Subungual hematoma: This condition occurs when blood collects under the nail due to injury, trauma, or a crush accident. It's the most common cause of blood under the fingernail and is often associated with a history of recent trauma to the finger.

Nail biting or picking: Frequent biting or picking at the nails can cause small injuries that lead to bleeding under the nail.
Onychomycosis: Fungal infections of the nail can cause lifting of the nail plate, leading to bleeding underneath.
Psoriasis: Nail psoriasis can cause changes in the nail, including pitting, thickening, and separation from the nail bed, potentially leading to bleeding.
Trauma: Any form of trauma to the finger, such as a blow or a crush injury, can cause bleeding under the nail.

Subungual melanoma: Although rare, this is a form of skin cancer that can present with bleeding under the nail. It's crucial to not miss this diagnosis due to its potential severity and the importance of early treatment.
Endocarditis: Infective endocarditis can cause embolic phenomena, including splinter hemorrhages under the nails, which might be mistaken for subungual hematoma.
Vasculitis: Certain types of vasculitis can affect the small blood vessels under the nails, leading to bleeding.

Scurvy: Vitamin C deficiency can lead to poor wound healing and bleeding under the nails, among other symptoms.
Sickle cell disease: This genetic disorder can cause vaso-occlusive crises, potentially leading to infarcts under the nail and subsequent bleeding.
Antiphospholipid syndrome: This autoimmune disorder can increase the risk of blood clots and may cause bleeding under the nails due to thrombocytopenia or vasculopathy.