Chyluria is the presence of chyle in the urine and is found when there is an abnormal communication between the lymphatics and the renal tract.

Globally, it is most commonly due to filariasis. In the non-tropical world it is most commonly iatrogenic, as in this case. It is seen with partial, but not complete nephrectomy. The theory is that as part of nephron-sparing surgery, the perirenal lymphatics end up communicating with the renal collecting system. Of course this does not happen when the kidney is removed in toto in more radical surgery.

Chyluria is usually asymptomatic in the post-surgical cohort. In filariasis it may lead to nephrotic syndrome due to massive urinary protein losses.

The important imaging learning point here is not to assume that every time you see low density anterior to urine in the bladder it is a gas-fluid level.