Are you certain it is not the lav drain that emits this odor?
This one would be hard to pinpoint without actually smelling this odor.
Because this is the only fixture in the house with an odor, I would eliminate the possible drain odor by pouring a mixture of baking soda and vinegar in the drain letting it sit for a few hours in the p-trap. This may solve the odor problem.
The Local Plumber
Tustin, California http://www.TheLocalPlumber.com
[Edited by TheLocalPlumber on 22 October 2000]