What causes brown streaks in fingernails?

Brown streaks in fingernails can be caused by various factors, including trauma or injury to the nail matrix, which can lead to bleeding under the nail. They may also indicate the presence of a fungal infection or a dermatological condition such as melanin deposits. In some cases, brown streaks can be a sign of more serious health issues, such as melanoma, particularly if they are new, changing, or accompanied by other symptoms. If brown streaks appear, it's advisable to consult a healthcare professional for proper evaluation.