A woman who shared a photo of faint black lines on her thumb and toe nails on Reddit was urged to see a doctor after commenters warned the marks can sometimes indicate melanoma. Medical sources say such lines, known as melanonychia, are often harmless but can signal aggressive skin cancer, highlighting the importance of professional assessment rather than online diagnosis.source https://timesofindia.indiatimes.com/etimes/trending/faint-black-lines-on-nails-harmless-or-sign-of-aggressive-skin-cancer-reddit-post-goes-viral/articleshow/126421840.cms