**Core Concept**
Obsessive-Compulsive Disorder (OCD) is a chronic anxiety disorder characterized by intrusive thoughts and repetitive compulsions. Treatment primarily involves selective serotonin reuptake inhibitors (SSRIs), which enhance serotonergic neurotransmission in the brain, particularly in the prefrontal cortex and limbic system.

**Why the Correct Answer is Right**
Fluoxetine is a first-line SSRI used in OCD due to its strong efficacy in reducing obsessions and compulsions. It works by inhibiting the reuptake of serotonin at synaptic clefts, increasing synaptic serotonin levels. Clinical trials have shown significant improvement in OCD symptoms with fluoxetine, especially in moderate to severe cases. It is preferred over other SSRIs due to its favorable side effect profile and long half-life, allowing for stable dosing.

**Why Each Wrong Option is Incorrect**
Option A: Clomipramine is an older tricyclic antidepressant with proven efficacy in OCD, but it is not the *drug of choice* due to significant side effects like anticholinergic effects, sedation, and cardiac toxicity. It is now used only in refractory cases.
Option C: Carbamazepine is an anticonvulsant used for seizure disorders and neuropathic pain, not for OCD. It has no significant serotonergic activity.
Option D: Chlorpromazine is a first-generation antipsychotic with weak serotonergic effects and is not indicated for OCD; it is used for schizophrenia and psychosis.

**Clinical Pearl / High-Yield Fact**
SSRIs like fluoxetine are first-line agents in OCD; if they fail, augmentation with clomipramine or cognitive behavioral therapy (CBT) with exposure and response prevention (ERP) is considered. Fluoxetine is especially useful in patients with comorbid depression.

✓ Correct Answer: B. Fluoxetine