A 58-year-old man with hypertension presents to the emergency department with acute chest pain radiating to the left arm. His 12-lead ECG shows the pattern marked **B** in the diagram: ST elevation in leads II, III, and aVF with reciprocal ST depression in leads I and aVL, accompanied by sinus bradycardia (heart rate 48/min). Which of the following is the most likely culprit vessel responsible for this acute coronary syndrome?
A. Left circumflex artery (LCX)
B. Right coronary artery (RCA)
C. Left anterior descending artery (LAD)
D. Left main coronary artery (LMCA)
Explanation
Why Right coronary artery (RCA) is right
The ECG pattern marked B — ST elevation in the inferior leads (II, III, aVF) with reciprocal ST depression in I and aVL — is pathognomonic for acute inferior wall STEMI. The RCA is the culprit vessel in 80–90% of inferior MIs, as it typically supplies the posterior descending artery (PDA) that perfuses the inferior left ventricular wall in the right-dominant circulation (present in ~85% of people). The RCA also supplies the SA node (60%) and AV node (90%), explaining the bradycardia and conduction disturbances seen in this patient. The reciprocal ST depression in aVL has >95% sensitivity for true inferior STEMI and confirms the diagnosis. (ACC/AHA STEMI Guidelines 2022; Harrison's 21e Ch. 269)
Why each distractor is wrong
Left anterior descending artery (LAD): LAD occlusion causes anterior STEMI with ST elevation in V1–V4 and reciprocal changes in II, III, aVF — the opposite of this patient's ECG pattern.
Left circumflex artery (LCX): While a dominant LCX can rarely cause inferior STEMI (~10–20% of cases), it is much less common than RCA. Additionally, when LCX does cause inferior STEMI, the ST elevation in lead II is typically ≥ that in lead III; in RCA occlusion, ST elevation in III > II, which is often seen here.
Left main coronary artery (LMCA): LMCA occlusion causes diffuse ischemia with ST elevation in multiple territories (anterior, lateral, and inferior simultaneously) and is a catastrophic event with cardiogenic shock; this patient's pattern is localized to the inferior wall.
High-YieldNEET PG
Inferior STEMI = RCA occlusion (80–90%); always check V4R for RV infarction and reciprocal aVL depression confirms the diagnosis.
