What is a sign that a beer may have been compromised by foreign microbes?

The presence of a ring of gunk at the neck of the bottle is a strong indicator that the beer may have been compromised by foreign microbes. This residue often results from yeast, bacteria, or other microorganisms that can either be inherent to the brewing process or introduced post-packaging. Such microbial activity can lead to various off-flavors, spoilage, and fermentation problems that negatively affect the beer's quality.

In the context of the other options, while unusual carbonation levels, a cloudy appearance, or a bitter aftertaste might suggest issues with the beer, they do not specifically indicate microbial contamination. Unusual carbonation could be due to over-carbonation during packaging or improper storage conditions. A cloudy appearance may stem from chill-haze or yeast still in suspension, which isn’t necessarily indicative of contamination. A bitter aftertaste might arise from various factors related to ingredients or brewing techniques rather than foreign microbes. Therefore, the presence of gunk is a more definitive sign of potential microbial influence in the beer.