A 64-year-old man with known anterior wall myocardial infarction 2 years ago presents with an episode of dizziness. ECG shows a PR interval fixed at 200 ms with sudden dropped QRS complexes occurring without any prior prolongation of the PR interval. There are two consecutive non-conducted P waves at one point. Which structure is most likely the site of this block?
- A His-Purkinje system ✓
- B AV node
- C Sinoatrial node
- D Left bundle branch only
Correct answer: A. His-Purkinje system
Explanation
Mobitz II second-degree AV block shows a constant PR interval before dropped beats, indicating intermittent failure of conduction below the AV node, in the His bundle or Purkinje system. It often follows anterior infarction and carries a high risk of progression to complete heart block with a slow, unreliable ventricular escape rhythm. In contrast, infranodal blocks do not show the gradual PR prolongation seen in nodal (Wenckebach) disease.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.