Studor valve, or more generic, air admittance valves. Save time and $$$ in home building and are allowed by code. They are mechanical with moving parts and a rubber seal which can fail. If they fail, you will get a sewer gas odor. Google search will give you some sense of the controversy but they are widely used. Apparently if all goes well they should last 20 to 30 years. I have no idea if this is your problem but the description fits.
Here are DIY instructions on replacement if you are up to it
How to Fix a Sewer Gas Smell
If it is really just the hot water, not a smell coming from beneath the sink, then anode buildup in the hot water tank is possible, but it should create the smell at every hot water faucet, not just one sink. Here is a discussion of that and solutions for one sink vs all sinks:
Solving rotten-egg odor in water heaters
Good luck, let us know the outcome