The answer to 'sink foul smell' is that a foul odor coming from your sink is typically caused by a buildup of organic matter, bacteria, or a blocked drain. This can lead to unpleasant smells that can be difficult to get rid of. Some common causes include food debris, grease, hair, and mineral deposits that accumulate over time.
The first step is to try some DIY solutions like using a sink plunger, baking soda and vinegar, or a drain cleaner to clear the blockage. However, if the smell persists or the drain is severely clogged, it's best to call in a professional plumber to properly diagnose and address the issue.