A 58-year-old man presents to the emergency department with acute onset severe substernal chest pain radiating to the left arm and jaw, associated with diaphoresis and dyspnea. His 12-lead ECG shows ST elevation ≥1 mm in leads I and aVL with reciprocal ST depression in leads II, III, and aVF. The pattern marked **A** in the diagram is consistent with acute lateral STEMI. Which of the following is the most likely culprit vessel responsible for this presentation?

Question

Accepted Answer

C. Left circumflex (LCx) coronary artery or first diagonal branch (D1) of the left anterior descending artery. ## Why Left circumflex (LCx) or first diagonal (D1) of LAD is right

Acute lateral STEMI with ST elevation in contiguous lateral leads (I, aVL = high lateral; V5-V6 = inferolateral) and reciprocal ST depression in inferior leads (II, III, aVF) is pathognomonic for occlusion of the LEFT CIRCUMFLEX (LCx) coronary artery or the FIRST DIAGONAL BRANCH (D1) of the LEFT ANTERIOR DESCENDING (LAD). These vessels supply the lateral wall of the left ventricle. The reciprocal ST depression in inferior leads reflects the opposing electrical vectors from lateral wall ischemia. Per the 2023 AHA/ACC STEMI Guidelines and Harrison's 21e, LCx occlusions are notably the 'electrocardiographically silent' STEMI and require high clinical suspicion, but when ST elevation is present in lateral leads with this distribution, LCx or D1 is the culprit in >95% of cases.

## Why each distractor is wrong

- **Right coronary artery (RCA)**: RCA occlusion causes INFERIOR STEMI with ST elevation in leads II, III, aVF — not lateral leads. This patient has the opposite pattern (ST elevation lateral, ST depression inferior), making RCA unlikely.
- **Left main coronary artery**: LM occlusion typically causes diffuse, severe ST elevation across anterior, lateral, and inferior leads with profound hemodynamic collapse. This patient's pattern is localized to lateral leads, inconsistent with LM disease.
- **Left anterior descending artery proximal to the first diagonal branch**: Proximal LAD occlusion causes ANTERIOR STEMI with ST elevation in V1-V4 and reciprocal changes in inferior leads. This patient lacks anterior ST elevation, making proximal LAD unlikely.

**High-Yield:** Lateral STEMI (ST elevation I, aVL ± V5-V6 with reciprocal ST depression II, III, aVF) = LCx or D1 occlusion; LCx is the "silent STEMI" — high suspicion needed even without obvious ECG changes.

[cite: 2023 AHA/ACC STEMI Guidelines; Harrison's Principles of Internal Medicine 21e]

Answer

A. Right coronary artery (RCA)

Answer

B. Left main coronary artery

Answer

D. Left anterior descending artery proximal to the first diagonal branch

Explanation

Why Left circumflex (LCx) or first diagonal (D1) of LAD is right

Why each distractor is wrong

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