Dr. Joseph M'Benga was a Starfleet doctor who performed his medical internship on Vulcan. This question asks why he didn't operate on the Vulcan Sarek in "Journey to Babel". The answer to that question establishes the timeline:
- M'Benga completes his internship on Vulcan.
- M'Benga is assigned as chief medical officer of the Enterprise under Captain Pike.
- "In 2267, however, M'Benga returned to Vulcan to further his studies."
- The events of Journey to Babel (S2E10) occur. M'Benga is not aboard to help Sarek.
- M'Benga is re-assigned to the Enterprise.
- The events of A Private Little War (S2E19) occur. This time, M'Benga is available and helps Spock.
But the chronology of season 3 raise another question.
- Spock's Brain (S3E1) is abducted. M'Benga does not appear in this episode, even though he clearly would be the best person to take care of Spock's body and reattach it to his brain. Instead, those duties are performed by Dr. McCoy.
- However, M'Benga appears again in That Which Survives (S3E17). There is no mention of him leaving the ship between his two appearances.
What is the in-universe explanation why Dr. M'Benga didn't take care of Spock in the episode Spock's Brain? (Support your answer with sources, no opinions.)