## Why option 1 is correct The rsR' pattern (M-shape) in V1 with QRS ≥120 ms represents complete right bundle branch block (RBBB). The pathophysiology is sequential: the intact left bundle branch first depolarizes the septum (small initial r-wave) and left ventricle (small s-wave), then the blocked right bundle forces the right ventricle to depolarize slowly via cell-to-cell myocardial conduction rather than rapid His-Purkinje fibers (tall delayed R' wave). This is the defining mechanism of RBBB and directly explains the characteristic rsR' morphology. [Harrison 21e Ch 235; Braunwald 12e Ch 12] ## Why each distractor is wrong - **Option 2 (Simultaneous depolarization via bypass tract)**: This describes Wolff-Parkinson-White (WPW) syndrome, which produces a short PR interval and delta wave, not an rsR' pattern. WPW involves an accessory pathway outside the AV node, not bundle branch pathology. - **Option 3 (Premature RV depolarization via accessory pathway)**: Again, this is WPW or other pre-excitation, not RBBB. The rsR' in RBBB is DELAYED, not premature, and occurs via myocardial conduction, not an accessory pathway. - **Option 4 (Abnormal LV repolarization secondary to hypertrophy)**: This describes left ventricular hypertrophy (LVH), which produces tall QRS complexes and secondary T-wave changes in lateral leads, not the characteristic rsR' in V1 with wide QRS. **High-Yield:** rsR' in V1 + QRS ≥120 ms = RBBB due to delayed RV depolarization via slow myocardial conduction; the single right bundle branch is easily disrupted (unlike the dual-fascicle left bundle), making RBBB a common finding in acute PE and anterior MI. [Harrison 21e Ch 235; Braunwald 12e Ch 12]
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