It could be his mother, but it could also be God, and it could also be his biological father/mother if he's adopted.

I passed, I'm smaaaaaarrrtttttttt!!

the mom 

It took some thinking though

i pictured it being his dad but his dad died in the car crash "/

......I can't believe I hesitated for a second.  Wtf haha!

Ummmmm. Isn't the asnwer obvious? Not everyone is sexist. I can clearly see that the doctor is.... DARTH VADER! Boy, "soy tu padre, mijo!"

Lol, in all seriousness it is the mother.

Actually you are all wrong. This test is to see if you oppose gay marriage. It's his other dad. :P

Assuming that the average child has two parents, a mother and a father, and that the father is dead.

That only leaves the mother.

Maybe its cos im a girl, but i "got it" instantly. i dont understand how that could ever be difficult.

Also, female doctors arent rare! havent any of you guys ever seen Scrubs??

*goes back to the kitchen*

Eh? I got that instantly, I got it so quickly I thought "His mother" would be the sexist answer. So I started to think of how this could be a trick question, eventually giving up I scrolled down and was disappointed by this non-riddle.