**Question:** A 25-year-old man with a history of hearing voices is brought to the emergency department by his family because the voices are telling him to kill his mother. He believes that aliens are trying to abduct him. These delusions and hallucinations have been present over the past 6 months. His family denies a history of depression or manic episodes. The patient is admitted to an inpatient psychiatry unit and started on haloperidol. Three hours after the initiation of therapy, he develops stiffness and muscle spasms of the neck and spine muscles.

Which of the following is the mechanism of action of the drug that should be used in the acute management of this patient?

A. Anticholinergic effects
B. Dopamine antagonism
C. Serotonin antagonism
D. Antipsychotic effects

**Correct Answer:**

**Core Concept:** Haloperidol is a dopamine antagonist, primarily targeting D2 receptors in the brain. In patients with psychotic disorders, excess dopamine levels in the mesolimbic pathway can lead to positive symptoms like hallucinations and delusions. By blocking D2 receptors, haloperidol helps to alleviate these symptoms.

**Why the Correct Answer is Right:**

* Haloperidol is a first-generation antipsychotic drug primarily used for the treatment of psychotic disorders, such as schizophrenia. It acts by blocking dopamine D2 receptors in the mesolimbic pathway, which is responsible for positive symptoms like hallucinations and delusions.
* The patient's symptoms of hearing voices and developing stiffness and muscle spasms after taking haloperidol are consistent with the known side effects of first-generation antipsychotics like haloperidol.
* The side effect experienced by the patient, extrapyramidal symptoms (EPS), is a result of the drug's blockade of dopamine D2 receptors, leading to reduced dopamine release in the basal ganglia. This can cause extrapyramidal side effects, such as rigidity, tremors, and muscle stiffness, which are seen in this patient.

**Why Other Options are Incorrect:**

A. Anticholinergic effects: While haloperidol does have anticholinergic effects due to its blockade of muscarinic receptors, these side effects are less severe than extrapyramidal symptoms and are not the primary mechanism of action of haloperidol.

B. Serotonin antagonism: Haloperidol does not primarily target serotonin receptors. It primarily acts as a dopamine antagonist, which is why it is the correct answer.

C. Antipsychotic effects: Although haloperidol is an antipsychotic drug, the question specifically asks about the mechanism of action related to the side effect experienced by the patient. Antipsychotic effects refer to the primary therapeutic effect of haloperidol, not the side effect being discussed.

**Clinical Pearls:**

1. In this scenario, haloperidol is the correct choice because it specifically blocks dopamine D2 receptors, causing the extrapyramidal side effects observed in the patient.
2. Extrapyramidal symptoms are a common