A hard lump growing at the root of the index finger may be caused by congenital developmental abnormality, tendon sheath cyst, tenosynovitis and so on. 1. Congenital developmental abnormality: this condition occurs after birth, mostly due to localized vasodilatation, and may not have obvious pain symptoms. 2. Tendon sheath cyst: It is a cystic swelling in the tendon sheath of the joint. The cause is not clear at present, but it may be related to the long-term damage and degeneration of the joints, mostly seen in young and middle-aged women. 3. Tenosynovitis: an acute and chronic inflammatory reaction in the tendon sheaths, most common in manual laborers, probably due to long-term excessive friction of the tendons. There may be other reasons for a hard lump at the root of the index finger, so we recommend that you go to the hospital in time, complete the examination to determine the cause of the problem, and then follow the doctor’s instructions for treatment.