Brief Answer:
It is called edema

Detailed Answer:
Hello and welcome to Ask A Doctor service.

We need to find cause of edema. One side swollen leg (edema of leg) that hurts could be due to deep venous thrombosis, lymphedema, or venous insufficiency. Causes of lymphedema include tumor, filaria, trauma, pelvic surgery, inguinal lymph node removal by surgery, and radiotherapy. Filaria is usually on both legs. Since filaria test is negative we need to find other causes. Has she got any surgery recently? Ultrasound of whole abdomen and pelvis can be done to find any tumor, etc
A venous duplex ultrasound will help find any venous cause.
A general surgeon will be able to help with these conditions.

Hope this helps. Let me know if I can assist you further.
Dr Vaishalee

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