Which finding on ECG may obscure STEMI diagnosis?

• A. Normal sinus rhythm
• B. Hyperkalemia
• C. New left bundle branch block
• D. Atrial fibrillation

Answer: (C)

New left bundle branch block changes the way electrical activity is conducted, producing a wide QRS and secondary ST-T changes that can mimic or hide the ST elevations seen with a myocardial infarction. The abnormal conduction alters the baseline relationship between QRS polarity and the ST segments, making it hard to interpret classic STEMI patterns on the ECG. Because of this masking effect, a new LBBB is considered a STEMI-equivalent scenario, prompting urgent reperfusion if clinical suspicion remains high. Other patterns, like normal sinus rhythm, hyperkalemia, or atrial fibrillation, do not inherently distort the ST-segment pattern in the same way as a new LBBB, so they’re less likely to obscure STEMI diagnosis.