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The ClueKnee, to an M.D. (4 letter answer)
NotesThe answer GENU is rarely seen, appearing only about once every 1800 puzzles.
Knee, to a zoologist
It's flexed by a kneeler
DefinitionGENU as a noun:
1. (knee, knee joint, human knee, articulatio genus, genu) = hinge joint in the human leg connecting the tibia and fibula with the femur and protected in front by the patella