A 56-year-old male with a 6-year history of precordial dull pain after exertion. For the past week, he has frequently been awakened by nocturnal chest pain. During episodes, the electrocardiogram (ECG) shows unidirectional curve-type ST-segment elevation of 0.2 mV in leads II, III, and aVF, which resolves after relief. Which medication is MOST contraindicated during an attack?