While I never rule out any possible solutions when it comes to plumbing, I basically told them my gut feeling was that they had a water system issue, not a drain system issue. My reasoning was that sewer odors usually don't smell like rotten eggs. So, before they started taking apart the drain lines, I felt they should have their water system tested first.

Well, it seems that I was not the only person with an opinion on this! Please see several of the many letters I received from plumbers, contractors and homeowners about their opinions on what could be causing a "rotten egg" smell in a kitchen sink:

Ed, I had a rotten-egg-like smell in my kitchen sink and a plumber friend of mine told me the problem was within my old flexible faucet supply lines. He changed the water supply lines and no more rotten egg smell! Dan (Virginia)

Dear Ed, I remember when I was very young and the water fountains at zoo we used to visit smelled like rotten eggs. We were told that it was because the water had a high sulfur content. Just a thought. (No name.)

Ed, from one master plumber to another, the other possibility for strange kitchen sink odors may be mold growing under the cabinet due to moisture. Sorry to say this happened to me and I had to open up the walls and traced the cause to a leaking water line to the ice maker. Steve (Chicago)

Ed, I know the question was about a kitchen sink, so forgive me for changing the topic a bit. But, please remind homeowners that natural or propane gas also has a "rotten egg" smell as well. This is from the mercaptan odor that is injected into the gas to detect leaks. Persistent rotten egg like odors could be a small gas leak and need to be checked by your local gas company right away. Chris (California)

Well-said Chris. And if we learned one thing with this question, it seems that with all the different opinions we've heard, finding the exact cause of a rotten egg smell in a kitchen may be difficult to one to sniff out!