Differential Diagnosis for Painful Lumps Under the Armpit

The patient presents with several painful lumps under the left armpit that have been present for approximately one and a half months. The lumps were initially painless, decreased in size, but have since acutely worsened in pain. The following differential diagnoses are considered:

• Single Most Likely Diagnosis
  • Infected Epidermal Inclusion Cyst (EIC) or Infected Sebaceous Cyst: The patient's history of shaving her armpits and the description of the lumps suggest a possible epidermal inclusion cyst or sebaceous cyst that has become infected, leading to increased pain and tenderness. The absence of systemic symptoms like fever and the localized nature of the pain support this diagnosis.
• Other Likely Diagnoses
  • Folliculitis: Given the patient's history of shaving, folliculitis is a plausible diagnosis. However, the lack of redness or visible inflammation makes it less likely compared to an infected cyst.
  • Hidradenitis Suppurativa (HS): Although HS can present with painful lumps under the arms, the acute worsening of pain and the absence of a history of similar lesions or skin conditions make it less likely in this scenario.
  • Lymphadenitis: Inflammation of the lymph nodes could cause painful lumps, but the absence of systemic symptoms such as fever and the specific location and description of the lumps make this less likely.
• Do Not Miss Diagnoses
  • Breast Cancer with Axillary Metastasis: Although rare and less likely given the patient's age and presentation, breast cancer metastasizing to the axilla could present as painful lumps. The absence of other symptoms like a breast mass or nipple discharge does not rule out this diagnosis.
  • Lymphoma: Similar to breast cancer, lymphoma could present with axillary lymphadenopathy. The lack of systemic symptoms such as fever, night sweats, or weight loss does not exclude this diagnosis.
• Rare Diagnoses
  • Cutaneous Metastasis: Metastasis to the skin from a distant primary cancer is rare but could present as lumps under the arm. The patient's lack of history of cancer makes this less likely.
  • Dermatofibroma: A benign skin growth that could be painful if irritated or inflamed, but typically does not present as multiple painful lumps.
  • Lipoma: A benign tumor of fat tissue that is usually painless and soft, making it an unlikely diagnosis given the patient's symptoms.

Each diagnosis is considered based on the patient's symptoms, history, and the potential for the condition to cause severe pain and tenderness in the axillary region. The single most likely diagnosis and other likely diagnoses are prioritized based on the combination of clinical presentation and commonality of the conditions. Do not miss diagnoses are included due to their potential severity and impact on patient outcomes, even if they are less likely. Rare diagnoses are considered for completeness, acknowledging their lower probability.