What is a key characteristic of a nail affected by onychomycosis?

Onychomycosis is a fungal infection of the nail that typically leads to noticeable changes in the nail's appearance. One of the key characteristics of a nail affected by onychomycosis is a crumbly free edge. This occurs because the fungal infection weakens the nail structure, causing it to break down and crumble rather than maintaining its usual smooth and solid appearance. As a result, the free edge becomes brittle and can flake off, which is a clear indicator of the presence of a fungal infection.

In contrast, the other options describe symptoms or characteristics that are not typically associated with onychomycosis. Pus and swelling are more indicative of bacterial infections or other inflammatory conditions rather than a fungal issue. The presence of red patches would suggest inflammation or irritation, which again is not specific to fungal infections. Longitudinal splits can occur for various reasons, including trauma or underlying health conditions, but they do not specifically indicate a fungal infection as onychomycosis does. Thus, the crumbly free edge is a distinctive sign of nail fungal infection.