Which condition is characterized by split or brittle nails with lengthwise ridges?

The condition characterized by split or brittle nails with lengthwise ridges is known as onychorrhexis. This condition typically presents as brittle nails that display multiple longitudinal ridges, which can be a result of various factors including trauma, exposure to harsh chemicals, or underlying health conditions. Onychorrhexis reflects a structural weakness in the nail plate that leads to its fragility, making nails prone to splitting or breaking.

Understanding onychorrhexis is critical for cosmetologists as it underscores the importance of nail care and the need for clients to be aware of their nail health. Proper nail care routines, including moisturizing and minimizing exposure to damaging substances, can help prevent or alleviate the severity of this condition.

The other options describe different conditions or terms: onychophagy refers to the habit of nail biting, plicatured nails involve an abnormal curvature of the nail plate, and lanthionine bonds are related to hair structure rather than nails, highlighting the importance of distinguishing between these terms in the realm of cosmetology.