**Core Concept**
Electric alternans is a cardiac phenomenon characterized by an alternating pattern of QRS complex morphology in the electrocardiogram (ECG), typically seen in conditions with abnormal ventricular filling or mechanical dyssynchrony. This phenomenon results from the varying orientation of the ventricular electrical axis during each cardiac cycle.

**Why the Correct Answer is Right**
The mechanism behind electric alternans is the result of ventricular contraction and relaxation. In conditions such as cardiac tamponade, massive pulmonary embolism, or severe left ventricular dysfunction, the ventricles experience varying degrees of mechanical dyssynchrony. This leads to a change in the electrical axis of the ventricles, causing an alternating pattern of QRS complex morphology on the ECG. The electrical axis of the ventricles is determined by the position and orientation of the ventricular septum and the direction of the electrical impulse through the ventricles.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not specified and cannot be evaluated.
**Option B:** This option is also not specified and cannot be evaluated.
**Option C:** This option is not specified and cannot be evaluated.

**Clinical Pearl / High-Yield Fact**
Electric alternans is a high-yield fact in cardiology, and recognizing this pattern on an ECG can be a crucial diagnostic clue for conditions such as cardiac tamponade or massive pulmonary embolism. Remember that electric alternans is often seen in conditions with abnormal ventricular filling or mechanical dyssynchrony.

**Correct Answer: C. Cardiac tamponade**