Why does the AC or sink drain smell bad in Egypt apartments/hotels and how to fix it?

If the smell only appears when the AC is turned on and disappears when it's off, the issue is likely inside the AC unit itself (clogged drain pipe or mold).

If the smell comes from the floor drain under the sink even with the AC off, it's a sewage odor caused by a missing water trap or a dried-out siphon.

A simple temporary fix: cover the drain hole with a piece of plastic/cellophane and place a water bottle on top to block the odor from escaping.