What Causes Thinning Of One Of The Thighs With History Of Arthritis?

my mother is 81 year old. Her left upper leg(thigh) is becoming thin as compare to other leg. She with great difficulty can move has arthrites problem. Also her creatnine is little high last test creatnine was 1.9 and she is under treatment of nephrology. so my question is what to do about her left upper leg getting thin as compare to right upper leg and which doctor shall i consultsk