Differential Diagnosis
- Single most likely diagnosis
- Folliculitis or Furuncle (Boil): The patient's history of scratching a small lump in her armpit, which then opened up and started draining, is consistent with a skin infection such as folliculitis or a furuncle. The location in the armpit, a warm and moist area prone to sweating, further supports this diagnosis.
- Other Likely diagnoses
- Abscess: Given the description of the lump opening up and draining, an abscess is a possible diagnosis. The patient's symptoms of a painful lump that eventually ruptured are consistent with an abscess.
- Infected Cyst or Sebaceous Cyst: The initial presentation of a small lump that the patient scratched, leading to it opening up, could also suggest an infected cyst or sebaceous cyst.
- Hidradenitis Suppurativa: Although less common, the location in the armpit and the nature of the lesion could suggest hidradenitis suppurativa, especially if the patient has a history of recurrent lesions in apocrine-gland-bearing areas.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Cat-Scratch Disease: Although the patient mentions scratching the lump, if she had been scratched by a cat and then touched the area, cat-scratch disease could be a consideration, especially if the lymph nodes are involved.
- Methicillin-Resistant Staphylococcus aureus (MRSA) Infection: MRSA can cause skin infections that resemble folliculitis, furuncles, or abscesses. It's crucial to consider MRSA, especially if the infection does not respond to standard treatments.
- Erysipelas or Cellulitis: While less likely given the localized nature of the symptoms, erysipelas or cellulitis could be considered, especially if there are signs of spreading infection or systemic symptoms.
- Rare diagnoses
- Mycobacterial Infection: Certain mycobacterial infections can cause skin lesions, although this would be less common and might be considered if the patient has been exposed to contaminated water or has a compromised immune system.
- Leishmaniasis: If the patient has traveled to areas where leishmaniasis is common, this could be a rare consideration, especially if the lesion does not respond to typical treatments for skin infections.