I have recently been looking at a past post that questioned what Simpsons characters have died. This got me to thinking, we never found out what happened to Dr. Marvin Monroe. They said he died in 3F31 "The Simpsons 138th Episode Spectacular," but how? Maybe someone can unlock the secret.
He had a massive inferiority complex, so he moved to Canada.
"I used to be with it, but then they changed what "it" was. Now, what I'm with isn't it, and what's "it" seems weird and scary to me" - Abraham Simpson
this has been talked about quite a bit since the first revelation that Dr. Monroe had died: in "Who Shot Mr. Burns Part 2", there's a "Marvin Monroe Memorial Hospital" in Springfield.
IIRC, the last time we saw Dr. Monroe alive was in "The Springfield Connection", when he was with Rev. Lovejoy when they were escorting Hans Moleman to the electric chair.
there have been some theories about what happened - my favorite has to be that a number of his disgruntled patients ganged up on him and locked him up in his own Monroe Box :)
Infinity Dr. Marvin Monroes to flush down Infinity Dr. Marvin Monroes... You flush one down, he spins around Infinity Dr. Marvin Monroes to flush down...