This one's actually kind of easy, but finding your way in pitch black darkness is annoying.
Start the Hospital memory. Pick up the flashlight.
Find the four "vital signs" indicators. (They light up red, and when you activate them, everything turns white, one of the four lights above the exit door turns green, and the vital signs indicator lights up green.) Commit to memory where they all are! (Or draw a map. Whatever works for you.)
Exit to the main menu, then Continue and reenter the Hospital memory. Do NOT pick up the flashlight.
Now go find and activate those four vital signs indicators. Once done, exit through the door. Finish the level.
Congratulations, you've unlocked the achievement!