Doctor Shortcoming

A medical student was out walking outside a large medical office complex one day, and came upon a little girl, playing with a pile of ****. "Missy, what are you doing?" asked the doctor. "I'm building a Nurse," said the girl.
The medical student, thinking this was quite funny, returned with a surgeon, and asked the same question. Again, the girl replied that she was building a Nurse.
The surgeon, also thinking it was funny, went back to the medical office building, and brought a cardiologist with him. Again the question was asked, and the reply was the same.
The cardiologist then asked the girl why she was building a Nurse. The girl replied, "Because I don't have enough **** to make a doctor."