**Core Concept**
Antidepressant drugs are a class of medications primarily used to treat depression and other mood disorders. They work by modulating neurotransmitter systems, particularly serotonin and norepinephrine, in the brain. Selective serotonin reuptake inhibitors (SSRIs) are a subtype of antidepressants that selectively inhibit the reuptake of serotonin, thereby increasing its availability in the synaptic cleft.

**Why the Correct Answer is Right**
Citalopram is a type of SSRI, which selectively inhibits the serotonin transporter, thereby reducing the reuptake of serotonin. This leads to an increase in the concentration of serotonin in the synaptic cleft, which in turn enhances serotonergic transmission and alleviates depressive symptoms. Citalopram's mechanism of action is distinct from other classes of antidepressants, such as tricyclic antidepressants, monoamine oxidase inhibitors, and atypical antidepressants.

**Why Each Wrong Option is Incorrect**
**Option A:** Pimozide is an antipsychotic medication primarily used to treat schizophrenia. It is a typical antipsychotic that blocks dopamine receptors, particularly D2 receptors, which is different from the mechanism of action of antidepressants.
**Option B:** Haloperidol is another antipsychotic medication that is used to treat schizophrenia and other psychoses. It is a typical antipsychotic that blocks dopamine receptors, which is not relevant to the treatment of depression.
**Option C:** Thioridazine is an antipsychotic medication that is used to treat schizophrenia and other psychoses. It is a typical antipsychotic that blocks dopamine receptors and has a higher risk of cardiac side effects compared to newer antipsychotics. It is not an antidepressant.

**Clinical Pearl / High-Yield Fact**
When choosing an antidepressant, it's essential to consider the patient's medical history, potential interactions with other medications, and the risk of side effects. SSRIs like citalopram are generally well-tolerated and have a favorable side effect profile compared to other classes of antidepressants.

**✓ Correct Answer: D. Citalopram**