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1.The location and nature of the lump suggest a
ganglion - which arises from the joint space, from tendon,ligament and joint space generally painless.
2.Possible infective cause with collection of pus with in - hair follicular infection.
3.In this case you need to get examined and request for an
ultrasound, that would help to know the nature of the lump and adjacent area, and its source and extensions.
4.In infective case, if large a small
incision and drainage will be helpful followed by antibiotic course.
5.So i would recommend to get an appointment and get evaluated and treated accordingly. Good luck.
Hope i have answered your query.
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http://doctor.healthcaremagic.com/doctors/dr-ganesh/62888 Thank you.