Relevant Findings: The correct answer is sinus rhythm with right bundle branch block. The underlying rhythm is sinus rhythm. This is determined via calculation of the heart rate, determination of the cardiac rhythm, and inspection of the relationship of the P waves and QRS complexes. The average heart rate is 200 beats per minute. The rhythm is regular. There is a P wave for every QRS complex and a QRS complex for every P wave providing a diagnosis of sinus rhythm. The primary alteration with this ECG is the abnormal morphology of the QRS complexes and the associated deviation of the mean electrical axis to 240 degrees (or -120 degrees). The QRS complexes display S waves in leads I, II, III and aVF suggesting either right ventricular enlargement or right bundle branch block. The QRS duration is prolonged at 0.07 seconds suggesting a conduction delay. Therefore right bundle branch block (as opposed to right ventricular enlargement) would be the most likely conduction alteration to produce S waves in the visible leads, right axis deviation, and prolongation of the QRS complex. This ECG is sometimes mistaken for ventricular tachycardia. Ventricular tachycardia typically displays AV dissociation without a fixed relationship between the P waves and QRS complexes. View an annotated image of Jack’s ECG highlighting the relevant findings.