A 34-year-old primigravida at 18 weeks gestation comes to the office for a routine prenatal examination. The patient's pregnancy has been uncomplicated. She takes a daily prenatal vitamin, and her laboratory results have been normal to date. The patient's personal and family medical histories are unremarkable. During the visit, a detailed fetal ultrasound reveals unilateral hydronephrosis. Male external genitalia are also visible. If the fetal hydronephrosis is caused by obstruction, which of the following is the most likely site?

Question

A 34-year-old primigravida at 18 weeks gestation comes to the office for a routine prenatal examination.  The patient's pregnancy has been uncomplicated.  She takes a daily prenatal vitamin, and her laboratory results have been normal to date.  The patient's personal and family medical histories are unremarkable.  During the visit, a detailed fetal ultrasound reveals unilateral hydronephrosis.  Male external genitalia are also visible.  If the fetal hydronephrosis is caused by obstruction, which of the following is the most likely site?

Quizplus · Accepted Answer

11ec160b_3752_d7f7_9249_5389b60f7289_MD0001_00 Fetal hydronephrosis is commonly detected in the second trimester during routine ultrasonographic anatomy survey. The finding is usually transient or clinically insignificant as the fetal renal pelvis has high compliance that makes the fetal kidney susceptible to dilation (hydronephrosis). The most common pathologic cause of unilateral fetal hydronephrosis is a narrowing or kinking of the proximal ureter at the ureteropelvic junction (UPJ). Newborns who were not diagnosed prenatally may present with a palpable abdominal mass reflecting an enlarged kidney. Embryologically, the fetal genitourinary tract is derived from the metanephric blastema and the ureteric bud (a dorsal outgrowth from the mesonephric duct). The metanephric blastema gives rise to functioning renal parenchyma by 10 weeks gestation while the ureteric bud develops into the renal pelvis and ureters through dilation and canalization. The UPJ is the last segment of the fetal ureter to canalize. The pathogenesis of UPJ obstruction may involve failure of canalization with abnormal development of circular musculature and/or collagen fibers. (Choices A, C, and D) Posterior urethral valves are the most common cause of bilateral fetal hydronephrosis in boys. They are caused by an obstructive, persistent urogenital membrane at the junction of the bladder and urethra. Other congenital anomalies that cause bilateral fetal hydronephrosis include urethral strictures, meatal stenosis, and bladder neck obstruction. Neurogenic causes of obstructive uropathy (eg, spinal cord damage) can also result in bilateral hydronephrosis. (Choice E) During normal bladder contraction, the intravesical portion of the ureter is compressed to ensure anterograde travel of urine. Vesicoureteral reflux results from incomplete closure of the vesicoureteral junction, allowing backward flow of urine into the ureter; however, this is a non-obstructive cause of fetal hydronephrosis. Furthermore, although the vesicoureteral junction is a very common location for kidney stones, it would be highly unusual for a fetus to have a kidney stone. Educational objective: Inadequate canalization of the ureteropelvic junction, the connection site between the kidney and the ureter, is the most common cause of unilateral fetal hydronephrosis. __________ References: Revised guidelines on management of antenatal hydronephrosis (http://www.ncbi.nlm.nih.gov/pubmed/23716913)

A 34-Year-Old Primigravida at 18 Weeks Gestation Comes to the Office