A patient in their 30s with a history of hypertension presented to the emergency department with acute chest pain that had begun 4 hours earlier. On examination, the patient’s blood pressure, heart rate, and respiratory rate were 117/78 mm Hg, 88 beats/min, and 20 breaths/min, respectively. A 12-lead electrocardiogram (ECG) was obtained (Figure) at the time of admission. The high-sensitivity troponin I level was 0.155 ng/mL (normal range: <0.034 ng/mL; to convert to μg/L, multiply by 1.0). The B-type natriuretic peptide and serum electrolyte levels were within normal limits. The patient was diagnosed with acute ST-segment elevation myocardial infarction (STEMI) and was immediately transferred to the catheterization laboratory.