On ECG, which pattern is described as gradually lengthening PR intervals with a block that eventually leads to a dropped QRS?

• A. Mobitz II
• B. Third-degree AV block
• C. Mobitz I (Wenckebach)
• D. Atrioventricular Dissociation

Answer: (C)

This pattern tests recognizing a Wenckebach type of second-degree AV block, where conduction through the AV node progressively slows and a beat is eventually dropped. On the ECG you see the PR interval lengthen with each successive beat, and then a P wave occurs without a following QRS complex. After that dropped beat, the cycle restarts with a shorter PR interval and the sequence repeats.

The reason this is the best answer is that the defining feature is the gradual prolongation of the PR interval leading to a non-conducted P wave (a dropped QRS). The block is at the level of the AV node, which is why the delay accumulates until transmission fails, rather than a sudden break.

For contrast, a pattern where a QRS is dropped without prior PR lengthening points to Mobitz II, which reflects a block below the AV node or in the His-Purkinje system and has more risk of progression. Third-degree AV block shows complete AV dissociation with no fixed relationship between P waves and QRS complexes. AV dissociation is a broader term that can describe several situations where atrial and ventricular activities are not coordinated.