The Dissection Room Key is an item you'll need to get early on in Resident Evil 7 as part of your escape from the Baker Family's Main House. To unlock the door to leave the Main House you need three dog head emblems which fit in the exit door in the Main Hall. Unfortunately, one of the emblems is in the Basement Processing Area in the Dissection Room. To unlock the Dissection Room, though, you'll need to find the location of the Dissection Room Key, which is located in the Incineration Room.
Getting to the Incineration Room in Resident Evil 7
Once you make it to the East Wing of the first floor of the Main House, you'll find the stairs leading into the Basement Processing Area. Take a left and carefully make your way to the first door you can open. There will be Molded in your path, so you'll have to choose whether to fight or run past them. Once you make it to the Incinerator Room you'll have to kill the Molded in there; then you can relax for a moment before you attempt to get the Dissection Room Key.
Finding the Dissection Room Key in Resident Evil 7
You'll find a note above the sink in the Incinerator Room that alludes to the solution to the puzzle presented before you. The key to solving the puzzle is held in the second paragraph: "Just remember 3 a's and a handprint."
If you look at the incinerators, you'll notice they have names taped on or inside of them and one has a red handprint on the outside of it. To solve the puzzle is simpler than it initially seems. You want to open the incinerators so that the total number of "a's" in the names they're labeled with equals three, and that the incinerator with the red handprint is open.
The solution to the puzzle is:
Open the last incinerator first labeled: Tamara.
Then open the third incinerator with the handprint and the name: Craig.
There you go, you've got your three "a's" and a handprint. The first incinerator will pop open, and you just have to inspect it to open it.
However, a Molded resides in it and you'll have to fight it before you can collect the Dissection Room Key which is laying on the slab of the first incinerator.
We hope this puzzle guide helped you. If you have trouble with the Birthday Cake Tape puzzle, make sure to check out our guide for that. For more helpful info on RE7, check out our Resident Evil 7 guide hub.