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A cyst like lesion in underarms that is painful and red can be due to multiple causes but the common one’s include folliculitis, which is a swelling involving the hair follicles or if it is recurrent and leaves a scar, HS could also be a possibility.
Next Steps
Get it checked by a Dermatologist or Surgeon to exclude other possibilities and for the required treatment.
Health Tips
If there is associated fever, chills or history of past tuberculosis infection, consult a doctor immediately.