The main cause of a hard nodule on one side of the anus is several reasons: 1. When a patient suffers from anal fistula disease will make a hard nodule on one side of the anus, mostly because an abscess occurs around the anus and develops subcutaneously, thus forming a subcutaneous fistula, there will be pressure and pain, the local skin will be red, swollen and painful, etc. After a long time, the abscess will rupture and the swelling will disappear, accompanied by a foul odor and visible pus-like secretions overflowing. Most of them will wet the underwear. 2. When patients have thrombosed external hemorrhoids, harder nodules will also appear around the anus, and the symptoms are more obvious, and the thrombosed masses can be touched. 3. varicose veins external hemorrhoids. This kind of varicose external hemorrhoid does not need any treatment, it is only formed by the thickening of blood vessels, it does not hurt and does not have any uncomfortable symptoms, it is not necessary to do any treatment.