A 55-year-old man with schizophrenia has a documented prolonged QT interval on ECG. Which antipsychotic must be avoided because it carries the greatest risk of torsades de pointes?
- A Thioridazine ✓
- B Risperidone
- C Haloperidol
- D Quetiapine
Correct answer: A. Thioridazine
Explanation
Thioridazine produces the most marked QT prolongation of any antipsychotic and has caused fatal torsades de pointes, so it is reserved for schizophrenia refractory to all other agents. Among atypical drugs, ziprasidone also prolongs QT substantially and would be a poor choice here. Haloperidol and risperidone carry comparatively modest QT risk, though intravenous haloperidol requires cardiac monitoring.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
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