Technically, she could be referring to the original meaning of the two-word phrase as opposed to the eponymous use.
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Charlie horse & Goose egg - commonality = animal
Bum knee & Green thumb - commonality = body part
Tennis elbow & Trick shoulder - commonality = body joint
Lead foot & ? - commonality = body extremity (from the conclusion that all the other commonalities are biological)
Therefore, your question could be read, "solve for x". (I thought you were into grammar, not mathematics)
General commonality = biological. Commonality sequence in list one = commonality word always second. Commonality sequence in list 2 = first word, second word, second word.... Since was no indication of a continuance, logically the missing word in the second list should switch back to being the first word. Truck is to lead (non biological) as hand is to foot (biological).
(humor)
I love you, Man.
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