A 26-year-old male with a history of 'A' type Wolff-Parkinson-White (WPW) syndrome presents with palpitations for 2 hours. Blood pressure (BP) is 16/9.5 kPa (120/70 mmHg), and electrocardiogram (ECG) shows WPW syndrome with atrial fibrillation and a ventricular rate of 170 beats/min. Which of the following drugs is contraindicated in this patient?