Could it be "Hand is to palm as what foot is to sole"?
I've found in a Wikipedia article, "Hand is to palm as foot is to ____?", and I wonder if the expression "A is to B what C is to D" is an omission of 'as' between the 'B' and 'what.' No?

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Answer

Hand/palm, sole/foot is correct. 'A is to B as C is to D' would work, the same as 'A is to B what C is to D.'; however, you would not add 'as' between 'what' and 'C'.  Use 'what' or 'as' but not both.