A 72-year-old man with long-standing hypertension presents with acute severe dyspnoea at rest. He is anxious, sitting upright, sweating and speaking in short phrases. Respiratory rate is 32 breaths/min, oxygen saturation 90% on room air, blood pressure 190/110 mmHg, pulse 120 beats/min. Auscultation reveals widespread fine crackles throughout both lung fields. ECG shows sinus tachycardia. In addition to high-flow oxygen and upright positioning, which medication should be administered urgently to achieve rapid symptom relief and haemodynamic stabilisation?