**Core Concept**
McCallum's patch is a characteristic radiographic finding observed in patients with a specific type of cardiac abnormality. It is associated with an area of increased density in the left atrium, which is a result of the abnormal flow of blood.

**Why the Correct Answer is Right**
McCallum's patch is seen in patients with left atrial myxoma. A myxoma is a type of benign tumor that arises from the endocardium of the left atrium. The tumor obstructs blood flow, leading to increased pressure and subsequent thickening of the atrial wall, which appears as a patch of increased density on a chest radiograph. This abnormality is a result of the body's attempt to compensate for the obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** Not applicable.
**Option B:** Not applicable.
**Option C:** Not applicable.

**Clinical Pearl / High-Yield Fact**
McCallum's patch is a classic radiographic finding that should raise suspicion for a left atrial myxoma, which is the most common type of primary cardiac tumor.

**Correct Answer: C. Left atrial myxoma.**