**Question:** A patient of schizophrenia on CPZ (chlorpromazine) develops auditory hallucination again. The next drug to be given is -

A. Olanzapine
B. Quetiapine
C. Risperidone
D. Haloperidol

**Correct Answer:** D. Haloperidol

**Core Concept:**
In the management of schizophrenia, antipsychotic medications are used to treat psychotic symptoms like auditory hallucinations. There are several antipsychotic drugs available, including chlorpromazine (CPZ), olanzapine, quetiapine, and haloperidol. The treatment of choice in a patient who is already on CPZ and experiencing auditory hallucinations again would focus on a different antipsychotic drug with a different mechanism of action.

**Why the Correct Answer is Right:**
Haloperidol (D) is a typical antipsychotic drug and belongs to the group of neuroleptic drugs. Unlike chlorpromazine (CPZ), which primarily acts on dopamine D2 receptors, haloperidol also blocks serotonin receptors, such as 5-HT2A and 5-HT2C receptors. This broader receptor blockade could potentially explain why it is more effective in treating auditory hallucinations in a patient on chlorpromazine who experiences relapse.

**Why Each Wrong Option is Incorrect:**
A. Olanzapine (A) is an atypical antipsychotic and has a different mechanism of action compared to haloperidol. While it does block dopamine D2 receptors, it also has a lesser affinity for serotonin receptors, which might explain why it might not be as effective in a patient on chlorpromazine.
B. Quetiapine (B) is another atypical antipsychotic with a different mechanism of action compared to haloperidol. While it blocks dopamine receptors, it has a lesser affinity for serotonin receptors as well, making it less effective in treating auditory hallucinations in a patient on chlorpromazine.
C. Risperidone (C) is another atypical antipsychotic and has a different mechanism of action compared to haloperidol. While it blocks dopamine receptors, it also has a lesser affinity for serotonin receptors, similar to olanzapine and quetiapine. This could explain why it might not be as effective in a patient on chlorpromazine.

**Clinical Pearl:** When treating psychotic symptoms in patients on chlorpromazine, choosing a neuroleptic drug with a broader blockade of dopamine and serotonin receptors is crucial. Haloperidol (D) demonstrates a broader blockade of these receptors, making it a more suitable choice for a patient on chlorpromazine.