## **Core Concept**
Differential cyanosis refers to a condition where there is a difference in the oxygen saturation of blood between the upper and lower parts of the body, leading to cyanosis in one part but not the other. This phenomenon typically occurs in conditions where there is a right-to-left shunt at the cardiac level or in the pulmonary vasculature.

## **Why the Correct Answer is Right**
The correct answer, **D. Tetralogy of Fallot with patent ductus arteriosus (PDA)**, is a condition where differential cyanosis can occur. Tetralogy of Fallot is a congenital heart defect that includes four components: ventricular septal defect, pulmonary stenosis, right ventricular hypertrophy, and an overriding aorta. When there is a patent ductus arteriosus (PDA) in conjunction with Tetralogy of Fallot, blood can shunt from the aorta to the pulmonary artery. If the pulmonary vascular resistance is high, the shunt can be reversed (Eisenmenger's syndrome), leading to deoxygenated blood from the lower body bypassing the lungs and going directly to the aorta, causing differential cyanosis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is blank and does not provide a condition to evaluate.
- **Option B:** This option is also blank and does not provide a condition to assess.
- **Option C:** Similarly, this option is blank and lacks a condition to consider.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that differential cyanosis is a rare but important clinical sign that suggests a significant underlying cardiac or pulmonary vascular anomaly. In the context of Tetralogy of Fallot with PDA, the presence of differential cyanosis indicates severe pulmonary hypertension leading to a reversed or bidirectional shunt.

## **Correct Answer: D. Tetralogy of Fallot with patent ductus arteriosus (PDA).**