Hello, I understand your concern for brown spot under armpit. After reading your query I can get that you have Erythrasma of the arm pit or it might be fungal infection of the area. For the best diagnosis a close examination is required to check for the area by close examination by a dermatologist or else you can send a good resolution picture fo the area at : bit.ly/drsanjaykanodia In my patients with your kind of problem I prefer an antibacterial cream if it is erythrasma or an anti-fugal cream if it is fungal infection. Stay in cool place and wear cotton clothes to reduce the further spread of the lesion. It is better o maintain a food hygiene to get better early and reduce the recurrence, All the best.