**Core Concept**
Obsessive Compulsive Disorder (OCD) is a psychiatric condition characterized by recurring, intrusive thoughts (obsessions) and repetitive behaviors or mental acts (compulsions) that an individual feels compelled to perform. The pathophysiology of OCD is believed to involve dysregulation of the cortico-striatal-thalamo-cortical (CSTC) circuitry, particularly the orbitofrontal cortex and anterior cingulate cortex.

**Why the Correct Answer is Right**
OCD is indeed associated with abnormalities in serotonin (5-HT) neurotransmission, particularly decreased activity of the serotonin transporter gene (SLC6A4) and altered expression of 5-HT2A and 5-HT1A receptors. These changes contribute to the development of OCD symptoms, which can be effectively treated with selective serotonin reuptake inhibitors (SSRIs).

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because OCD is indeed associated with abnormalities in serotonin neurotransmission.
**Option B:** Incorrect because while there is some evidence of dopamine involvement in OCD, it is not the primary neurotransmitter system implicated in the disorder.
**Option C:** Incorrect because OCD is not typically associated with abnormalities in dopamine transporter gene (DAT1) expression.
**Option D:** Incorrect because OCD is indeed associated with abnormalities in serotonin neurotransmission, particularly decreased activity of the serotonin transporter gene (SLC6A4).

**Clinical Pearl / High-Yield Fact**
OCD is often comorbid with other psychiatric conditions, such as major depressive disorder, anxiety disorders, and attention deficit hyperactivity disorder (ADHD).

**Correct Answer:** B.