors (SSRIs), which increase the availability of serotonin in the synaptic cleft. Serotonin is a neurotransmitter that plays a crucial role in regulating mood, anxiety, and obsessive-compulsive behaviors. By enhancing serotonergic activity, SSRIs can help alleviate symptoms of OCD.

**Why Each Wrong Option is Incorrect**
**Option A:** Dopamine is a neurotransmitter involved in reward processing, motor control, and motivation, but it is not the primary target of first-line OCD treatment.
**Option B:** GABA (gamma-aminobutyric acid) is an inhibitory neurotransmitter that helps regulate anxiety, but SSRIs do not primarily affect GABA levels in the treatment of OCD.
**Option C:** Norepinephrine is a neurotransmitter involved in attention and stress response, but it is not the primary target of first-line OCD treatment.

**Clinical Pearl / High-Yield Fact**
SSRIs, such as fluoxetine (Prozac), are the first-line treatment for OCD due to their efficacy in reducing symptoms and their relatively favorable side effect profile.

**Correct Answer:** B. Serotonin.