kid knee
A child's knee. originally thought to be a subdivision of kidney but was later discovered to be an entirely differant kettle of fish, the only similarity being in prenounciation. Unfortunately, few anatomists and surgeons are able to differentiate between the two. Kidney failure is the number one cause of death amongst small children in hospital.
"I'm afraid the operation will require one of your child's knees to be removed"
"but billy's knees are fine! The problem was with his urination!"
"quiet you! surgeons! commence kid knee amputation!"

(some time later)

"billy is dead"