What compound contributes to the skunky aroma found in certain beers?

The skunky aroma found in certain beers is primarily attributed to the presence of methyl or isopentyl mercaptan. This compound is a result of a chemical reaction that occurs when beer is exposed to light, particularly ultraviolet (UV) light. The ultraviolet exposure causes the breakdown of hop compounds, specifically the iso-alpha acids that develop during the brewing process. When these compounds oxidize, they can form mercaptans, leading to the characteristic skunky aroma that can be detected even at very low concentrations.

Other options such as isohumulones, phenolic compounds, and esters contribute to the overall aroma and flavor profile of beer but do not specifically account for the skunky smell. Isohumulones relate to the bitterness derived from hops, phenolic compounds are often associated with spicy or medicinal aromas common in certain styles, and esters typically provide fruity aromas. Thus, the unique skunky aroma is distinctly linked to methyl or isopentyl mercaptan, making it the correct answer.