Differential Diagnosis for a Red, Painful Knot Under the Armpit

When approaching a patient with a knot under their armpit that is red and painful, it's crucial to consider a wide range of potential diagnoses due to the variety of structures and tissues present in the axillary region. Here's a structured differential diagnosis:

• Single Most Likely Diagnosis
  • Infected Axillary Lymph Node or Folliculitis: This is often the most common cause of a painful, red knot under the armpit. It can result from a bacterial infection of the skin or from an infected lymph node reacting to a localized or systemic infection.
• Other Likely Diagnoses
  • Cat Scratch Disease: Caused by Bartonella henselae, this condition can lead to swollen and painful lymph nodes, especially in the axilla if the scratch occurred on the arm.
  • Hidradenitis Suppurativa: A chronic skin condition characterized by recurrent, painful abscesses and nodules in the apocrine-gland-bearing areas of the body, such as the armpits.
  • Lipoma or Sebaceous Cyst: While typically not painful unless infected, these benign growths can become inflamed and present as a painful knot.
• Do Not Miss Diagnoses
  • Breast Cancer with Axillary Lymph Node Involvement: Although less common, a hard, painless lump under the arm could be a sign of breast cancer that has spread to the lymph nodes. The presence of pain and redness might suggest an inflammatory breast cancer, which is a rare but aggressive form of breast cancer.
  • Lymphoma: Both Hodgkin and non-Hodgkin lymphoma can present with lymphadenopathy (enlarged lymph nodes), which might be painful, especially if there is an associated infection.
  • Tuberculosis or Atypical Mycobacterial Infection: These infections can cause lymphadenitis (inflammation of the lymph nodes) that might present as a painful, red knot under the arm.
• Rare Diagnoses
  • Melanoma Metastasis: Rarely, a metastasis from a melanoma could present as a lump under the arm, though this would typically be associated with a known history of melanoma.
  • Kikuchi Disease (Histocytic Necrotizing Lymphadenitis): A rare, self-limiting condition that primarily affects the lymph nodes in the neck but can occasionally involve axillary nodes, presenting with painful lymphadenopathy.

Each of these diagnoses requires careful consideration of the patient's history, physical examination findings, and potentially, diagnostic tests such as imaging studies or biopsies to determine the underlying cause of the symptoms.