Brief Answer:
Different possible causes

Detailed Answer:

Hi,

Weakness in one limb can be due to different possible causes such as:

(1) Nerve injury or abnormal nerve function of the nerves that go to that leg - EG if there is arthritis of the lower back affecting the nerves on the left that come out and go to the legs- this can cause weakness

(2) Arthritis of the back or left hip or left knee can also cause weakness but this would be associated with pain.

(3) Muscle injury or abnormal muscle function of the left lower limb. If there is injury to any of the ligaments of the knee, this can also cause it to give away

Because only one lower limb is involved, the weakness is not likely due to a systemic cause such as anemia (low blood count) or low levels of thyroid hormone or systemic infection.

To find out the exact cause, you would need to be seen by your doctor. Imaging of the back , hip and knee would be needed.

Bloods would depend on examination findings.

Treatment would depend on the cause found but if the knee is unstable, a brace may help, you would also need to make sure to use a walking aid- walking stick or walker

I hope this helps, feel free to ask any other questions.

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