Final answer:
Renal vein thrombosis is a serious complication of nephrotic syndrome which may present with side pain, decreased kidney function, and increased swelling.
Step-by-step explanation:
A serious complication of nephrotic syndrome is renal vein thrombosis, which is when a blood clot forms in the renal vein, the vessel that drains blood away from the kidney. This complication can lead to worsening kidney function and may present with symptoms such as pain in the side, decreased kidney function, and potentially a worsening of the swelling that is often associated with nephrotic syndrome. Patients with nephrotic syndrome are at an increased risk for this complication due to changes in blood clotting mechanisms and increased blood cholesterol levels that can promote clot formation.