**Core Concept**
Congenital heart disease refers to structural abnormalities of the heart present at birth, which can lead to various complications, including heart failure in infancy. These defects can range from mild to severe and are often associated with genetic or environmental factors.

**Why the Correct Answer is Right**
Congenital heart disease is the most common cause of heart failure in infancy due to the presence of structural anomalies that disrupt normal heart function. These anomalies can include septal defects, outflow tract obstructions, or other abnormalities that impede blood flow and lead to increased workload on the heart, ultimately resulting in heart failure. The most common types of congenital heart disease associated with heart failure in infancy include ventricular septal defects, atrial septal defects, and tetralogy of Fallot.

**Why Each Wrong Option is Incorrect**
**Option A:** Myocarditis, an inflammatory condition of the heart muscle, is a rare cause of heart failure in infancy, as it typically presents in older children or adults. While myocarditis can lead to heart failure, it is not the most common cause in infancy.
**Option B:** Rheumatic fever, an autoimmune condition that can cause heart valve damage, is a rare cause of heart failure in infancy, as it typically occurs in response to group A beta-hemolytic streptococcal infections, which are uncommon in infants. Rheumatic fever is more commonly associated with heart valve problems in older children and adults.
**Option C:** Cardiomyopathy, a disease of the heart muscle, can cause heart failure in infancy, but it is less common than congenital heart disease. Cardiomyopathy can be caused by various factors, including genetic mutations, infections, or toxins.

**Clinical Pearl / High-Yield Fact**
Infants with congenital heart disease may present with symptoms such as tachypnea, tachycardia, and feeding difficulties, which can be subtle and nonspecific. A high index of suspicion is essential for early diagnosis and management of congenital heart disease in infancy.

**✓ Correct Answer: D. Congenital heart disease**