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After speaking to Flossy about the Altered Biocell in Cell Block A and agreeing to his delivery request, you'll receive a secondary objective that will task you with bringing the item to Red Shoes in Cell Block B. At this point, follow the objective waypoint and it will lead you to a small outdoor courtyard in between both cell blocks. There is a Maintenance Shed on the eastern side of this courtyard, but its keypad is locked. If you were successful in getting a Multitool in Cell Block A for the "Quid Pro Quo" achievement, then sneak your way to the shed's door and use this item to open the keypad and enter the room. In the back of the shed, you will find a Cell Block B jumpsuit inside a semi-opened locker. Simply interact with it in order to put it on and unlock the achievement.

If you don't have a Multitool in order to open the door to the Maintenance Shed, however, you can still reach this room another way. Make your way to the showers on the ground floor of Cell Block A. Jump on top of the lockers in the corner of the room and then jump onto the showers. You should be able to find a ceiling vent above a large heating conduit. Head inside and make a manual save. When you exit into the large conduit, make an attempt to run through the electricity field on the right and past the closed gate. Now quickly find and activate the breaker switch on the right-hand side of the vent to disable the electrical current. From here, you can turn around and crawl underneath the second fan to find another ventilation shaft on the left. Climb through, open the grate on the other side, and you will now find yourself inside the Maintenance Shed. Again, interact with the Cell Block B jumpsuit inside the locker and the achievement will unlock.