Cat Scratch Disease: This is a common condition caused by the bacterium Bartonella henselae, typically transmitted through scratches from domestic or feral cats. It often presents with a painful lymph node in the axilla (armpit) on the same side as the scratch, which can come and go.

Viral Infections (e.g., Infectious Mononucleosis, Cytomegalovirus): These can cause lymphadenopathy (enlarged lymph nodes) that may be painful and can fluctuate in size. The presence of systemic symptoms like fever, sore throat, and fatigue may accompany the lymph node enlargement.
Bacterial Infections (e.g., Staphylococcus aureus, Streptococcus pyogenes): Skin infections in the arm or chest area can lead to axillary lymphadenitis, which is inflammation of the lymph nodes in the armpit. These infections can cause painful lymph nodes that may come and go as the infection resolves or worsens.
Breast Cancer or Other Malignancies: Although less common, malignancies, especially of the breast, can metastasize to the axillary lymph nodes, causing them to become enlarged and sometimes painful.

Lymphoma: Both Hodgkin and non-Hodgkin lymphoma can present with lymphadenopathy. While lymphoma is less common than infectious causes, missing this diagnosis can have severe consequences due to the need for prompt treatment.
Breast Cancer: As mentioned, breast cancer can cause axillary lymphadenopathy. Given the potential for serious outcomes if left untreated, it's crucial not to miss this diagnosis, especially in individuals with risk factors for breast cancer.
Tuberculosis (TB): TB can cause lymphadenitis, and although it's less common in some regions, it's a critical diagnosis not to miss due to its public health implications and the need for specific treatment.

Sarcoidosis: This is a condition characterized by the growth of granulomas (inflammatory cells) in various parts of the body, including the lymph nodes. It can cause lymphadenopathy but is less common than other diagnoses listed here.
Kikuchi-Fujimoto Disease: A rare, self-limiting condition that primarily affects the lymph nodes in the neck but can also involve axillary lymph nodes, causing pain and enlargement.
Castleman Disease: A rare disorder that involves an overgrowth of cells in the lymphatic system, which can cause lymphadenopathy among other symptoms.