**Core Concept**
Cyanotic heart diseases are characterized by congenital heart defects that result in reduced oxygenation of the blood, leading to cyanosis. These defects often involve abnormal connections between the right and left sides of the heart or involve the great arteries.

**Why the Correct Answer is Right**
PDA (Patent Ductus Arteriosus) is a congenital heart defect where the ductus arteriosus, a normal fetal connection between the aorta and the pulmonary artery, fails to close after birth. This results in a left-to-right shunt, where oxygenated blood from the aorta flows into the pulmonary artery. In contrast to cyanotic heart diseases, PDA typically presents with signs of increased pulmonary blood flow, such as widened mediastinum, cardiomegaly, and pulmonary hypertension, but not cyanosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Tetralogy of Fallot (TOF) is a cyanotic heart disease characterized by four components: ventricular septal defect, pulmonary stenosis, right ventricular hypertrophy, and an overriding aorta. TOF is a classic example of a cyanotic congenital heart defect.
**Option C:** Tricuspid atresia is another cyanotic heart disease where the tricuspid valve is absent or abnormally developed, resulting in reduced blood flow from the right atrium to the right ventricle. This leads to decreased oxygenation of the blood and cyanosis.
**Option D:** Eisenmenger's complex is a cyanotic heart disease that occurs when a left-to-right shunt, such as a ventricular septal defect, becomes reversed due to pulmonary hypertension. This results in deoxygenated blood flowing from the right side of the heart to the left, leading to cyanosis.

**Clinical Pearl / High-Yield Fact**
Cyanotic heart diseases can be remembered using the "4 Ts": Tricuspid atresia, Tetralogy of Fallot, Transposition of the great arteries, and Truncus arteriosus. These conditions are all characterized by cyanosis and often require surgical intervention to correct.

**Correct Answer:**
✓ Correct Answer: B. PDA.