A hard lump under the armpit. In the first case, the most common form of paronychia is a localized raised lump that is a congenital developmental abnormality. It often occurs in both armpits at the same time, and the appearance of the parametrium is due to incomplete degeneration of the breast tissue during the embryonic period, and sometimes the patient may experience localized pain. The second condition is the enlargement of axillary lymph nodes, which requires patients to go to the hospital for timely examination because there are many reasons for the enlargement of lymph nodes, the most serious one being the enlargement of axillary lymph nodes due to the metastasis of tumors. For example, breast cancer can cause hard lumps in the armpit, so patients must pay attention to this situation and go to the hospital for timely examination and treatment.