**Core Concept**
Differential cyanosis is a clinical phenomenon where there is a difference in the oxygen saturation of blood between the upper and lower limbs, or between the two sides of the heart. This occurs due to a significant difference in the systemic vascular resistance between the two circulations, often seen in conditions with patent ductus arteriosus, coarctation of the aorta, or in severe left-to-right shunts.

**Why the Correct Answer is Right**
Differential cyanosis is typically seen in conditions where there is a significant difference in the systemic vascular resistance between the two circulations, such as in coarctation of the aorta, where the lower limbs are more cyanotic due to decreased systemic vascular resistance in the lower limbs. Another classic example is patent ductus arteriosus, where the aorta is perfused through the ductus, resulting in differential cyanosis due to the mixing of oxygenated and deoxygenated blood. This phenomenon can also be seen in severe left-to-right shunts, such as in ventricular septal defects, where the decreased systemic vascular resistance in the lower limbs results in increased flow and decreased oxygen saturation.

**Why Each Wrong Option is Incorrect**
**Option A:** Tetralogy of Fallot - While tetralogy of Fallot is a congenital heart disease that can result in cyanosis, differential cyanosis is not a characteristic feature of this condition.
**Option B:** Aortic stenosis - Aortic stenosis can result in decreased cardiac output and increased systemic vascular resistance, but it is not typically associated with differential cyanosis.
**Option C:** Pulmonary embolism - Pulmonary embolism can result in acute right heart failure and decreased systemic vascular resistance, but it is not typically associated with differential cyanosis.

**Clinical Pearl / High-Yield Fact**
Differential cyanosis is often associated with significant left-to-right shunts or coarctation of the aorta, and it is essential to recognize this clinical phenomenon to differentiate it from other causes of cyanosis, such as cardiac arrest or severe respiratory disease.

**Correct Answer:** C. Coarctation of the aorta