A 65-year-old man presents to a rural emergency department 45 minutes after the sudden onset of central crushing chest pain radiating to his left arm. His ECG shows 3 mm ST elevation in leads V2-V5 with reciprocal ST depression in the inferior leads. He has no history of peptic ulcer disease and is not on anticoagulants. In addition to oxygen if required and intravenous access, which single medication should be given immediately?