Since there are 206 bones (and not a lot else in that exact quantity) in the typical adult human body, it's no secret that we seek a 3-letter code for the word BONE. This would be:
ALN
Why? Because the method used to encode body parts in this puzzle involves...
...finding the name of a country (in English) that contains the letters of the body part in question (not necessarily in order), removing those letters, and then alphabetising what remains!
For the examples given:
AH (1) = BRAIN from B(ah)RAIN
MN (2) = EYE from YE(m)E(n)
AIT (1) = NOSE from ES(t)ON(ia)
A (1) = CHIN from CHIN(a)
BIMU (2) = LEG from (b)ELG(ium)
AHIIP (10) = TOE from ET(hi)O(pia)
So to get BONE we need:
ALN (206) = BONE from (l)EB(a)NO(n)
:)
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