Metal Gear 2: Solid Snake (2023 Ver.): Part 11: The Trial of Poisonous Zanzibar Hamsters
So it was revealed in Part 10 of this playthrough that Dr. Madnar was a double agent for Zanzibar Land. The deaths of both Gustava and Dr. Marv were of his doing, and he attempted to kill Snake for his Brooch from Gustava, too.
The MSX cartridge containing a copy of the late Marv's OILIX plans was inside at the locker in the room the doctors are lying dead in, Snake needs to use Gustava's Brooch in a different way to unlock that door.
Remember going into a sauna room? Causing the Brooch to morph into a special Key from the heat? Why not instead take that Brooch to a previous freezer room to turn it into a different kind of Key.
Unfortunately, this means going ALLLLLLL the way back to the Zanzibar Building again.
Also, one more time, Colonel's third and last Frequency should be 140.24 if you're wondering about that.
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Before reaching the Zanzibar Building floor that contains the freezer room, on 1F go to the same section that had the Red and Blue Cards and now try going UP. This should lead you to the Green Card, which merges Cards 7 through 9 together, which should be your final card upgrade in this game.
Now head to Zanzibar Building's 4F.
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At 4F, make your way through the whole floor and towards the Mess Hall. Go back into the freezer room. Just make sure you're well-healed again as this will instantly freeze all of your Rations again.
Equip Gustava's Brooch again. After a few seconds, it should morph into the key you need to unlock the locker in the dead Madnar and Marv's room in the Detention Center's basement. You're ready to go ALL the way back to there!
To get there, head back to the Tower Building, take the door at 13:40, then transform into a Cardboard Box and make your way into the truck. This will have a soldier deliver you past the broken bridge and back into the area of the Detention Center.
Time to go back to that basement.
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The ice key Brooch finally unlocks that locker! However...
There's more to check out in it... There's a hole on the other side. Crawl into it and...
FINALLY, Marv's MSX cartridge! Almost...
That room is somehow full of DEADLY POISONOUS ZANZIBAR HAMSTERS!!!
Don't joke around in this room. You can't get up from crawling, so you can't even shoot them right there. You have to lure these mice out of that hole, and thankfully there's a way how to. If not, you're screwed, because ONE SINGLE TOUCH from one of these rodents instantly kills you! I'm not kidding!
Crawl back out of the hole and this time equip B3 Rations (the yellow ones). These Rations contain cheese, which will slowly-but-surely draw those mice out of that hole and towards you.
As the squeaking little fur nuggets come crawling towards you to murder you cold, SHOOT THEM DEAD!
Very interesting that this game gave us alternative Rations and different uses for all three of them rather than just healing Snake. B1/Red ones for the sulfuric acid, B2/Green ones for the pigeon, and B3/Yellow ones for the toxic mice.
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So then Snake can finally get that MSX Cartridge. Now what? The two doctors are dead now...
Well... turns out that Madnar was just almost dead. With his dying breath, Madnar warned Snake that Gray Fox is planning to use Metal Gear D against him before he can leave Zanzibar Land.
Through his daughter Ellen, Madnar decided to tell Snake the way he can destroy this version of Metal Gear: Blow up its legs with Grenades!
Presumably, Madnar finally passed away after this info, but Snake couldn't stick around to see this as he was then suddenly dropped through a long trapdoor.
The end is coming...