The whole riddle is set up to be intentionally difficult. The way it primes you with "father" and "son" as parent comment mentions. I think there's some validity to it, but I think it's a clever linguistic trick more than anything.

I actually read the parent comment before going to the article and I still had a hard time figuring out the answer. It may be because I was primed with the fact it's a "riddle"; the entire time I was trying to come up with a clever answer rather than thinking the surgeon was the mother. Father+daughter vs father+son didn't have much of an effect, but bias was probably present, nevertheless.