A pea-sized lump under the armpit, non-mobile and tender upon palpation.

A small, pea-sized lump under the armpit that does not move and is painful upon pressing may be caused by folliculitis, swollen lymph nodes, or lymphadenitis, among other conditions, and timely medical treatment at a hospital is necessary. Specific causes are as follows:

1. Folliculitis

This condition is usually caused by friction from external substances or bacterial or fungal infections leading to hair follicle damage. Inflammatory irritation may result in the aforementioned symptoms. Treatment may include topical medications such as compound polymyxin B ointment or mupirocin ointment, used according to medical advice.

2. Lymphadenopathy (Swollen Lymph Nodes)

This typically results from reactive proliferation of lymphocytes and histiocytes within lymph nodes due to various injuries or stimuli. It may present as a hard lump accompanied by tenderness upon pressure. Treatment under medical guidance may include medications such as Yanlixiao capsules or thalidomide tablets.

3. Lymphadenitis

This is commonly an infection caused by Staphylococcus aureus or hemolytic streptococci. When the skin is damaged, pathogens can invade through the broken skin or mucous membranes, resulting in lumps and pain. Under medical supervision, medications such as Qingre Sanjie tablets or Xihuang pills may be used for treatment.

In addition to the above causes, similar symptoms may also result from sebaceous cysts, lipomas, or schwannomas. Patients are advised to seek prompt medical evaluation and treatment at a reputable hospital. Regular moderate physical exercise may also help enhance personal immunity.