I think I've finally figured out the answer to this age old question, and I hope that others have come to the same conclusion. As we've seen in The Trouble With Tribbles and DS9's Trials and Tribbleations, our favorite "southern country doctor" is able to pick out a Klingon spy disguised as a human using a bioscan, yet in TUC he has no clue how to treat Chancellor Gorkons injuries because 'he doesn't know (Gorkons) anatomy. I believe that how this can be has been finally answered by ENTs explanation of how Klingons with and without forhead ridges can exist in the same universe. The virus that was created using human augment DNA to make Klingon augments not only smoothed out the foreheads but also jumbled the internal organ arrangement into something close to human standards. Throughout the five year mission, McCoy has studied Type Two Klingons (TOS no ridges) but never Type One Klingons (spinoffs ridges) because almost none existed during TOS. By the time of TMP, the effects of the virus had been reversed and no more Type Two Klingons were being born. Possibly any surviving Type Two Klingons undergo 'cranial reconstruction' to fit into Klingon society better. By the time of TUC, Type One Klingons are the only Klingons sculking about the quadrant, the ones McCoy never studied and thus, could not save Gorkon because of his 'unknown anatomy'.
I know, I know. There are those fans who don't wish to include ENT as canon. You're all entitled to your own opinions, of course. I'm just stating how I see the Star Trek universe revolving and evolving. Also if any mods out there think this thread belongs somewhere else, I respectfully apologise for putting it here.
Now, what do you all think?