Question: A man and his son are in a terrible accident and are rushed to the hospital in critical care. The doctor looks at the boy and exclaims "I can't operate on this boy, he's my son!" How could this be?

Answer: It's the boys mother.

Other Answer(s):

The son was adopted, his father who died in the car crash was his adoptive father and the doctor was his birth father.

The mom wants her son to die because she's such a wonderful parent!

The father who died wasn't the father, it was a kidnapper who abducted the son and claimed that he was the father.

The original answer is kinda obvious though