**Core Concept**
McCallum's patch refers to a characteristic electrocardiographic (ECG) finding associated with a specific cardiac condition. It represents a region of depolarization in the right atrium, often seen in patients with atrial enlargement or Wolff-Parkinson-White (WPW) syndrome.

**Why the Correct Answer is Right**
McCallum's patch is a distinctive feature on the P wave of an ECG, indicating abnormal atrial conduction. In WPW syndrome, this patch is a result of an accessory electrical pathway between the atria and ventricles, leading to premature activation of the ventricles. This aberrant pathway causes the characteristic delta wave and slurred upstroke in the QRS complex.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not related to McCallum's patch, as it is not a characteristic ECG finding for this condition.
**Option B:** While atrial enlargement can cause changes in the P wave, McCallum's patch is a more specific feature associated with WPW syndrome.
**Option C:** This option is incorrect, as McCallum's patch is not a characteristic feature of atrial septal defects.

**Clinical Pearl / High-Yield Fact**
Remember that McCallum's patch can be a subtle finding on ECG, but it's a crucial clue in diagnosing WPW syndrome, a condition that can lead to tachyarrhythmias and sudden cardiac death if not recognized and treated promptly.

**Correct Answer: C.**