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1.The symptoms and history suggest the most possible cause could be - enlarges
lymph nodes.
2.These lymph nodal enlargement is seen in case of the any inflammation / infection, can be localised or systemic cause.
3.Other possible cause could be lump from the breast tissue or lactational changes with dilated ducts.
4.For better evaluation i would suggest to get an
ultrasound examination - ultrasound will exactly helps to evaluate the nature of the lesion, its source and extensions.
5.If necessary - an FNAC - fine needle aspiration cytology to evaluate the cells involved and can be treated accordingly.
Good luck.
Hope i have answered your query,
any thing to ask ? do not hesitate to ask.
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