Final answer:
In a child with nephrotic syndrome who suddenly develops flank pain, the most likely cause is renal vein thrombosis. The next step would be to perform imaging studies to confirm the diagnosis.
Step-by-step explanation:
In a child with nephrotic syndrome who suddenly develops flank pain, the most likely cause is renal vein thrombosis. Renal vein thrombosis occurs when a blood clot forms in the vein that drains the kidney, causing pain in the flank area. The next step in evaluating this child would be to perform imaging studies, such as a renal ultrasound, to confirm the diagnosis of renal vein thrombosis.