**Core Concept**
Acute cor pulmonale, often resulting from a pulmonary embolism, is characterized by a sudden increase in pulmonary artery pressure, leading to right ventricular strain and potential failure. This condition is associated with specific electrocardiographic (ECG) changes that reflect the altered hemodynamics.

**Why the Correct Answer is Right**
The ECG feature of acute cor pulmonale is a right bundle branch block (RBBB) pattern, manifesting as a widened QRS complex (> 120 ms) with a distinctive "M" or "M-shaped" appearance in the QRS complex. This occurs due to the delayed activation of the right ventricle, which is overloaded and under strain. The right bundle branch block is a result of the increased resistance in the pulmonary vascular bed, which causes a delay in the electrical conduction through the right ventricle.

**Why Each Wrong Option is Incorrect**
**Option A:** Sine wave pattern - This is typically associated with hyperkalemia, not acute cor pulmonale.
**Option B:** P pulmonale - While this can be seen in pulmonary disease, it is not specific to acute cor pulmonale.
**Option C:** Left axis deviation - This is more commonly associated with left ventricular hypertrophy or posterior myocardial infarction.

**Clinical Pearl / High-Yield Fact**
In the context of acute cor pulmonale, it's essential to remember that the RBBB pattern can be a harbinger of impending right ventricular failure. This is a high-yield fact for exam purposes, as it highlights the importance of recognizing this ECG pattern in the setting of acute pulmonary embolism.

**Correct Answer: C. Left axis deviation**