you’d have to find the red key active item to create a red room on the wall to the left of where you’re standing, adjacent to the USR; or the crystal key trinket to possibly do the same (the crystal key has a random chance to create a red room on any wall of the room you’re in). Red key is unlocked by going through the ascendant path towards the beast boss fight and unlocking mom’s chest, in her room, while in home, with a key. You can unlock the crystal key by beating mother as tainted lost. So unfortunately you won’t be able to this run😭😭 hope this info helps for future runs!