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1.The history and symptoms suggest the possible cause could be superficial
skin infection / inflammation, next possible cause will be hair
follicle infection.
2.The other possible cause internal / testicular mass lesion.
3.The treatment in the case would be antibiotic course with local hygiene.
4.To confirm the diagnosis
ultrasound scrotum will be investigation of choice.
5.No blood alone will notable to catch hold of all the STD's, some time swab from the discharge will be helpful.
6.The ultrasound will give the source and its nature if the lesion and extensions. Good luck.
Hope i have answered your query,
any thing to ask do not hesitate to ask.
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