well if you trust the mechanic that tested the cylinder then it might be a cracked ring, cracked head,bad head gasket or valves. i would do my own compression test. if it is low then add some oil to the cylinder and see if it improves. if it does then its the rings, if not its something else. it is possible the mechanic is lying, you do have a misfire and it might be a simple fix. plug, wire, coil.