o3 got this one right when I tried it, but o4-mini and GPT-4.5 did not. o3's response:

Because the surgeon is the patient’s parent. Here’s one way it lines up:

1. The patient (“the cousin”) is the surgeon’s son.

2. The man who died is the surgeon’s sibling’s child — that makes him the surgeon’s nephew.

3. Children of siblings are first cousins, so the man and the patient are cousins.

Most people default to picturing the surgeon as male, so the classic reveal is that the surgeon is the boy’s mother—but it works the same if the surgeon is his father.