What could be causing the foul smell in my bathroom sink?
A blocked vent is probably the most likely culprit. Other things to consider: leaking plumbing from a low point in the trap under the sink letting the trap drain, evaporation of the water in the trap if this sink is rarely used. 

Some sinks do not have a true vent, and may have in place aa AAV (Air admittance valves-AAVs or Durgo valves or Studor vents). In my experience, these are poorly designed and allow sewer gas to seep through.

Sometimes really old hair that has been in a drain pipe for a long time will smell pretty bad. Not a sulfur smell like sewer gas, but more like a poop smell.