What condition may develop in a child with nephrotic syndrome experiencing loin pain and an enlarged kidney?

In a child with nephrotic syndrome who presents with loin pain and an enlarged kidney, the most likely condition to develop is renal vein thrombosis. Nephrotic syndrome is associated with a number of complications, one of which is an increased risk of thrombosis due to hypercoagulability, a state where the blood has a higher tendency to clot.

In nephrotic syndrome, the loss of proteins in urine leads to a decrease in serum albumin, which subsequently can cause changes in blood volume and pressure. As a result, the renal veins may become obstructed, leading to renal vein thrombosis. This condition often presents with flank pain (loin pain) and can cause an enlarged kidney due to swelling and decreased venous outflow, which may happen as a result of the thrombosis.

The other conditions listed may occur in children but do not typically present in the same way. Acute pyelonephritis would primarily cause systemic symptoms like fever due to a kidney infection, and hydronephrosis involves dilation of the renal pelvis and calyces due to obstruction, which may not always present with loin pain in the context described. Glomerulonephritis refers to inflammation of the glomeruli in the kidneys and is usually