Show Explanatory Sources

This patient's CT scan shows fusion of the right and left renal parenchyma, or horseshoe kidney, a congenital anomaly caused by fusion of the metanephros in utero.

The metanephros is an embryologic structure that comprises the metanephric blastema, which gives rise to the renal parenchyma, and the ureteric bud, which gives rise to the renal collecting system and ureters.  Abnormal migration of the right and left metanephric blastema can lead to fusion of this developing renal parenchyma along the inferior poles, forming a U or horseshoe shape.  The fused kidneys begin their physiologic ascent from the pelvis into the abdomen until the isthmus (central fused portion) is physically restricted by the overlying inferior mesenteric artery at its origin from the aorta.  As a result, compared to normal, horseshoe kidneys lie lower in the abdomen.  The isthmus also displaces the renal pelvises superiorly, often causing the ureters to insert abnormally high and pass over the anterior portions of the kidneys or the isthmus.

Although horseshoe kidney is often an incidental or asymptomatic finding, patients are prone to intermittent ureteropelvic junction obstruction and urinary stasis due to the abnormal orientation of the kidneys and location of ureteral insertion.  These consequences increase the risk for nephrolithiasis as well as retrograde urine flow (ie, reflux) and, therefore, recurrent urinary tract infections.

(Choices A, B, and D)  The pronephros is a rudimentary nephric system that degenerates in early gestation.  Similarly, much of the rudimentary mesonephros degenerates, although a portion (the mesonephric duct) persists as the ureteric bud.  This patient has horseshoe kidney, a congenital defect involving the renal parenchyma, which is derived only from the metanephric blastema, a component of the metanephros.

(Choice E)  Although the nephric systems (including pronephros, mesonephros, and metanephros) are all derived from the urogenital ridge, this structure also gives rise to the gonads, reproductive tract, and adrenal cortex in addition to the kidney.  Horseshoe kidney is due to fusion of the metanephros alone.

Educational objective:
Horseshoe kidney is a congenital anomaly in which fusion of the metanephros (specifically, metanephric blastema) results in fused renal parenchyma along the inferior poles.  Most patients are asymptomatic, but some may develop recurrent urinary tract infections or nephrolithiasis due to abnormal urine flow through the malpositioned kidney and ureters.