The answer to 'why is my sink foul smell' is that a sink drain can develop unpleasant odors for several common reasons. This includes food debris buildup, mold growth, or a dried-out P-trap. The symptoms are typically a persistent, unpleasant smell coming from the sink drain.
To address a foul sink smell, you can try DIY solutions like using a drain cleaner, baking soda and vinegar, or manually removing any visible debris. However, if the odor persists or you're unsure of the cause, it's best to call a professional plumber to properly diagnose and fix the issue.