Occasionally the USMLE may present a case in which the lower limbs are cyanosed but the upper limbs are not. A condition referred to as Differential Cyanosis.
Here's the differential diagnosis;
Persistent Ductus Arteriosus + Pulmonary hypertension
So when the pulmonary tree pressure increases the left to right shunt in the PDA will be reversed and "cyanosed" desaturated blood from the pulmonary circulation will shunt through the PDA to the descending aorta causing the cyanosis to be seen in the lower limbs only. (sometimes the PDA connects before the left subclavien artery and in this case the cyanosis can also be seen in the left hand nails).
Preductal Coarctation (Infantile type) + PDA
In this case the PDA is linking after the constriction and since blood pressure drops downstream from a constriction (check your physiology notes) then the blood will likely shunt from the pulmonary side to the descending aorta even without the need for pulmonary hypertension and so babies will have blue feet but pink hands.
Transposition of the great vessels (TGA) + PDA + Pulmonary hypertension
In this case the saturated blood enters the pulmonary artery from the left ventricle and then shunted through the PDA to go down the lower part of the body and resulting in Reverse Differential Cyanosis where the lower limbs are pink and the upper limbs are blue!