What would cause a foul smell from the bathroom sink when no other sinks bathtubs or toilets smell?

Could be mold in the space inside that sink intended to prevent overflow from overfilling. The inside of the sink is actually hollow so that any water that flows into the little overflow hole runs down into the drain. If you look down into the drain you should see spaces intended to let that water into the drain. Any black color there could be mold and would need to be bleached.