A 58-year-old man with hypertension presents to the emergency department with acute-onset chest pain radiating to the back. His 12-lead ECG shows horizontal ST-segment depression ≥0.5 mm in leads V1–V3 with tall, wide R waves (R/S ratio >1) in V2. The structure marked **A** in the diagram is suspected. Which of the following coronary arteries is MOST likely occluded in this clinical scenario?
A. Acute marginal branch of the right coronary artery
B. Left circumflex artery (in a left-dominant system) or distal right coronary artery (in a right-dominant system)
C. Diagonal branch of the left anterior descending artery
D. Left anterior descending artery with septal branch involvement
Explanation
Why "Left circumflex artery (in a left-dominant system) or distal right coronary artery (in a right-dominant system)" is right
The clinical presentation—horizontal ST-segment depression in V1–V3 with tall, wide R waves and R/S ratio >1 in V2—is the classic ECG mirror image signature of posterior MI (marked A). According to Braunwald Heart Disease 12e, Chapter 56, posterior (inferobasal/posterolateral) MI is caused by occlusion of the left circumflex artery in 80–90% of left-dominant systems or the distal right coronary artery (via the posterior descending artery) in right-dominant systems. The anterior precordial leads record the injury current in mirror image because there are no leads directly over the posterior wall. This patient requires posterior leads (V7–V9) to confirm ST elevation and establish STEMI-equivalent status, mandating emergent reperfusion.
Why each distractor is wrong
Left anterior descending artery with septal branch involvement: LAD occlusion causes anterior STEMI with ST elevation in V1–V4 and reciprocal changes in inferior leads, not the mirror-image pattern of posterior MI. Septal involvement does not produce tall R waves in V2 with the characteristic posterior MI signature.
Diagonal branch of the left anterior descending artery: Diagonal branch occlusion causes anterolateral STEMI with ST elevation in I, aVL, and V1–V4, not the isolated horizontal ST depression and tall R waves in V1–V3 characteristic of posterior wall ischemia.
Acute marginal branch of the right coronary artery: The acute marginal branch supplies the right ventricular free wall and lateral RV; occlusion causes right ventricular infarction with ST elevation in V4R, not the posterior wall injury pattern seen here.
High-YieldNEET PG
Posterior MI is under-recognized on standard 12-lead ECG because the posterior wall is "behind" the anterior leads—mirror-image ST depression and tall R waves in V1–V3 are the diagnostic clue; always obtain V7–V9 to confirm STEMI-equivalent status and guide emergent reperfusion.
Braunwald Heart Disease 12e, Ch 56
Practice similar questions
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.
