A 58-year-old man from Delhi presents to the emergency department with palpitations and syncope. His ECG shows a prolonged QT interval (520 ms) and he is found to have hypokalaemia (K+ 2.8 mEq/L). On further questioning, he has been taking a new antiarrhythmic drug for atrial fibrillation. Which phase of the cardiac action potential is most critically affected by this electrolyte disturbance, leading to early afterdepolarizations and torsades de pointes?