**Core Concept**
Congenital heart disease can be classified based on the direction of blood flow through the heart. One such classification is left-to-right shunt, where blood flows from the left heart to the right heart. This can be further divided into two subcategories: increased pulmonary blood flow and decreased pulmonary blood flow. The question focuses on the latter, where there is a reduction in pulmonary blood flow.

**Why the Correct Answer is Right**
A left-to-right shunt with decreased pulmonary blood flow typically involves an obstruction to blood flow from the right heart to the left heart. This can be due to various reasons such as pulmonary stenosis, atresia, or other obstructive lesions. In such cases, the right heart pumps blood into the pulmonary arteries against increased resistance, resulting in decreased pulmonary blood flow. The most common congenital heart disease associated with decreased pulmonary blood flow is Tetralogy of Fallot (TOF).

**Why Each Wrong Option is Incorrect**
**Option A:** Transposition of the Great Arteries (TGA) is a congenital heart disease associated with increased pulmonary blood flow, not decreased. In TGA, the aorta arises from the right ventricle and the pulmonary artery from the left ventricle, resulting in parallel circulation.

**Option B:** Tricuspid Atresia is a congenital heart disease characterized by the absence of the tricuspid valve, resulting in decreased pulmonary blood flow. However, it is not the most common congenital heart disease associated with decreased pulmonary blood flow.

**Option D:** Ebstein's Anomaly is a congenital heart disease characterized by the downward displacement of the tricuspid valve into the right ventricle. This can result in decreased pulmonary blood flow, but it is not the most common congenital heart disease associated with decreased pulmonary blood flow.

**Clinical Pearl / High-Yield Fact**
The classic clinical features of Tetralogy of Fallot include cyanosis, clubbing, and a harsh systolic ejection murmur. The cyanosis is due to the decreased pulmonary blood flow, which results in a right-to-left shunt. The clubbing is due to chronic hypoxia.

**Correct Answer:** C. Tetralogy of Fallot