**Core Concept**
The most common cause of heart block in infants involves a congenital or acquired abnormality affecting the electrical conduction system of the heart. This condition arises due to a disruption in the normal sequence of electrical impulses that regulate heartbeats, often resulting from anatomical or functional defects in the atrioventricular (AV) node or its surrounding tissues.

**Why the Correct Answer is Right**
The most common cause of heart block in infants is congenital complete atrioventricular block (CAVB). This condition is often associated with maternal autoantibodies, such as anti-Ro (SS-A) and anti-La (SS-B), which can be transferred to the fetus during pregnancy. These autoantibodies may bind to the AV node and surrounding tissues, leading to a disruption in the electrical conduction system and resulting in heart block. In some cases, CAVB can also be caused by genetic mutations or other congenital anomalies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify the most common cause of heart block in infants.

**Option B:** This option is incorrect as it refers to a different condition affecting the heart, but not the most common cause of heart block in infants.

**Option D:** This option is incorrect as it is a potential cause of heart block, but not the most common cause in infants.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that congenital complete atrioventricular block is often associated with maternal autoantibodies, and a thorough maternal history is crucial in diagnosing this condition.

**Correct Answer:** C.