On a standard 12-lead ECG, a patient shows a PR interval of 280 ms that remains constant with occasional dropped beats following every third P wave. Which conduction disorder best explains this finding?

• A First-degree AV block
• B Mobitz type I (Wenckebach) second-degree AV block
• C Mobitz type II second-degree AV block ✓
• D Third-degree (complete) AV block

Explanation

Mobitz type II second-degree AV block is characterized by a constant PR interval (often prolonged) followed by sudden non-conducted P waves without progressive lengthening of PR. In Wenckebach (Mobitz I), the PR progressively lengthens before a dropped beat. Mobitz II has a fixed PR with intermittent blocks, indicating a lower conduction defect in the bundle of His or bundle branches.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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