The first time Dr. Marvin Monroe's death was referenced in a Simpsons episode is when a character was taken to the Marvin Monroe Memorial Hospital.

If you didn't read the hospital's sign, you were at a loss in the later-that-season clip show where Troy McClure asks the trivia question "Which popular characters have died in the last year?" (Answer given after commercial: "If you guessed Bleeding Gums Murphy and Dr. Marvin Monroe, you were wrong! They were never popular!")