Class III antiarrhythmic drugs like amiodarone prolong the QT interval by blocking:
- A The rapid delayed rectifier potassium channel (IKr, encoded by hERG gene) ✓
- B The inward rectifier potassium channel (IK1)
- C The transient outward potassium channel (Ito)
- D The slow delayed rectifier potassium channel (IKs) exclusively
Correct answer: A. The rapid delayed rectifier potassium channel (IKr, encoded by hERG gene)
Explanation
Class III agents prolong phase 3 repolarisation primarily by blocking IKr (rapid delayed rectifier K+ current), encoded by the hERG (KCNH2) gene. This lengthens the action potential duration and QT interval. Many drugs causing acquired long QT (e.g., sotalol, dofetilide, domperidone, certain antihistamines) also block IKr. IK1 maintains resting potential; Ito mediates phase 1 notch; IKs is a secondary target.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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