Medical Exam Wordplay

This is a wordplay joke. The setup presents a seemingly straightforward question about rearranging letters to form a body part. The expected answer, 'penis', is implied by the phrase "most useful when erect." However, the punchline reveals that those who answered 'spine' are doctors, suggesting that doctors might be too professional to think of the more obvious answer and implying that they do not have a sense of humor. It plays on the contrast between what is expected and what doctors would likely answer.