I agree, seek help and get your knee, hip, back and neural structures examined as Bob indicated.
The Straight leg raise your doctor asked you to do being so painful indicates that some neural component is involved and thus likely neural glides and manual therapy of the spine would be beneficial.
My thought was how did this come about. Originally it started just in the knee and then started to spread. I would keep in the back of your mind that a biomechanical issue from the foot/ankle may have been the original cause and the tibial torsion and associated imbalances have travelled up into the spine to cause the problem you now see. It of course highly possible the problem came from the knee or back originally - Hence the reason to seek someone skilled to give you a 45min to hour assessment and initiate some treatment.

Puddin'