**Question:** All are side effects of clozapine EXCEPT:

A. Somnolence
B. Dry mouth
C. Orthostatic hypotension
D. Weight gain

**Core Concept:** Clozapine is an atypical antipsychotic drug used in the treatment of schizophrenia and other psychotic disorders. Like other antipsychotic medications, clozapine can cause various side effects due to its pharmacological actions on dopamine, serotonin, and histamine receptors.

**Why the Correct Answer is Right:** Somnolence (drowsiness) is a well-known side effect of clozapine, as it primarily targets dopamine D2 receptors in the brain. This blockade leads to a decrease in dopamine activity which can cause sedation and drowsiness.

**Why Each Wrong Option is Incorrect:**

A. Dry mouth (xerostomia): Clozapine does inhibit the parasympathetic nervous system, reducing saliva production and leading to dry mouth.
B. Orthostatic hypotension (postural hypotension): Clozapine can cause orthostatic hypotension due to its effect on alpha-1 receptors and autonomic nervous system. It reduces blood pressure when standing up (orthostatic) which can lead to lightheadedness or fainting.
D. Weight gain: Clozapine can cause weight gain due to increased appetite and altered lipid profiles, leading to obesity and related comorbidities.

**Why D is Right and the others are Wrong:** Weight gain is a common side effect of clozapine, primarily caused by increased appetite and alterations in lipid profiles. These factors contribute to obesity and related comorbidities, making it the correct answer.

**Clinical Pearl:** Monitoring for and managing side effects of clozapine are essential for optimal patient care. Frequent weight assessments and lifestyle modifications can help mitigate weight gain without compromising therapeutic effectiveness.

In summary, the correct answer (D) is due to the specific pharmacological actions of clozapine, while the incorrect options (A, B, and C) are consequences of its effects on various receptors and systems in the body.