Hi, thanks for this interesting question.
Lung diseases are known to be chronic- long standing diseases. They are also known to lead to cardiac complications which can manifest in the form of edema of lower limbs
stone obstruction can lead to back pressure on kidney and it can also lead to limb swellings. Now on e needs to find out the exact cause of edema. Your mother needs complete urological and cardiac evaluation to pinpoint the cause of limb swellind. Since BUN and creatinine
are normal, there is no worry of kidney failure. Heart failure
and Kidney stones needs to be investigated. Treatment will finally depends on the cause. Best luck.