Which of the following is a sign of nail fungus?

Thickened nails are a recognized sign of nail fungus, also known as onychomycosis. When a fungal infection occurs, it can cause changes in the texture and appearance of the nails. This often includes thickening, which happens as the fungus disrupts the normal growth of the nail and leads to an accumulation of keratin. As the infection progresses, the nail may also become discolored, brittle, or develop a distorted shape.

The other options presented do not typically indicate the presence of a fungal infection. Brightly colored nails, for example, may be caused by various non-fungal factors, such as nail polish or other cosmetic products. Perfectly smooth nails usually signify good nail health and are not associated with fungal issues. Shiny nails can indicate healthy nails but are not a sign of a fungal infection either. Thus, thickened nails stand out as a clear symptom of nail fungus.