**Core Concept**
Cyanosis is a clinical sign characterized by a bluish discoloration of the skin and mucous membranes, indicating a decreased oxygen saturation in the blood. It can result from various congenital heart defects, respiratory conditions, or circulatory issues.

**Why the Correct Answer is Right**
Tetralogy of Fallot (TOF) is a congenital heart defect that involves four main abnormalities: ventricular septal defect, pulmonary stenosis, right ventricular hypertrophy, and an overriding aorta. This condition leads to a significant reduction in pulmonary blood flow, resulting in decreased oxygen saturation in the blood and consequently causing cyanosis. The right-to-left shunt in TOF allows deoxygenated blood to bypass the lungs and directly enter the systemic circulation, further contributing to cyanosis.

**Why Each Wrong Option is Incorrect**
**Option B:** Patent ductus arteriosus (PDA) typically results in a left-to-right shunt, increasing pulmonary blood flow and oxygenation, rather than causing cyanosis.

**Option C:** Tricuspid atresia is a congenital heart defect characterized by the absence of the tricuspid valve, which can lead to cyanosis. However, it is not the most common cause of cyanosis compared to TOF.

**Option D:** Eisenmenger's complex is a condition that results from uncorrected left-to-right shunts, leading to pulmonary hypertension and eventual reversal of the shunt to right-to-left. While it can cause cyanosis, it is a secondary consequence of other congenital heart defects, rather than a direct cause.

**Clinical Pearl / High-Yield Fact**
Cyanosis is a late sign of congenital heart disease, often appearing after the first few weeks of life. Early recognition and diagnosis of congenital heart defects, like TOF, are crucial to prevent long-term consequences.

**✓ Correct Answer: A. TOF**