**Core Concept**
Alternating right bundle branch block (RBBB) with left anterior hemiblock (LAHB) is a characteristic ECG pattern of a bi-fascicular block, involving conduction abnormalities in both the right bundle branch and the left anterior fascicle.

**Why the Correct Answer is Right**
This pattern reflects a block in two conduction pathways: the right bundle branch and the left anterior fascicle. The presence of alternating RBBB and LAHB indicates a bi-fascicular block, which is a conduction system disturbance involving the right bundle branch and the left anterior fascicle. It is distinct from first-degree AV block (which shows prolonged PR interval without blocked beats), complete heart block (with no atrioventricular conduction), or Mobitz type II block (with intermittent non-conducted P waves). Bi-fascicular block is clinically significant due to increased risk of sudden cardiac death and often represents a precursor to more serious conduction disease.

**Why Each Wrong Option is Incorrect**
Option A: First-degree AV block shows uniformly prolonged PR interval without blocked beats or bundle branch blocks, so it does not present with RBBB or LAHB.
Option B: Complete heart block shows no ventricular activation from atria, with a regular ventricular rhythm (often a ventricular escape rhythm), not alternating bundle blocks.
Option C: Mobitz type II block presents with dropped P waves and a constant PR interval before the drop, not with alternating RBBB and LAHB.

**Clinical Pearl / High-Yield Fact**
Bi-fascicular block is a high-risk conduction abnormality and a predictor of progressive heart disease; it often precedes complete heart block and requires close monitoring, especially in patients with ischemic heart disease or hypertension.

✓ Correct Answer: D. Bi-fascicular block