**Core Concept**
Amyloidosis of the heart involves the deposition of amyloid fibrils in the cardiac tissue, leading to restrictive cardiomyopathy and dysfunction. This condition can affect the heart's electrical conduction system, causing arrhythmias.

**Why the Correct Answer is Right**
The deposition of amyloid fibrils in the heart can disrupt the normal functioning of the cardiac conduction system, leading to arrhythmias. This is because amyloid fibrils can infiltrate the cardiac tissue, including the His-Purkinje system, and disrupt the normal electrical pathways that control heart rhythm. This disruption can lead to a range of arrhythmias, including atrial fibrillation, ventricular tachycardia, and heart block.

**Why Each Wrong Option is Incorrect**
**Option B:** AV block is a possible complication of amyloidosis, but it is not the most common presentation. While amyloidosis can affect the cardiac conduction system, it typically presents with more diffuse conduction abnormalities rather than a fixed AV block.

**Option C:** Cardiac enlargement or increased mass/voltage can be seen in a variety of cardiomyopathies, but it is not a specific feature of amyloidosis. In fact, amyloidosis typically presents with a normal or decreased cardiac silhouette on chest X-ray.

**Option D:** Aortic stenosis (AS) is a valvular heart disease that can present with symptoms of heart failure, but it is not directly related to amyloidosis. While amyloidosis can affect the heart's function, it does not typically cause valvular disease.

**Clinical Pearl / High-Yield Fact**
Amyloidosis of the heart often presents with non-specific symptoms such as fatigue, shortness of breath, and peripheral edema. A key diagnostic clue is the presence of a low voltage QRS complex on the electrocardiogram (ECG), which can be seen in up to 90% of patients with cardiac amyloidosis.

**✓ Correct Answer: A. Arrhythmia**