Urine smell in bathroom sink
The drain pipe on your sink goes straight down, but then it should go through a U-shape and go back up again for a few inches, and then go on its merry way.  

This U-shaped bit of pipe has a name, and Admiral Ackbar really wants to say it... 

Anyway, the idea of this "trap" is that the U is supposed to stay full of water all the time, all the way up to the nape if the "U".  That way, this water blocks the pipe, so sewer gases cannot come up the pipe.  A couple things can go wrong with that.  

First, the water ultimately drains to somewhere else.  If that final exit is below the "nape" of the U, the U won't be full of water, and it won't seal the pipe. 

If a sink is rarely used, that "U" will actually dry up from normal evaporation, and the sewer gases will get through.  This is especially a problem with dual sinks where no one ever uses one of them, tubs nobody ever uses, basement slop sinks, or unused washing machine connections. 

Lastly, sewage makes gas by decomposition.  That gas has to go somewhere.  The sewer-pipe system in your house is supposed to be ventilated, via a "stack" / exhaust pipe that goes to the top of your house.  If that is plugged or malfunctioning, the sewer pipe will pressurize a bit and push the water up out of the trap enough for gases to get through.