A 58-year-old man with hypertension and smoking history presents to the emergency department with 2 hours of epigastric pain, nausea, and diaphoresis. His blood pressure is 95/60 mmHg and heart rate is 48 bpm. A 12-lead ECG is obtained. The structure marked **A** in the diagram shows ST elevation in leads II, III, and aVF with reciprocal ST depression in I and aVL. Which of the following is the most likely culprit vessel responsible for this acute myocardial infarction?