A 50-year-old man presents with syncope. ECG shows PR interval gradually prolonging over several beats followed by a dropped QRS (Wenckebach phenomenon), and this cycle repeats. The level of block is most likely:
- A Bundle of His — below the AV node
- B Both bundle branches simultaneously
- C AV node — above the bundle of His ✓
- D Sinoatrial node
Correct answer: C. AV node — above the bundle of His
Explanation
Mobitz Type I (Wenckebach) second-degree AV block shows progressive PR prolongation with eventual non-conducted P wave and typically indicates block at the AV node level, which is above the bundle of His. This is generally a benign conduction disorder with a stable junctional escape rhythm and often does not require pacing unless symptomatic. Mobitz Type II block occurs at or below the bundle of His and carries a worse prognosis, requiring pacemaker implantation.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.