Clozapine's superior efficacy in treatment-resistant schizophrenia is most closely attributed to which pharmacological property compared to other second-generation antipsychotics?

• A Higher D2 receptor occupancy at therapeutic doses
• B Lower D2 occupancy (40–60%) combined with high affinity for multiple receptors including 5-HT2A, H1, M1, alpha-1, and D4 ✓
• C Selective blockade of mesolimbic D2 receptors sparing nigrostriatal pathway
• D Potent NMDA receptor antagonism reducing glutamatergic excitotoxicity

Explanation

Clozapine occupies D2 receptors at only 40–60% occupancy (compared to 70–90% for other antipsychotics), and its 'loose binding' or fast-off kinetics at D2 receptors minimise EPS and tardive dyskinesia. Its efficacy derives from its broad receptor profile including high-affinity blockade of 5-HT2A, H1, M1 muscarinic, alpha-1 adrenergic, and D4 receptors. This multimodal binding, particularly D4 and 5-HT2A activity, is believed to underlie its superior efficacy for positive and negative symptoms in treatment-resistant cases. Clozapine does not selectively block mesolimbic versus nigrostriatal D2.

Reference: Kaplan & Sadock's Synopsis of Psychiatry, 11th ed.

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