CLINICAL HISTORY.—A white male infant, born at the Bernalillo County Medical Center after an uneventful pregnancy, was found to have a mobile, firm, right lower quadrant mass. An excretory urogram, followed for 24 hours, confirmed the presence of the mass and established the diagnosis (Fig 1, 2, and 3). Prior to the time of surgical exploration at 10 days of age, the child manifested clinical evidence of urinary tract infection which was subsequently controlled by antibiotic therapy.
Denouement and Discussion
Ureteropelvic Junction Obstruction
Hydronephrosis is the most common upper abdominal mass encountered in a newborn infant.1 The pathogenesis of congenital ureteropelvic obstruction is not understood clearly and a variety of etiological explanations have been set forth. Intrinsic stenosis may be caused by collagenous replacement of muscular tissue or by fibrous scarring secondary to inflammation.2-5 Extrinsic obstruction may result from adhesions, ureteral kinks, high anomalous ureteral insertion, and renal