Presumably, the original quote that would _not_ stump an LLM is "A father and a son are involved in a car accident. The father dies, and the son is taken to the emergency room. At the emergency room, the surgeon remarks "I cannot operate on this person, he is my son. How is this possible?"

Where the original gotchya is that the Surgeon can be the son's mother or other adoptive parent.

The modification catches the LLM because with the modification, the surgeon could just be the cousin's parent -- father or mother -- so there is no gender/sex at play here but the LLM continues to remark that there is, therefor exposing its statistical training sets.

The original, well-known version of the riddle starts "A man and his son..." so that it appears to present a paradox if your instinctive assumption is that the surgeon must be a man. The op's prompt alters this so that there is no potential paradox, and it tests whether the model is reasoning from the prompt as written, regardless of the presence of the original riddle in its training data.

the unaltered question is as follows:

A father and his son are in a car accident. The father dies at the scene and the son is rushed to the hospital. At the hospital the surgeon looks at the boy and says "I can't operate on this boy, he is my son." How can this be?

to spoil it:

the answer is to reveal an unconscious bias based on the outdated notion that women can't be doctors, so the answer that the remaining parent is the mother won't occur to some, showing that consciously they might not still hold that notion, but they still might, subconsciously.