Differential Diagnosis for a Red and Painful Lump under the Armpit

Single Most Likely Diagnosis

• Infected Epidermal Inclusion Cyst or Furuncle: This is a common cause of a painful, red lump under the armpit. The size being bigger than a pea and the presence of pain and redness suggest an infectious process, likely involving a cyst or a hair follicle that has become infected.

Other Likely Diagnoses

• Cat Scratch Disease: Caused by Bartonella henselae, this infection can lead to a painful lymph node swelling, often in the axilla, following a cat scratch or bite. The lymph node can become suppurative and form an abscess.
• Hidradenitis Suppurativa: A chronic skin condition characterized by recurrent, painful abscesses and nodules in apocrine-gland-bearing areas, such as the axillae. It can present with a single painful nodule or abscess.
• Lymphadenitis: Inflammation of the lymph nodes, which can be caused by bacterial, viral, or fungal infections. The axillary lymph nodes can become enlarged, red, and painful due to infection.

Do Not Miss Diagnoses

• Breast Cancer with Axillary Metastasis: Although less likely, a new lump under the armpit could be a sign of breast cancer that has spread to the axillary lymph nodes. This would be particularly concerning in older patients or those with risk factors for breast cancer.
• Tuberculosis: TB can cause lymphadenitis, including in the axillary lymph nodes, and is a critical diagnosis not to miss due to its public health implications and the need for specific treatment.
• Lymphoma: Both Hodgkin and non-Hodgkin lymphoma can present with lymphadenopathy, including in the axilla. This diagnosis is crucial due to its significant implications for treatment and prognosis.

Rare Diagnoses

• Actinomycosis: A rare bacterial infection that can cause abscesses or masses in the skin, including under the arm. It's often associated with a history of trauma or surgery.
• Nocardiosis: A rare infection caused by Nocardia species, which can lead to abscesses or cellulitis, including in the axillary region, particularly in immunocompromised patients.

When considering antibiotic treatment, it's essential to cover common skin pathogens such as Staphylococcus aureus and Streptococcus pyogenes. However, the choice of antibiotic should be guided by the suspected diagnosis and local resistance patterns. For example, a first-generation cephalosporin or dicloxacillin could be considered for initial treatment of a suspected skin infection like an infected epidermal inclusion cyst or furuncle.