A word like renally can be formed by construction, and be a genuine English word regardless of whether it is listed in any dictionaries. Nevertheless, when an adverb is used occasionally, dictionaries - at least a few of them - are liable to include it, even if only as a run-on entry to the adjective it is derived from, with no specific definition. The other words you mention, MK, can all be found in a few dictionaries, but renally cannot, as far as I can see.
The word has been used at times, often in "renally impaired", but also in "renally excreted". But the few examples I found were all in academic journals or other technical publications like the Oxford Textbook of Clinical Nephrology (see page 1405).
I feel this word falls below my threshold of obscurity for acceptance in the puzzle.