A new mother was concerned b/c just before they discharged t…

A new mother was concerned b/c just before they discharged the mother and the newborn infant from the hospital the nurse administered a “heel prick” test.  It was explained to the mother the heel prick and the blood sample taken was to look for indications in the blood of rare medical conditions, such as phenylketonuria or PKU.  What region of the baby’s body was this blood taken?