Cathedral Key

Cathedral Key is a quest item in Pathologic. It is obtained by the Bachelor on Day 4 as part of the main quest.

The Bachelor will attempt to examine the Cathedral, but be unable to do so because the building is locked. A young woman near the entrance will tell him where to find her relative, a young man, who has the Cathedral Key. The Bachelor must find and escort the young man out of the indicated infected district in order to obtain the Cathedral Key.

Quote
"Now the building can be inspected and appropriated for sanitary needs if it really does have water. Hopefully, it won't be seen as sacrilege — the Cathedral doesn't seem to be functional… Maybe this is why things turned out as they did?"