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?

Explanation

## Why Right coronary artery (RCA) is right The ST elevation pattern marked **A** in leads II, III, and aVF with reciprocal ST depression in I and aVL is the classic ECG signature of INFERIOR STEMI. According to Harrison's Principles of Internal Medicine and the 2022 ACC/AHA/HRS STEMI guideline, the RCA is the culprit vessel in 80–90% of inferior STEMI cases, particularly in the ~85% of the population with right-coronary dominance where the RCA supplies the inferior (diaphragmatic) wall and the posterior descending artery. The clinical presentation of epigastric pain, nausea, bradycardia, and hypotension further supports RCA territory infarction, as the RCA supplies the AV node in 90% of patients (explaining the bradycardia) and can trigger the Bezold-Jarisch reflex (explaining nausea and hypotension). The RCA is the dominant culprit in inferior STEMI unless specific ECG clues (e.g., ST elevation II ≥ III with lateral ST elevation) point to LCx occlusion. ## Why each distractor is wrong - **Left anterior descending artery (LAD)**: The LAD supplies the anterior wall (V1–V4) and anterior septum. ST elevation in II, III, and aVF is not consistent with LAD territory; LAD occlusion would produce anterior STEMI with ST elevation in V1–V4, not inferior leads. - **Left circumflex artery (LCx)**: While the LCx can cause inferior STEMI in left-dominant systems (~7–10% of population), it is responsible for only 10–20% of inferior STEMI cases overall. LCx occlusion typically presents with ST elevation II ≥ III, often WITH ST elevation in lateral leads (V5–V6, I, aVL), not the isolated inferior pattern shown in **A**. - **Left main coronary artery (LMCA)**: LMCA occlusion causes massive, diffuse ischemia affecting anterior, lateral, and inferior territories simultaneously, with widespread ST elevation and ST depression. It does not produce the isolated inferior STEMI pattern shown in **A** and is incompatible with the patient's relatively stable presentation (no cardiogenic shock yet). **High-Yield:** Inferior STEMI = II, III, aVF ST elevation + reciprocal I, aVL depression = RCA occlusion in ~85% (right-dominant population). Always check V4R for RV infarct and avoid nitrates/diuretics if RV involvement is present. [cite: Harrison's Principles of Internal Medicine 21e, Chapter 274; 2022 ACC/AHA/HRS Guideline for the Management of Patients with STEMI]